EEVblog Electronics Community Forum

Electronics => Metrology => Topic started by: Andreas on June 15, 2014, 04:19:35 pm

Title: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:19:35 pm
Hello volt-nuts,

in this thread I will describe temperature coefficient (T.C.) measurements on precision resistors like Z201, UPW50, 8E16 ... i.e. with ageing <50 ppm/year and T.C. <= 5ppm/deg.

Motivation:

I got the idea from Mickle in the LM399 thread with his statistical divider. The idea is to select from several individual resistors a matching pair with nearly same tempco for either the temperature setpoint of a LTC1000 reference (12K5 + 1K) or a output voltage divider 10V/7V for a output buffer of a precision reference voltage. My temperature range for this is the "extended room temperature" range which is from 18 to 33 degrees C at minimum or with some reserve around 10-40 deg C.

There are many different informations regarding T.C. on the web especially for the Z201 metal foil (Z-foil) resistors. While the manufacturer gives a typical spec of 0.2ppm/deg or even  0.05ppm/deg for a limited 0-60 deg C range other "typical" measurement values from users of Z-foil resistors (VHP202Z) are more in the range from -0.3 up to -1ppm / deg for a  around 10K resistor which is not very far from wire wound resistors with 3-5ppm/deg.
https://www.eevblog.com/forum/reviews/precision-resistor-standard/msg420396/#msg420396 (https://www.eevblog.com/forum/reviews/precision-resistor-standard/msg420396/#msg420396)
https://www.eevblog.com/forum/projects/precision-dc-current-source-diy/msg382583/#msg382583 (https://www.eevblog.com/forum/projects/precision-dc-current-source-diy/msg382583/#msg382583)

So I wanted to make my own picture and do some comparisons at least with Z201 and UPW50

Also from other articles I got the impression that the metal foil resistors have from principle more hysteresis than good wire wound resistors. The metal foil is bonded do a ceramic substrate and the bond may create some hysteresis which is not available on wire wound resistors. So it is interesting for me how large the hysteresis actual is for the 10-40 degrees range. Dr. Frank reports 5ppm hysteresis for a 25 deg to 125 deg excursion for a hermetically 10K VHP202Z which is the hermetically tight upgrade of Z201. Others report soldering drift (hysteresis?) of even 50 ppm.
https://www.eevblog.com/forum/reviews/precision-resistor-standard/msg420729/#msg420729 (https://www.eevblog.com/forum/reviews/precision-resistor-standard/msg420729/#msg420729)
https://www.eevblog.com/forum/projects/vishay-bulk-foil-drift-after-soldering/msg445297/#msg445297 (https://www.eevblog.com/forum/projects/vishay-bulk-foil-drift-after-soldering/msg445297/#msg445297)

Further questions:

Another question is wether the resistor value of a Z201 resistor has a influence on T.C.
Is there a sweet spot resistor value which really is able to be below 0.1ppm/deg for selected resistors?
So perhaps it is better to use for e.g. all values created out of 2K or 5K resistors with zero T.C. instead of using a 1K with large positive T.C. and a 10K with large negative T.C.

Humidity sensitivity:

On the other side a volt-nut friend of mine has reported a humidity sensitivity between 5-50ppm for 30% RH change of 8E16 resistors which are similar to the UPW50 wire wound resistors. I hope I will get further information on this topic from him when he is back from his trip.
My problem is that I do not have any hermetically resistors up to now so that I cannot measure the RH sensitivity with my equipment. But 50ppm is really serious compared to 1ppm/K over some 10K temperature change.

If you have any measurement data on these topics: Feel free to contribute.
The minimum numbers that I need is the type of resistor, the resistor value and the measured values preferably as temperature cycle from around 25 down to 10 up to 40 and back to 25 degrees C so that a hysteresis can be easily shown.

For my part I will need some days to post the first results and update the thread. So don't be too impatient.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:20:00 pm
first pre tests:

The first test that I did is simply a voltage divider built of a 1K Z201 resistor and a 1K UPW50 resistor.
The resistors are soldered to 1.5m long lines (twin 0.14qmm) so that they can easily put into a temperature controlled environment.
Of cause the soldering is done at the end of the Resistor pins using aligator clips of a "3rd hand" to keep the resistor elements cool during soldering.

Because of the line length which consumes around 0.5mV (100ppm) for each line I need a 4 wire connection for each resistor.
The current path is fed from the 5V voltage reference of a 24 bit ADC.
From one to the other resistor. And finally to the star ground of the voltage reference.
The voltage path measures the voltages by a MAX4052A multiplexer which is attached to the ADC.
So I have a quasi differential measurement over the resistors.
Since the 0.5mV on the copper lines are not constant over temperature they are not usable for T.C. calculation.

The 24 bit ADC is similar to branadics hardware:
https://www.eevblog.com/forum/projects/oshw-24bit-adc-measurement-system-for-voltage-references/ (https://www.eevblog.com/forum/projects/oshw-24bit-adc-measurement-system-for-voltage-references/)
The main differences are: my FT232R and the pre regulators for the reference voltage are outside of the ADC board. So the design of branadic is more compact. I also use different voltage references. But since I do all measurements in a ratiometrical manner the stability of the voltage reference has a minor effect on the T.C. measurement results in my setup. My highest quality ADCs with AD586LQ and LT1236AILS8-5 are needed for my daily ageing measurements.

First measurement was simply taking the Z201 between my fingers to heat up from room temperature (around 25 deg C) to around 35 deg C.
This gave a around +9 uV voltage increase measured over the 1K Z201 resistor (ADC_CH1 - ADC_CH2). (minute 5 to 9 in the measurement).
On the other side a warming up of the 1K UPW50 gave a -27uV decrease over the Z201.
So both measured resistors have a positive TC and the UPW50 is only about a factor of 3 worse than the Z201.



Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:20:26 pm
measurement setup:

the whole setup is built without using pricey instruments.

IMG1625w shows an overview:
The (blue) temperature chamber on the floor is a 40 EUR (price may vary) 12V/230V (car) cool box.

Within the box there is a aluminium sheet (~1mm thick) as heat spreader for the heater foils.
(IMG1627w = bottom view).
http://www.reichelt.de/Heizfolien/THF-77110/3//index.html?ACTION=3&GROUPID=5159&ARTICLE=108462&SEARCH=THF-77110&SHOW=1&OFFSET=30& (http://www.reichelt.de/Heizfolien/THF-77110/3//index.html?ACTION=3&GROUPID=5159&ARTICLE=108462&SEARCH=THF-77110&SHOW=1&OFFSET=30&)

In the middle between the foils a small NTC 33K (red legs + black line) glued to the heat spreader.
http://www.reichelt.de/Heissleiter-Varistoren/NTC-0-2-33K/3//index.html?ACTION=3&GROUPID=3114&ARTICLE=13561&SEARCH=NTC-0%2C2%2033K&SHOW=1&OFFSET=30& (http://www.reichelt.de/Heissleiter-Varistoren/NTC-0-2-33K/3//index.html?ACTION=3&GROUPID=3114&ARTICLE=13561&SEARCH=NTC-0%2C2%2033K&SHOW=1&OFFSET=30&)

Edit: the fan + plastic cap is now obsolete and is no longer used (see below)
On the other side (IMG1635w) there is a low noise fan (low power) under a plastic cap (CD-spindle against direct cold draft + dripping water from the cooler).
http://www.reichelt.de/Luefter/FAN-ML-6015-12-S/3//index.html?ACTION=3&GROUPID=6215&ARTICLE=110414&SEARCH=FAN-ML%206015-12%20S&SHOW=1&OFFSET=30& (http://www.reichelt.de/Luefter/FAN-ML-6015-12-S/3//index.html?ACTION=3&GROUPID=6215&ARTICLE=110414&SEARCH=FAN-ML%206015-12%20S&SHOW=1&OFFSET=30&)

The resistor under test is mounted under the air current of the fan together with 2 additional NTCs to sense the temperature of the resistor rather than the aluminium sheet. The fan shall keep the NTCs and the resistor at the same temperature.
Unfortunately there is no possibility to place the sensor within the resistor. So when ramping up + down the temperature there will be always small differences between actual and meaured temperature.

On the desk in foreground from left to right:
ADC3 (grey box) measuring the NTC (27K pull up) of the heat spreader. The black cable is a USB/RS232 converter connected to the laptop.
ADC18 (grey box) with a MAX4052A multiplexer attached measuring the resistors with 4 channels (4 wire pseudo differential).
ADC14 (grey box) with a MAX4052A multiplexer with 27K multiplexed pull up measuring the 4 NTCs of the 2 resistors under test.
All 24 bit ADCs (LTC2400) are isolated with photocouplers on RS232 line and battery supplied to keep mains noise away.

The measurement setup uses only ratiometric measurement. So all reference voltages cancel out.
On the resistors under test the offset of the ADC is also canceled out by the pseudo differential measurement.
The noise of the ADC (around 10uVpp for a single measurement) is reduced by averaging over 1 minute to around 1uVpp.
The LTC2400 does a self adjustment of offset and full range at every conversion so it is very stable over temperature compared to other 24 bit converters. With the LTC2400 a temperature controlled environment is not necessary.

On the right: Instrument to measure the battery state before and after measurement.

Middle left:
2 Power stages (VNP5N07 + 1K series at the gate + 100K pull down) for the heater foils.
http://de.rs-online.com/web/p/transistoren-mosfet/3133080/ (http://de.rs-online.com/web/p/transistoren-mosfet/3133080/)
The heater foils are connected in series to a 17-18V unregulated DC supply (not visible)
 (transformer 2*12V/30VA with middle tap, MBR20100CT schottky, 4700uF/35V Cap)
The power stages are connected to the RTS-lines of the USB/RS232 converters and can be.
The 3rd USB/RS232 converter is connected to a 230V SCR relay (not visible) for the cooler.

Background left:
Cardbox with temperature controlled (27.5 deg C) reference resistor between 2 heat spreaders with heater foil
together with 2 NTCs. See also IMG1631w.

The cooler is switched on for negative slopes below 32 deg C and switched off at 15 deg C for rising slopes.
The temperature control is done by the heaters.
The temperature range is around 10 - 45 degrees. (10 degrees only below 25 degrees room temperature).

The laptop is generating temperature setpoints calculating the temperature control loops + sampling all measurement data as average values over 1 minute.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Enhancements to the measurement setup.

1)
the most essential tip came from Emmanuel:
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg600033/#msg600033 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg600033/#msg600033)

Previously I had the body from the resistor within a metal block and contacted the wires outside the block.
But this gives unequal temperatures between the 2 wires (thermoelectric) and additional to the body.
(different hysteresis curve when placing resistor face down or face up).
I tried a 12V fan for temperature equalizing but with little positive effect.

The most heat transfer from / to the resistor is through the wires.
So it is most essential to keep the wires at the same temperature.
My approach from 14.02.2015 on is with aluminum sheets + silicone foil (see pictures).
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg615638/#msg615638 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg615638/#msg615638)
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg637341/#msg637341 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg637341/#msg637341)
The whole setup is now in a shoe box (card box) within the car cooler to keep air drafts away.

2)
AC-multiplexer
See also:
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg693460/#msg693460 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg693460/#msg693460)

This was a approach to eliminate completely the thermocouple voltages.
I tried this with low ohmic resistors where it works good.
But with higher ohmic resistors 12K and 70K the filter capacitors which
I need to compensate EMI noise from the long lines had a bad effect on the measurements
since they have to be completely charged/discharged every measurement..
With the enhancement 1) the thermocouple voltages play nearly no role,
so I discarded the AC-Multiplexer.

3)
For the 70K resistors the noise level of the measurement increased dramatically.
Usually the LTC2400 has around 10uVpp noise level.
But with the 70K resistors the average level increased to about 50uVpp
with single events far above the average level.
Increasing the EMI capacitors was useless:
So I changed from unshielded wires to shielded lines and grounded
the heat spreaders of reference and DUT resistor.
With shielded lines + EMI filters the noise level is somewhere between
12-18uVpp as average level with the 70K resistors.

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------

error estimation:

resistor self heating:
--------------------------
since there are no values of thermal resistance I use the power derating curves for a worst case estimation.
self heating of the resistor is relative constant over temperature so only the whole curve is shifted.
-> has only (a small) influence on the 25 degree related values.

Z201/S102 1K self heating
---------------------------------
Z201 derating 0.3W / 50K -> 6mW/K
power 1K 6.25mW -> 1K
due to thermal grease / fan a reduction of factor 3-5 can be estimated
-> resulting 0.2 - 0.33 K error


UPW50 1K self heating
-----------------------------
UPW50 derating 0.5W / 20K -> 25mW/K
power 1K 6.25mW -> 0.25K
due to thermal grease / fan a reduction of factor 3-5 can be estimated
-> resulting 0.1 - 0.05 K error


NTC-02 33K self heating with 27K pull up @ 5V reference
------------------------------------------------------------------------
Datasheet: dissipation factor 7mW/K

max power (power match): 2.5V @ 27K = 0.23 mW
-> heating with 1:4 multiplex for NTC1-NTC4 = 8.3mK average
will be further reduced due to thermal grease.


Error due to T.C. of reference resistor Z201#1
----------------------------------------------------------
Temperature is controlled to 27.5 degrees C nominal.
There are slight variations (or ADC noise) during measurement of up to 0.05 K.
Together with the T.C. of 0.9 ppm/K this gives a
maximum error of 0.05 ppm for the whole temperature span.





to bee continued...
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:20:53 pm
Z201 results:

Datasheet values:
typical +/-0.2ppm/K +/-0.6ppm/K max. spread from -55 .. +125 deg C
typical +/-0.05ppm/K from 0 .. 60 deg C

http://www.vishaypg.com/docs/63187/zseries.pdf (http://www.vishaypg.com/docs/63187/zseries.pdf)

first pictures: explanation + further evaluation will follow

----------------------------------------------------------------------------------

Z201 #1 1K 0.01% datecode B0940-

to measurements of 14.06.2014 with Z201 #1 with 0.3 K / minute slope

Regression curve

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -5.7873783799925560E+0000
A 1 =  8.9822403793771305E-0001
A 2 = -1.0237801410870104E-0002
A 3 = -1.6531589738371225E-0004

max. deviation to regression curve:  1.6041069332224885E+0000 ppm

So measured T.C. at 25 degrees is  0.9 ppm / K. (= A1)

----------------------------------------------------------------------------------

Z201 #2 1K 0.01% datecode B0940-

measurement of 27.07.2014 with 0.12 K / minute slope

T.C. with box method: -0.27 ppm / K  (-9.75 ppm / 36 K)

Regression curve

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -8.21920230407688E-0002
A 1 = -2.59088412027236E-0001
A 2 = -4.62391535615482E-0003
A 3 =  1.60378673959659E-0004

max. deviation to regression curve: 1.58584984755668E+0000

So measured T.C. at 25 degrees is  -0.26 ppm / K. (= A1)

----------------------------------------------------------------------------------

Z201 #3 1K 0.01% datecode B0940-

----------------------------------------------------------------------------------

Z201 #6 1K 0.01% datecode B1305-

----------------------------------------------------------------------------------

Z201 #7 1K 0.01% datecode B1315-

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:21:22 pm
UPW 50 results:

Datasheet values:
- typical +/-3 ppm/K  (0..85 deg C)
- max. +/-5 ppm/K (-55..125 deg C)

http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773299%7FB%7Fpdf%7FEnglish%7FENG_DS_1773299_B_UPW-0312.pdf%7F1624323-6 (http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773299%7FB%7Fpdf%7FEnglish%7FENG_DS_1773299_B_UPW-0312.pdf%7F1624323-6)

--------------------------------------------------------------------------------------------------------
UPW50 #1 1K 0.1% Datecode 0816 resistor value @25 deg about -100 ppm against Z201#1

First results from 17.06.2014

on 17.06.2014 the 2 NTCs are fixed to the UPW50 on opposite sides (top + bottom) with tape.
The resistor is taped to the heat spreader directly above the NTC which is used for the temperature controller of the heating foils.
The Fan is installed. (But obviously I have used too much tape).

There is a relative large difference between the temperature of the 2 NTCs visible.
see: 20140617_UPW50_Tdiff_NTC2_NTC1.PNG
And the difference is also dependant on temperature slope.
The difference between +0.3K/minute and -0.3K/minute is up to 0.8K between the 2 NTCs.
Whats the real temperature of the resistor then?
It is not possible to tell wether the observed hysteresis of the resistor curve
is really from the resistor or if it is simply a temperature measurement error.
So for hysteresis I will have to improve the measurement setup thermal coupling (see discussion below).

For the T.C. measurement the thermal coupling has less influence,
since I have waiting times at the temperature extremes for temperature settling.
So the final results will not significantly differ from that what I have up to now:

Picture: 20140617_TC_UPW50_1K_1_raw_temp.PNG
For the UPW50 1K sample #1 this is after the box method:
   dev (ppm)   temp   (deg C)
min   -42.04121928   8.4112
max   32.44987977   44.5383
diff   74.49109905   36.1271
TC = 2.061917482

The measured curve is not straight linear with slightly decreasing T.C. at higher temperatures.
So the T.C. after box method is dependant on measured temperature span.

I calculated a 3rd order regression curve after normalisation to 25 deg C.
See picture 20140617_TC_UPW50_1_LMS.PNG
So the A1 coefficient is the linear part of T.C. at 25 degrees.

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg)

A 0 = -7.1365771688644140E-0001
A 1 =  2.0716957976478127E+0000
A 2 = -2.2325669247677248E-0002
A 3 =  1.1149432124690903E-0004

max. deviation to regression curve:  1.4751894259493142E+0000

So measured T.C. at 25 degrees is  2.1 ppm / K.

23.06.2014: further results from 20.06.2014:
change in setup: UPW50 mounted between 2 stacked SK09 heat sinks with thermal grease for the UPW50 + 2 NTCs.
NTC1 is between bottom heat sink and UPW50. NTC2 is above UPW50. Some silicone foil between upper and lower heat sink for better thermal contact. Ramp is 0.3 K / minute.
The difference between NTC2 and NTC1 is much smaller between rising and falling temperature.
But no significant effect on UPW50 hysteresis.


further results from 21.06.2014:
same setup as on 20.06.2014 only setpoint ramp changed to 0.12K / minute.
The "hysteresis" did not change significantly.
If there was a thermal settling time I would have expected a factor 2.5 lower hysteresis for UPW50.
But this is not visible in the measurement.

T.C. curve coefficients:

A 0 =  7.5167802876286093E-0001
A 1 =  2.0586430917904010E+0000
A 2 = -2.2608480477597587E-0002
A 3 =  1.6552949180267930E-0004

max. deviation  1.6919232571042099E+0000 ppm


further results from 22.06.2014:
same setup as on 20.06.2014+21.06.2014 only setpoint stepwise starting from 30 degrees (over night) then
3 hours 25 degrees,
2 hours 8.5 degrees (not reached),
3 hours 25 degrees,
2 hours 46 degrees,
4 hours 25 degrees.

still a remaining hysteresis of around 1.3 ppm (zoomed picture)

By the way: NTC1 below UPW50 shows some overshoot on setpoint changes. NTC2 lags somewhat. So the true temperature might be somewhere inbetween.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------

measurements of 29.06.2014
-------------------------------------
Against 21.06.2014 2 isothermal blocks where used.
UPW50 #1 is mounted in the brass cube 40mm of branadic.
NTC1 in a hole below the UPW50.
NTC2 near the wire of the UPW50 in the 6.5 mm dia hole for the UPW50
Thermal grease is used. Fan is within the plastic cap.

the Z201 is mounted in a smaller aluminium 19.5x19.5x40mm together with NTC3+4
Also with thermal grease.

The results are calculated with linearisation correction for the NTCs.
They do not differ much from all previous results (except for NTC temperature tracking).
So why all the effort? (I hate thermal grease).

Resistance: about -100 ppm @ 25 deg C against Z201 #1

Average T.C. from box method:

Drift UPW50 #1
min -38.96856779
max 33.85651927
diff   72.82508706

temperature
min 8.778501769
max 45.22255957
diff   36.4440578

T.C. = 1.99827054

so average T.C. from box method = 2.0 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -1.1959881148839021E+0000
A 1 =  2.0982250580817275E+0000
A 2 = -2.3221150504742209E-0002
A 3 = -5.7552059855277272E-0005

max. deviation to regression curve:  1.7295248354935827E+0000

So measured T.C. at 25 degrees is  again 2.1 ppm / K. (= A1)
Hysteresis not changed against previous measurements.

--------------------------------------------------------------------------------------------------------
UPW50 #2 1K 0.1% Datecode 0841 resistor value @25 deg about -580 ppm against Z201#1

Measurement of 17.08.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: +0.59 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -2.80998469524063E-0001
A 1 =  6.29693967898549E-0001
A 2 = -1.70300759394405E-0002
A 3 =  1.32133545088887E-0004

max. deviation to regression curve: 2.02996451023940E+0000 (hysteresis + noise)

So measured T.C. at 25 degrees is  +0.63 ppm/K

So UPW50#2 has only 1/3 of the T.C. of UPW#1

--------------------------------------------------------------------------------------------------------
UPW50 #3 1K 0.1% Datecode 1220 resistor value @25 deg about -100 ppm against Z201#1

Measurement of 23.08.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: +1.85 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 =  5.45175269191540E-0001
A 1 =  1.95948365674911E+0000
A 2 = -1.56801576131755E-0002
A 3 = -1.03623042954573E-0004

max. deviation to regression curve: 2.02918100035813E+0000  (hysteresis + noise)

So measured T.C. at 25 degrees is  +1.96 ppm/K

--------------------------------------------------------------------------------------------------------
UPW50 #4 1K 0.1% Datecode 1345  resistor value @25 deg about -50 ppm against Z201#1

Measurement of 28.08.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: +4.76 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -9.06731926559255E-0001
A 1 =  5.09683804579340E+0000
A 2 = -2.83692334469616E-0002
A 3 = -4.77182333446752E-0004

max. deviation to regression curve:  2.41485072237296E+0000  (hysteresis + noise)

So measured T.C. at 25 degrees is  +5.10 ppm/K

This candidate is also one (the first wire wound) where a ageing drift of about 1.2ppm in 3 days is visible.
Maybe due to the high T.C. value.

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:22:27 pm
Metal film resistors (for comparison only).

----------------------------------------------------------------------------------------------------------------

UPF50 results (seems to be relatively new)

Datasheet values:
- T.C.: +/- 5 ppm/K (20..80 deg C)

unfortunately there is no data for shelf life.

http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773299-1%7FA%7Fpdf%7FEnglish%7FENG_DS_1773299-1_A_UPF-1113.pdf%7F2176163-6 (http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773299-1%7FA%7Fpdf%7FEnglish%7FENG_DS_1773299-1_A_UPF-1113.pdf%7F2176163-6)

---------------------------------------------------------------------

UPF50 #1 1K 0.1% Datecode CA051  resistor value @25 deg about +490 ppm against Z201#1

Measurement UPF50 #1 of 31.08.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: +1.81 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -1.11413825943835E-0001
A 1 =  2.01228648499642E+0000
A 2 = -2.18642911697272E-0002
A 3 = -2.09357556163436E-0004

max. deviation to regression curve: 9.57837118906888E-0001  (hysteresis + noise)

So measured T.C. at 25 degrees is  +2.01 ppm/K

This is the first time that I have a resistor with no visible hysteresis. (hysteresis if any is below noise level).
There was a warm / cold / warm cycle on 30.09.2014 after soldering.
---------------------
Measurement UPF50 #1 of 01.09.2014: (fast ramp 0.3 deg / minute)

Average T.C. from box method: +1.84 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -3.19004540786116E-0002
A 1 =  2.02623371441288E+0000
A 2 = -2.12673046301004E-0002
A 3 = -2.45915701944138E-0004

max. deviation to regression curve: 1.55865402178475E+0000  (hysteresis + noise)

So measured T.C. at 25 degrees is +2.03 ppm/K

So with fast ramp some hysteresis shows up for the first temperature cycle (heating from min to max temperature).
The two following cycles have lesser hysteresis.
---------------------------------------
Measurement UPF50 #1 of 02.09.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: +1.82 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 =  3.30940300218205E-0001
A 1 =  2.00605814295572E+0000
A 2 = -2.15233506217060E-0002
A 3 = -2.15338666959163E-0004

max. deviation to regression curve: 9.80511753771691E-0001  (hysteresis + noise)

So measured T.C. at 25 degrees is +2.01 ppm/K

And again nearly no hysteresis with slow temperature ramp.

---------------------------------------------------------------------

UPF50 #2 1K 0.1% Datecode CA051  resistor value @25 deg about +310 ppm against Z201#1

Measurement UPF50 #2 of 03.09.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: +2.17 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 =  2.17091994721987E-0001
A 1 =  2.35167500256426E+0000
A 2 = -2.13936252691839E-0002
A 3 = -2.85364735533639E-0004

max. deviation to regression curve: 1.37799986290797E+0000  (hysteresis + noise)

So measured T.C. at 25 degrees is +2.35 ppm/K

UPF50 #2 shows some hysteresis in the cold cycle.
Against UPF50 #1 I did only a warm cycle on 02.09.2014 after soldering.

--------------------------

Measurement UPF50 #2 of 04.09.2014: (fast ramp 0.3 deg / minute)

Average T.C. from box method: +2.15 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 =  3.98056255275583E-0001
A 1 =  2.36173851561087E+0000
A 2 = -2.16714481631818E-0002
A 3 = -2.90793789458481E-0004

max. deviation to regression curve: 1.36220327821289E+0000  (hysteresis + noise)

So measured T.C. at 25 degrees is +2.36 ppm/K
Again some (moderate) hysteresis.

-----------------------------------------

Measurement UPF50 #2 of 05.09.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: +2.13 ppm/K

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 =  5.39247317441195E-0001
A 1 =  2.33157020163661E+0000
A 2 = -2.18089184079106E-0002
A 3 = -2.32087081335189E-0004

max. deviation to regression curve: 1.15652373887686E+0000  (hysteresis + noise)

So measured T.C. at 25 degrees is +2.33 ppm/K
Again some (low) hysteresis.

----------------------------------------------------------------------------------------------------------------

RC55Y results: (is not really high precision but one of the best easily available metal film)

Datasheet values:
- T.C.: +/- 15ppm/K (20..70 deg C)

Shelf life typ: 300ppm/year
Shelf life max: 1000ppm/year

http://www.welwyn-tt.com/pdf/datasheet/rc.pdf (http://www.welwyn-tt.com/pdf/datasheet/rc.pdf)

----------------------------------------------------------------------------------------------------------------
RC55Y #1 1K 0.1% no Datecode  resistor value @25 deg about -250 ppm against Z201#1

Measurement RC55Y #1 of 06.09.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: -8.13

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -3.80443706117962E+0000
A 1 = -8.88999969660920E+0000
A 2 =  5.61290695727147E-0002
A 3 =  1.10751465297150E-0003

max. deviation to regression curve: 3.57710183792049E+0000  (hysteresis + noise + ageing)

So measured T.C. at 25 degrees is -8.89 ppm/K

relative large ageing drift (4-5ppm) during measurement.

-----------------------------------------

Measurement RC55Y #1 of 07.09.2014: (slow ramp 0.3 deg / minute)

Average T.C. from box method: -8.26

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -9.90226686827984E+0000
A 1 = -8.80002275616066E+0000
A 2 =  5.95676322397589E-0002
A 3 =  6.15950680297529E-0004

max. deviation to regression curve: 5.87999450789868E+0000  (hysteresis + noise + ageing)

So measured T.C. at 25 degrees is -8.80 ppm/K

relative large ageing drift (8-9 ppm) during measurement.

-----------------------------------------

Measurement RC55Y #1 of 08.09.2014: (slow ramp 0.12 deg / minute)

Average T.C. from box method: -8.10

Regression curve:

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -1.28036495163400E+0001
A 1 = -8.77294237200453E+0000
A 2 =  5.15156773448553E-0002
A 3 =  1.05699451708786E-0003

max. deviation to regression curve:  2.63539353853565E+0000  (hysteresis + noise + ageing)

So measured T.C. at 25 degrees is -8.77 ppm/K

total ageing drift over 3 days: around -14 ppm.


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:22:52 pm
reserve 8E16
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:23:21 pm
Vishay S102JT results  (C-alloy)

datasheet values
- typical +/-2 ppm/K +/-2.5ppm/K max. spread
- max  ( = +/- 4.5ppm/K max ????)

http://www.vishaypg.com/docs/63001/63001.pdf (http://www.vishaypg.com/docs/63001/63001.pdf)

-----------------------------------------------------------------------------------------------
S102JT #1 1K 0.01% date code B1312-

Measurements of 13.07.2014:

From the Z201 datasheet: "Vishay Foil Resistors’ new Z-Foil technology provides an order of magnitude reduction in the Bulk Metal Foil element’s sensitivity to temperature changes - both external and internal."

When I compare the T.C. of Z201 and S102J then I really ask me: what do they really mean?
Ok I do not test at the temperature extremes. And also the hysteresis is much larger than on every tested resistor up to now.
In my temperature range T.C. is almost zero but with a large hysteresis of +/-5.5 ppm

Deviation against Z201#1 near zero ppm.

Now its getting difficult. Up to now I could define the "box method" as min/max drift span divided by min/max temperature span.
With this definition I will get: 11.1 ppm / 37.2 K = 0.3 ppm / K.
If I calculate with the 3 cardinal points method the T.C. gets around doubled values.

Regression curve

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -2.11289497717753E-0001
A 1 = -1.63006512501367E-0002
A 2 = -1.19211117177579E-0002
A 3 =  2.81642336236479E-0004

maximum deviation to regression curve:  5.49062470812308E+0000

So measured T.C. at 25 degrees is  -0.016 ppm / K. (= A1)

So the main improvement seems to me that the packaging or bonding (hysteresis) has been largely improved on Z201. But T.C. at least on these samples got worse.

Measurement 14.07.2014 on S102 #1: temperature setpoint jump from 21-30 degrees.

shows that the thermal time constant is very short.
The hysteresis is issued by another (2nd) much slower time constant.

-----------------------------------------------------------------------------------------------
S102JT #2 1K 0.01% date code B1312-

Measurements of 16.07.2014:

this guy has similar hysteresis than #1 but additional positive T.C. of around 0.55ppm/K

Deviation against Z201#1 @25 deg around -65 ppm

T.C. with box method: +0.55 ppm/K

Regression curve

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 = -2.98885401103763E-0001
A 1 =  5.45546004290231E-0001
A 2 = -1.30854146011424E-0002
A 3 =  4.17914762203182E-0005

maximum deviation to regression curve:  6.25321618704226E+0000 (hysteresis +(noise))

So measured T.C. at 25 degrees is also +0.55 ppm / K (= A1)

Hysteresis on #2 (+/- 6.3 ppm) is slightly higher than on #1 (+/- 5.5ppm)

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:23:47 pm
UPW25 results:

Datasheet values:
- typical +/-3 ppm/K  (0..85 deg C)
- max. +/-5 ppm/K (-55..125 deg C)

http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773299%7FB%7Fpdf%7FEnglish%7FENG_DS_1773299_B_UPW-0312.pdf%7F1624323-6 (http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773299%7FB%7Fpdf%7FEnglish%7FENG_DS_1773299_B_UPW-0312.pdf%7F1624323-6)

I got 2 UPW25 1K last week (couldnt resist since they were cheeeap at RS)
and made a isothermal block for them this weekend.
So here is the result of the first of them:


UPW25 #1 1K  0.1% date code 1244 measured on 08.07.2014

Resistance: about +630ppm @ 25 deg C against Z201 #1

T.C. with box method: -4.047

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg)

A 0 = -1.89847058482001E+0000
A 1 = -4.27204490196377E+0000
A 2 = -1.35428364585836E-0002
A 3 =  8.08056748472276E-0004

Max deviation: 2.59646041441657E+0000 (hysteresis)

I did not expect such a large deviation of the values against UPW50.
Since it is the same manufacturer.
Ok the UPW25 are much "newer" than the UPW50 from my drawer.
Temperature gradient is doubled in amount.
And this is the first resistor measurement with negative T.C.
The curve is relative "linear" against the others.
Hysteresis is slightly increased.

At 25 degrees the gradient is around -4.3K/deg

----------------------------------------------------------------------------------------------

UPW25 #2 1K  0.1% date code 1244 measured on 10.07.2014

Resistance: about +10ppm @ 25 deg C against Z201 #1

T.C. with box method: -4.117

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg)

A 0 =  2.17582422582792E-0001
A 1 = -4.29694066234232E+0000
A 2 = -1.36897923948147E-0002
A 3 =  7.03952866822879E-0004

Max deviation: 3.67844904416664E+0000 (hysteresis)

So hysteresis is again higher than UPW50 #1 and UPW25 #1

At 25 degrees the gradient is around -4.3K/deg


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:24:06 pm
Riedon USR2 results

Datasheet values:
- typical +/-3 ppm/K  (-55..+125 deg C)
- max. +/-5 ppm/K (-55..+125 deg C)
- upon request +/-1ppm/K (0..60 or -55..+125 deg C)

http://www.riedon.com/media/pdf/USR2-0808.pdf (http://www.riedon.com/media/pdf/USR2-0808.pdf)

I got two 1K 0.01% TCR1 versions (whatever this means)

http://www.digikey.com/product-detail/en/USR2G-1KX2/USR2G-1KX2-ND/1650022 (http://www.digikey.com/product-detail/en/USR2G-1KX2/USR2G-1KX2-ND/1650022)

Oh I see "country origin" GERMANY

So the resistors had to travel from Germany to Minnesota and back to get them.

---------------------------------------------------------------------------------------------

USR2 #1 1K  0.01% date code 1309 measured on 03.08.2014

Due to the "sticker" for the resistor value which makes the total thickness of the
resitor larger than that of the usual 8mm housing I had to use some force to
put the resistor into the slot of the isothermal block.
I do not know how the influence on T.C. is for the mechanical stress.
So perhaps I have to repeat the measurement.

Its also the first time that I see a large "ageing" on the measurement.

T.C. with box method: -1.2 ppm / K  (-43.2 ppm / 35.9 K)

Regression curve

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg C)

A 0 =  3.41653005510780E+0000
A 1 = -1.09885912220308E+0000
A 2 =  1.12101896460950E-0002
A 3 = -2.66093117429204E-0004

max. deviation to regression curve:  7.49589680302222E+0000

So measured T.C. at 25 degrees is  -1.1 ppm / K. (= A1)
(or somewhat higher if I did not have the ageing).

Update from 08.08.2014 for USR2 #1

I have done a measurement outside the iso thermal block on 08.08.2014 just to see wether the sticker has some influence.
There was no significant difference to the previous measurements (06.08.2014) except some higher "hysteresis" due to difference between sensor and resistor.
During ageing the T.C.  (A1 coefficient) decreased in amount from -1.1 down to -0.95 ppm/K

I have added a ageing curve.
Directly after soldering the resistor had +13 ppm against my "reference" Z201.
6 days later the value was nearly 42 ppm -> a change of 29 ppm within 6 days.

Question: is this ageing or is this humidity change of a resistor after freshly opening the polyethylene bag?

---------------------------------------------------------------------------------------------

USR2 #2 1K  0.01% date code 1309 measured on 09.08.2014 + 10.08.2014

Resistance: about  -90 ppm @ 25 deg C against Z201 #1

T.C. with box method: -1.354

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg)

A 0 = -1.41707170509333E-0001
A 1 = -1.41352317040293E+0000
A 2 =  9.45352247510392E-0003
A 3 =  4.40837707667485E-0005

Max deviation: 2.21010836378330E+0000 (hysteresis)

so T.C. at 25 degrees is -1.4 ppm/K which is somewhat higher than "TK1"

Ageing drift seems to be much smaller than on USR2 #1.
Only the first "fast hysteresis" cycle on 09.08.2014 shows about 7-8 ppm shift.
But this candidate came out of the same polyethylene bag than #1
so it had now one full week to acclimate.

---------------------------------------------------------------------------------------------
to be continued

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 15, 2014, 04:24:26 pm
reserve 3
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on June 16, 2014, 03:17:30 am
Looking forward to your results.
Title: Re: T.C. measurements on precision resistors
Post by: quarks on June 16, 2014, 04:30:47 am
Will be interesting,  thanks for sharing
Title: Re: T.C. measurements on precision resistors
Post by: jpb on June 16, 2014, 12:56:44 pm
I too am looking forward to reading your results as you produce them.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 16, 2014, 04:14:06 pm
I was pointed to this thread by DiligentMinds, there are a lot methods and questions to be answered.  I will address some of them here.

Hysteresis of resistor characteristics can be caused by a number of effects and also depends on the type of resistor being used.  Hysteresis can be caused by temperature excursions, power cycling, humidity and soldering.  A correctly made precision wire wound resistor (PWW) exhibits near zero hysteresis effects from the aforementioned effects.  Large temperature excursions (- 55°C to +125°C) whether by thermal shock (5 cycles) or power, should cause very small changes in the resistor value, somewhat less than 15 PPM.  A correctly made PWW should be essentially immune to humidity without hermetic sealing (if it is, this is a result of poor design, i.e. foil resistors for example).  Soldering should cause no significant shift in value if done according to specification, no more than 10 seconds at normal soldering temperature.  Any resistor which shows significant shifts in value from any of these effects are flawed by design or its own limitations.  Soldering limits are different for foil resistors, they do exhibit a higher sensitivity to soldering temperatures due to their very small mass and inability to handle high heat for even brief periods.  Check the data sheet for the particular foil resistor to find the soldering specification.

There is no 'sweet spot' in resistors, their characteristics are determined by the materials and manufacturing processes used to make them.  Any given type of resistor has limitations, no perfect resistor can be achieved unfortunately even though everybody chases after one and tries to specify them, in practice it cannot be done, despite what Vishay's marketing BS may state or anybody else's who claim otherwise.

Personally, I am very wary of using statistics to come up with a 'typical' value for most resistors, the fact is that any given TCR specification does have a +/- limit over a given temperature range and that TCR will vary between those limits in a limited random manner.  I have not found any method of accurately predicting just how many resistors in a given batch will produce a given TCR range.  The tighter the specification, the more likely that selection has to be used to find those resistors, no matter what may be said to the contrary.  Metallurgy cannot control the parameters of a given batch of alloy that closely, I've talked with metallurgists in the industry and they all agree with that statement.  The only way to get a given batch of alloy close to zero TCR is by a hit and miss heat treating procedure which is used on all resistor alloys.  The difference here is that this hit and miss procedure can produce alloys with near zero TCR but it is costly to do and few if any alloy suppliers will agree to this procedure.  To my knowledge, it must be done in-house and that costs a lot.  The current level of heat treating by the alloy supplier does produce a fairly consistent yield of a given TCR range but the tighter the TCR specification, the lower the probable yield of a given TCR range and you are going to pay a higher price for that tighter TCR spec, this is true of foil or wire forms.

Primary resistors standards, such as the SR-104 uses this procedure to tweak the alloy down to very low TCR values and you all know how expensive an SR-104 is.  This procedure is not the only way to do it, given a really good resistor design, a very good yield of low TCR resistors can be had without the expensive in-house heat treatment.  What this means is that for any given batch of resistors, a certain percentage will yield low TCRs which must be selected out of the batch.  This does cost in labor but is cheaper than the other procedure and actually takes less time.

In the real world, a resistor should be able to operate under any given conditions within its specifications without suffering significant changes to it characteristics.  Yes, there will always be some effect produced, particularly from extreme conditions (i.e. maximum specified limits) but those effects should be minimal in a good resistor design.  With the exception of primary standards, a resistor should not have to be handled with kid glove care like a standard to maintain its specifications.  Unfortunately, foil resistors are closer to standards handling in this respect than not.

Foil resistors still, after decades of development, have a 'wavy' TCR curve, even though they have been able to bring that 'lumpy' curve down to a pretty darn good level, it is still not linear by any means.  Vishay has a habit of drawing graphs which has ridiculous ranges to compare against, for instance, a resistor with a TCR of say 0+/- 1 PPM/°C will have its TCR drawn on a graph with a 0+/- 100 PPM/°C range, making the graph of the resistor appear far flatter than it really is.  This illusion has been used by Vishay for many years to confuse customers.  If your TCR is good, then draw a graph that shows it in detail, not some intentionally obfuscated nonsense.
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on June 16, 2014, 04:48:50 pm
Glad to see your input E.G Pettis.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 16, 2014, 06:47:08 pm
Thank you, Vgkid, I've worked with resistor alloys, i.e. precision resistors since before 1975.  I've worked with some of the best people in the industry in precision wire wound resistors.  I am the co-author of the articles mentioned in the thread:

http://www.edn.com/design/analog/4427151/The-last-half-century--Wirewound-resistors-Part-one (http://www.edn.com/design/analog/4427151/The-last-half-century--Wirewound-resistors-Part-one)

http://www.edn.com/design/analog/4427940/The-last-half-century--Wirewound-resistors-Part-two (http://www.edn.com/design/analog/4427940/The-last-half-century--Wirewound-resistors-Part-two)

I am also the co-designer of the precision wire wound resistors I allude to here in comparison to other resistors.  The PWW resistors I co-designed are, in my opinion (which is backed up by extensive testing by other laboratories) are the best precision wire wound resistors.  The specifications are real world, not best case marketing hype.  I do not claim a typical TCR, that tends to be mostly statistical manipulation, they are specified by a range of TCR which is guaranteed by design.  I will not claim a TCR of 0.2 PPM/°C as typical because it isn't.  What I do claim in my resistor specifications is that the standard product TCR is 0 +/-3 PPM/°C which is easily and repeatably achievable and in any given batch of resistors, approximately 60% of the yield will be within 0 +/- 1 PPM/°C.

What I cannot predict with any certainty is how many of those resistors will be within a given range of TCR within the +/- 1 PPM/°C yield.  If a customer wants a sub-PPM TCR, it will have to be selected from the given batch, pretty much like anybody else.  The exception here is that I don't have to fiddle around with any heat treating, time consuming and expensive alloy tweaking like everybody else.  You will, of course have to pay a bit more for the selection process, but then, you are already doing that with the other suppliers and getting sub-par resistors.

You want long term stability and reliability?  My resistors were put through 50 (not 5, but 50!) thermal shock cycles by a major aerospace contractor, according to MIL-STD 202, -55°C to +125°C, most resistors changed less than 10 PPM (that's everybody else's shitting on a shelf spec) with zero failures, which no other resistor has ever achieved.  You can expect similar changes running at full rated power and slightly higher changes if you're going to run full power at 125°C.

There are other differences between a good wire wound and a good foil resistor.  Foil does have very low parasitics  compared to PWWs, however the parasitic card tends to be overplayed; except at very low values, the 'Q' of wire wound resistors is well below 1 which means that they have far less effect in a circuit than would be thought.  Yes, PWWs do have parasitics like all components, but depending on the application, they can be dealt with and in many instances, they actually have little or no effect.

Noise is another area where a good PWW excels, wire wound resistors do not produce shot noise, it is not inherent in wire but foil resistors do have shot noise and tend to have slightly higher Josephson noise levels than wire wounds, this is because the etching of the foil paths causes irregularities along the edges of the paths, this causes more noise because of the increased irregularity in the electron movements along the 'rough' edges.  It is a lot like water flowing in a stream, even if the bed of the stream was perfectly smooth, the rough edges of the stream would cause irregular eddies in the water.

Foil resistors have inherently higher drift than PWWs (well, at least mine), maybe even sitting on a shelf, even sitting in hermetically cans.  I don't make hermetic resistors, they are a waste of time and money, they are only a band-aid for a poor design.

Foil resistors are affected by humidity in their common molded packaging, hence the hermetic can to protect the foil inside.  The humidity, if it gets inside and onto internal connections or foil, directly affects the metal, with oxidation occurring which affects the characteristics.  It is also possible that the bonding agent between the foil and ceramic substrate absorbs water as well.  In a correctly designed and built PWW, humidity has exceedingly little effect on the resistor, the wire is coated with Pyre ML which is good to 220°C to 240°C, well above most resistor encapsulation ratings and does not easily absorb water (and if the resistor is under power, what little humidity may get in is evaporated by the heat) plus the wire is welded (well, at least mine are) in which case water will have little effect there as well and the weld joints are encapsulated too.  This has been verified by tests in a humidity chamber, both under power and not powered.

I could go into more details about why other PWWs don't seem to work as well as one would think but that is another lengthy story, suffice it to say that their resistor designs are flawed, hence the less than stellar performance.

Vishay does make good resistors but their predilection for buttering up the specifications is annoying at best and borders on lying at the worst.  They just need to cut out the marketing hype and publish real world specifications (which sometimes are buried in the fine print somewhere).  While Caddock may not seem to make as good as Vishay foil resistors, I find their specifications much more closer to the real world with little hype thrown in.  As a design engineer, it makes it a bit easier when you know just what you are working with instead of what statistical manipulation produces.  Sorry if I sound like I'm beating up on Vishay, in some ways I am, but it is mostly of their own making and it is ingrained in the company.

One more thing, the 'lumpy' TCR curve of foil resistors is inherent in the design, you should have seen the original TCR curves 30 or 40 years ago, it would make you seasick.  My TCR curves are linear across the entire temperature range (well, there is a slight change at the extremes, but then what wouldn't?), it is part of the design.  Whatever TCR you get from a foil, it is going to be 'lumpy' and that may give your circuits fits, it has in the past.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 16, 2014, 09:23:32 pm
Hello,

I am very surprised on the resonance to my experiment.
Sorry that I will not read+answer all posts at the moment.
I am still busy with doing measurements and generating the calculations for the results.
One measurement has around 300-800 data points each averaged out of 1 minute of the ADC values to reduce the noise of the ADC.
So one measurement lasts for up to 1 Day + several hours evaluation.
Sometimes I have to repeat the measurement if it is obvious that something went wrong ...
My wife is very tolerant, but all has its limits.

I am also not shure if it is a good idea to work with (possibly hand picked) samples which are not easily avaliable from stock of common suppliers.
How can I be shure that this would not give a wrong picture? And how can other hobbyists get those resistors?

I also still do not know if my setup will work with higher ohmic resistors or if I will have to do some changes due to the input impedance of the LTC2400.

so long

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on June 16, 2014, 10:39:12 pm
For E.G. Pettis, and others.
What are your thoughts on the precision vintage resistors on ebay.ERC/NRC/Mepco/Ohmite.
What about the various standard resistors(you might have some insight, others can chime in)?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 17, 2014, 12:29:43 am
Hello Andreas,

To quote you:  "I am also not sure if it is a good idea to work with (possibly hand picked) samples which are not easily available from stock of common suppliers.  How can I be sure that this would not give a wrong picture? And how can other hobbyists get those resistors?

I also still do not know if my setup will work with higher ohmic resistors or if I will have to do some changes due to the input impedance of the LTC2400."


My, you do seem to be a bit skeptical.....let's see if I can put your mind at rest. 

1. All of my resistors are custom made to order, even my 'standard' grade resistors are made to order, I generally do not have very many 'stock' resistors lying around.  While some manufacturers do make a point of having some 'standardized' values in stock, I don't, my order turn around time is faster than most with the exception of stocked values on a shelf somewhere.  I do not charge a lot more for making a 12.5K resistor instead of a 10K resistor, that is just silly, the resistor price should be a function of the actual costs, a 12.5K resistor only costs a little bit more in wire and a few more seconds of labor than a 10K resistor given the same bobbin style.  You are being taken advantage of by some(?) resistor sources.  Non-stocked resistor values.....quite laughable, from where I sit, that is just an excuse to charge more money than necessary.

2. If I have the materials in stock to make a given resistor order, I do not usually put any minimum on the order, you pay the quoted price plus shipping and you've got your resistors.   Unlike many manufacturers, I will not make a customer buy some unwanted quantity or quote some sky-high price for the part(s) just because you are a small quantity annoyance.  The only exception is if I do not have the required materials in stock, particularly the wire, I am usually forced to buy a certain minimum amount which is often in the hundreds or even thousands of dollars, in which case I will either suggest another resistor type that I do have or I will simply refuse the order with an explanation why.

3. My 'standard' resistor turn around time, given materials stock, is about 5 days.  There are various options which will lengthen that out some by days, not weeks like the other guys.  Number two above is the only exception to number three.  The 'standard' part is 0 +/- 3 PPM/°C TCR, see my earlier post for more details.  If you want tighter TCR than that, they will have to be selected from the batch and depending on how tight that TCR is determines a given yield.  This of course involves more time and does carry a price point with it.  Not to put too fine a point on it, I do not select resistor TCR unless the customer specifically asks for it and agrees to the price.  If a customer is asking for free samples, they will get the 'standard' resistor, not any hand picked part, it takes too much time and effort to cull out lower TCR parts to give them away for free.  The samples are to verify the major characteristics of the parts, unless the customer is a potentially large user of 'special' parts (say NASA, etc.,) and requests a special part for qualification, then and only then would I supply 'selected' parts.  This is pretty much standard policy among resistor manufacturers.  I am sympathetic to hobbyists as I am one too, which is why I am willing to provide reasonable samples under the right circumstances to hobbyists (no I can't give you a set of resistors to build your prototype but I am willing to provide them at a competitive price).

4. I can enhance the stability (which is already better than just about anybody else's) even further if required, naturally that will take a little longer and add a bit to the price but usually hobbyists do not really need that much stability.  (I am a bit curious that hobbyists think that they need state-of-the-art components that are rather expensive, this is usually left to places like R&D labs, the military or aerospace).  I can see wanting to try and make a really ultra stable voltage reference, etc., but wanting it and needing it are two different things.

Andreas, I am capable of producing resistors over a very wide range, none of the resistors you have mentioned in your posts are unusual, I do not understand your last sentence above.  If I do send you any samples, they will be 'standard' grade, there will be no 'hand selection' of the TCR.  Since many people in these various posts and threads are talking about wanting sub-PPM TCR resistors, it may not be of any use to provide you with samples that do not meet these 'ultra' requirements.  RhoPoint and any of the other wire wound resistor manufacturers named here do not provide sub-PPM TCR resistors and if they say they do, they're not only fooling you but themselves.  They have enough trouble getting under 3 PPM/°C to 5 PPM/°C TCRs, anything much less than that is by accident.  You didn't seem to have any problem with testing their sub-standard parts so why are you questioning mine?

I have some reservations about the measurement methods being used in these threads, there are uncertainty terms, for instance, which do not seem to be observed.  Even ratio measurements have uncertainties, even short term measurements have uncertainties.  I have seen the HP 3458A referred to in several threads being used as a ratio measurement, it does have very good linearity but linearity is not accuracy, there are still uncertainties to be observed.  Additionally, the smaller the delta temperature deviation, the greater the likely error in the TCR measurement, particularly when attempting to measure small sub-PPM TCR values.  Just because the error appears to be repeatable, doesn't make it accurate.

I also have some reservations about your ADC test setup as well, from your statements, it sounds like you have to use a lot of math on those 'measurements' due to various influences such as noise, I am very leery of measurement results that use statistics to iron out garbage.  It calls into question the results accuracy.  Unless the known accuracy plus the known uncertainty is at least 5 times better than what you are trying to measure, your measurements are invalid, even if they repeat.  For example, if you are trying to measure a 0.2 PPM/°C TCR and you delta shift the part (an accurate) 30°C from ambient for an expected 6 PPM in resistance, your system accuracy plus the uncertainty must be no bigger than 1.2 PPM but that still leaves a fairly large error possibility, the 6 PPM reading could be off by +/-20%, it actually might be as much as 7.2 PPM or as little as 4.8 PPM, that is a very large error in fact when trying to measure a 0.2 PPM TCR and it just gets even worse as you try to measure even smaller TCRs.

Granted, a hobbyist does not usually have access to a high quality resistor bridge but that method coupled with an appropriate delta temperature range gives a more accurate and repeatable TCR measurement.  I have an ESI 242D (yes, they are very expensive too), one of the most accurate resistor bridges made, the only method which is more accurate than a 242D is a Direct Comparison Current bridge and those are real heavy weights in the price category.

I am not trying to throw cold water on these projects, on the contrary I think they are quite commendable and a valuable teacher but I still question a hobbyist's need for such component performance.

If you want to know more about my resistors you can ask here or I can tell you how to contact me more directly.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 17, 2014, 01:04:29 am
Hi VgKid,

Most of the old vintage 'precision' resistors do not meet today's more rigorous specifications and most, if not all of the vintage resistors have likely drifted out of tolerance after all these years.  Of course, the resistors are still eminently usable if you don't mind them being out of their stated tolerance.  Their other specifications are likely to still be as good as when original, but beware that TCRs can creep up or down over time with these old parts.  Alloys were not as good as today's are and were not as stable so don't expect TCRs much better than 10 PPM/°C at best.  The original Evanohm alloy was originally specified at 0 +/- 20 PPM/°C which dates back to being introduced in 1943.

The vintage power resistors are just as good as today's for the most part and while I wouldn't pay any extra for them, they have no drawbacks to using them.

For those of you who may be thinking of buying an old General Radio or early ESI resistor decade box, these also suffer the same fate, they may have drifted out of tolerance over the years and the really old ones will have higher TCRs than the newer 5 PPM/°C models today.  As long as you are not expecting these great old vintage pieces to perform like new, they deserve a place on the display shelf at the very least and may be good enough for your purposes to boot.
Title: Re: T.C. measurements on precision resistors
Post by: quarks on June 17, 2014, 09:51:33 am

Granted, a hobbyist does not usually have access to a high quality resistor bridge but that method coupled with an appropriate delta temperature range gives a more accurate and repeatable TCR measurement.  I have an ESI 242D (yes, they are very expensive too), one of the most accurate resistor bridges made, the only method which is more accurate than a 242D is a Direct Comparison Current bridge and those are real heavy weights in the price category.


If you want to know more about my resistors you can ask here or I can tell you how to contact me more directly.

Hello Edwin,

great to have you here.

About ESI 242D, when I searched for the best possible accuracy it was top of my list. But unfortunately when I tried to buy one, I ran into trouble with missing stupid European Union CE mark on the mains powered DC Generator 801. So I tried to "make" my own kind of 242 around a beautiful ESI Kelvin Ratio Bridge 240C. My first setup with a Burster high precision Decade and a DC Calibrator as Generator showed very good results. Later I tried to find RS925D but only got a RS925A. And finally looked into Fluke 8508A.

Can you suggest a setup to do a (in your opinion) propper TCR measurement with "242" and Fluke 8508A?
Because the absolute value accuracy will not be that important, can you judge if the Burster Decade (with very good <1ppm TK) will be the better choice than RS925 in the "242" setup?

If I find enough spare time I would like to check/compare different technologies (wire wound, VPG Z-Foils, and other goodies in my collection).
 
Also I would like to know more about your resistors. If you have a link, datasheets, where to buy, ... please share.

bye
quarks
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 17, 2014, 02:32:47 pm
Hello quarks,

Thank you, glad to be here, I was not aware of this forum until recently.  I usually only visited the professional forums and after being told about this one, I thought I could be of assistance.

It is difficult to find any ESI 242D (or earlier versions) here in the USA as well and prices for even used beat up units have become quite high.  A brand new 242D can still be had by special order, the price is about $30.000,00; you would be lucky to find a decent condition used one for under $10.000 plus some hefty shipping charges.  While I do see an ESI RS925 for sale now and then, it isn't very often any more and a 925D can set you back a good $4.000,00; a RS925A is still a very good decade standard, while it can't be calibrated like the 'D' version, it still has excellent accuracy.

I agree, many of the EU regulations are just plain stupid without much thought.  The original ESI 801 generator/detector is not the greatest and they tend to break down more often than one would like.  The DC power supply in the 801 is a very simple, unregulated DC supply circuit with various current limiting resistors in series with the output.  It is junk, its output varies from 600V at 6 mA on the high range (>100K ohms) to 2 volts at 2A on the 1 ohm range.  There is a variable rheostat which is used to "adjust the approximate power" going to the bridge.  Nothing complicated in the design, because it is a bridge, the voltage does not have to be regulated but should be clean of noise.  It is very important that you use the lowest POWER setting to get a stable resistance reading, too much power will cause a drift in the readings and a reading should be taken as quickly as possible.

The foregoing is standard calibration procedure, you may want to check a resistor's characteristics at something higher than minimum power levels.  The problem here is that your standard resistor (such as the RS925) will be dissipating the same power as your resistor of interest which is generally not good for the standard, even though it is rated to dissipate a few watts spread over its decades (this is spelled out in the RS925's operating manual).  The increased power will cause the standard to drift some as well, hence the reason for minimal power levels.  You can use a higher power level if it is done as quickly as possible, it will not damage the standard (within its ratings).

The 801 detector is about average in performance, you can substitute a better null detector such as a Fluke 845A or Keithley 155 or other similar null meter.  The HP 419A is also better than the ESI but I prefer the other two over it.

The Fluke 8508A is an excellent DMM, in many instances I have been able to calibrate Flukes to much better accuracy than the specifications would indicate and they are very stable instruments.  Over the mid-range of resistance, a properly calibrated Fluke could outperform an RS925A in accuracy although the lowest range may not be quite as good.  The use of a DMM with a resistance bridge setup would limit its usefulness to setting the bridge voltage (no real accuracy needed there) and as a null detector which I am not fond of using a digital instrument for that function as the digit flipping is annoying and may not give you as good as an analog null detector reading.  It can be more difficult to 'find' the best null with a digital DMM.  The ESI 240C and RS925A forms the most important parts of a Kelvin bridge and it is there that your accuracy in resistance readings resides.  The power supply is not critical but a good analog null detector is, you are 85% of the way to where you need to be with the two Kelvin bridge components you already have, in conjunction with your Fluke for the higher resistance readings, you are very well set for good accurate readings.

I am not familiar with the Burster Decade but if it has an actual TCR of 1 PPM/°C then it would be better than the 925A in that respect.  As I recall, the 925A is specified at an initial accuracy tolerance of +/-20 PPM.  The RS925D can be calibrated to as good as +/- 1 PPM except on the lowest and highest ranges where it can be as loose as +/- 3 PPM.

One other slightly less important detail, I know it is difficult to control temperatures in the typical hobby environment but at least knowing the temperature reasonably accurately and tracking it can help minimize those pesky errors that creep into measurements.  Measurements, particularly TCR, require accuracy in temperature as well as resistance to give an accurate result and as I have stated elsewhere, the lower the TCR being measured, the greater the demands on measurement accuracy become and the wider the temperature shift must be to accurately measure the TCR.

I am going to post again a bit later here about the TCR values being talked about here on several threads.

Currently, I have some restrictions on producing resistors, I am limited to the material stock on hand for the time being.  There has been substantial price increases in many of the materials necessary to manufacture these resistors.  I must wait until I can purchase additional materials which will likely cost in the thousands of dollars unfortunately.  I sell directly at this time with no distributors as yet.  Distributors add to the buyer's cost for one thing and I do not operate like most other resistor houses.  I also ran into a major problem with my bobbin supplier who apparently decided they didn't want to bother with making bobbins any more and upped the prices to the "I don't want to bother with you (or anybody else)" level, in other words, they priced the bobbins so high that they are too expensive, it causes the resistor prices to be too high to even compete.  So I am working off of my inventory and working on a solution which is going to take some time.

If I can make the resistors I would certainly quote them, otherwise I would explain why I can't.  I will post more details shortly, have some other things to attend to, thank you for asking and watch for further posts soon to answer your other questions.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 17, 2014, 06:25:05 pm
Hello gazelle,

Yes I can make dividers with matched TCRs, they are in the form of individual resistors with an option of encasing.  After some study of the LTZ1000/A, the basic circuits given in the LTZ1000?A have nominal values which can be varied to improve LTZ1000/A performance, of course these values are not shelf stock, it is also possible to improve the performance even more by modifications to the basic circuitry.  As I've mentioned elsewhere, all the resistors I make are, by definition, custom made, but I don't indulge in the pricing games other resistor houses do.  I charge only for the cost of the resistor by specification and the other related costs.  Just for your information, I do have stock on the materials required to make all of the precision resistors in the LTZ1000/A circuits including matched TCR performance for the dividers.

I suppose this is as good as any time to post the notes concerning these resistors and associated TCR requirements.  I have seen a lot of posts fussing over extreme low TCR values, sub-PPM values, this is foolish for two reasons; first is the cost of sub-PPM resistors and secondly, a voltage divider, in this LTZ1000/A application in particular, does not require sub-PPM TCR resistors to achive the needed ratio performance.  What is needed are resistors whose TCR track each other closely, the absolute TCR is of no real concern only the matching TCR.  When you are not trying to get absolute  'zero' TCRs, the resistors not only become easier to obtain but are also lower in cost as it is easier to match higher TCRs than sub-PPM TCRs.  While sub-PPM TCR matching is often called for, few circuits really need sub-PPM TCRs as many other circuit components will cause bigger errors than the resistors.

As such, I can provide matched TCRs in these voltage dividers, no matter the values, to sub-PPM matching where it really counts.  My standard TCR specification is 0 +/- 3 PPM/°C but in the case of these dividers, it is essentially irrelevant.  I have known a fair number of engineers who have made this same mistake, over specifying a parameter, such as TCR because they did not fully understand the function of the circuit.  The other non-divider resistors should have reasonably low TCRs, but again, extremely low TCR specification is unnecessary.

I believe that by tweaking the nominal values and the circuit configuration in the LTZ data sheet, even better performance can be achieved but I will leave that up to the person doing the design work.  Proper layout of the circuit is also critical, particularly for the external components, any resistors that need to track any other resistor should be mounted as close to each other as possible and shielded from air currents, even a little air movement can cause uneven deviations in temperature, a Styrofoam cup works really well and can't be beat for cost.  There are two options here, one is to put the resistors inside a small case and encapsulate them in a thermally conductive material or apply a thermally conductive material to the resistors on the circuit board.  The latter costs the least to do.  To anticipate a question, no, hermetic resistors are not really any better, they will have a longer 'tail' as we say, the response to changes in temperature are much slower, both in warming and cooling, unless the resistors are inside the same can and close together, if you check on pricing, this will cost you dearly for this option.

The best thing to do is to talk with your (potential) resistor supplier about your application and what options and costs are available.  Most often, the best solution is not to be found on a shelf or in a high priced component or exceptional specifications.  Just because the 'big boys' did it one way doesn't mean that is the only way to do it and get the same results.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 17, 2014, 06:37:03 pm

As promised, my General Resistance Specification:


Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 17, 2014, 10:58:32 pm
Hi gazelle,

I forgot to add one more detail, particularly in the case of matching resistors.  It won't do you much good to order two (or more) 'matching' TCR resistors if they are dissipating different power levels, as is the case in the LTZ1000/A circuits.  If both resistors are dissipating the same power levels, then the self-heating will be the same and the TCR will track as expected, all else being equal.  If the power levels are different, then the self-heating is going to cause an apparent TCR mis-tracking which will be blamed on the resistors.  Surprisingly, I've seen this mistake made by engineers pretty often.

So in the case of the 12.5K / 1K divider, there is roughly a 15:1 power ratio, if this is not taken into consideration when selecting the resistors....it isn't going to work like the man (or lady) said it would.

I will get back to you shortly on specific sizing and other details, sorry for the delay.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 18, 2014, 12:16:05 am
Hello gazelle,

To accommodate the materials in stock, these are the best sizes; for the 1K ohm resistor, my 704 style, axial lead, 24 AWG, body size 0.125" D x 0.250" L, for the 12.5K ohm resistor, my 807 style, axial lead, 20 AWG, body size 0.250" D x 0.750" L, the 70K ohm resistor would be a 807 style, the 120 ohm resistor would be an 805 style, axial lead, 20 AWG, body size 0.250" D x 0.500" L, the 30K ohm resistor would be a 805 style.  Any values near these would use the same style.

The resistor sizes for the 12.5K and 1K resistors are for best power balance and also best matching  tracking TCR.

One other note, the standard spacing for measurements of the resistor is 0.375" from the resistor body, this can be changed at the request of the customer if longer spacing is needed.

 I just received a new pricing list for some of the wire I use, the increase was a jaw dropper.  Thank you commodity materials and speculators, I hope that sink hole under your house is big enough when it opens.
Title: Re: T.C. measurements on precision resistors
Post by: CaptnYellowShirt on June 18, 2014, 02:22:30 am
Subscribed...
Title: Re: T.C. measurements on precision resistors
Post by: branadic on June 18, 2014, 07:33:26 am
Quote
I am also not shure if it is a good idea to work with (possibly hand picked) samples which are not easily avaliable from stock of common suppliers.
How can I be shure that this would not give a wrong picture? And how can other hobbyists get those resistors?

I appreciate that, not because to blame Edwin G. Pettis, but as expected there is somewhat emotion in the posts which make them sound less objective and objective results on resistors are what Andreas want to present.

I guess if we had some guy from Vishay in here there would be a flame war and fairy tale exchange. There is still some needless myth around the welding process of wirewound resistors and the CTE / TC compensation of metal foil resistors. Resistor manufactor act as if they had understood all the effects, but I bet they don't and that they are only on the right way of understanding it. But it would be a shame to agree such a statement.
Instead some marketing guy creates stories, foil resistor manufactor fight against wire wound resistor manufactor and vice versa, but both of you agree that thin film, thick film and carbon resistors are bad. But hey, I understand that, you guys want to sell resistors. You don't want to offer and share your knowledge, you don't want to tell us the disadvantages of your resistor technology and I'm sure you can tell some, because there is always some imperfection and parasitics is at least only one of them.

So Andreas, keep going on in your fully independant research, even if the test environment and the test conditions are not perfect. I don't know of any independant and objective research on the topic yet.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 18, 2014, 05:21:07 pm
Gentlemen,

If I sound like I'm attacking anyone here personally, it was not my intention and I apologize if I offended anyone, my intention here is to draw everyone's attention to the facts (not personal opinion) of both resistors and measurement techniques.  If you will carefully read my posts on page 2, I do talk about characteristics and parasitic comparisons between PWW and foil in both general and specific terms.  These 'facts' have been verified by independent engineers and laboratories, not just by me.  Have I personally examined every possible PWW manufacturer's components on the planet, of course not, however the resistors of nearly all major resistor manufacturers have been tested and examined in detail by my colleague (who worked for many of them over the years), myself or the laboratories of the top aerospace, military and precision instrumentation suppliers which has included some EU facilities as well.

When measurements involve PPM and sub-PPM accuracy, no matter who is doing it, it requires the utmost care to prevent the many possible sources of error from sneaking in, it has happened to the best of us, measuring nanovolts is no easy task, even for a standards laboratory let alone smaller, less well equipped facilities and even though I will encourage it quite heartily for the experimenter as a learning tool, the best of intentions and efforts may not produce accurate results, it may produce apparent consistent results but not accurate results.

Just because a DMM offers 6, 7, or 8 1/2 digits of resolution, that does not mean all of the digits are accurate, in fact, even in the short term, they are not.  The same facts applies to ADCs and DACs, just because there are 24 digits of resolution doesn't mean there are 24 digits of accuracy.  Sorry if I sound preachy here, I am just trying to draw attention to just some of the error sources present in the measurements being attempted here.  Careful examination of the specifications will show that there are several sources of internal errors which can and do add up to significantly higher figures than the resolution of the unit.  In the case of a DMM, usually the DCV is the most accurate function and is limited to 6 or 6 1/2 digits of accuracy, the other functions have even less accuracy.  The ratio function does tend to have terrific linearity, the top DMMs can possess very high linear specs of around 0.1 PPM but that does not guarantee accuracy of the measurement in itself.  Many of the sources of error are external to the DMM and are very 'sneaky', thermal EMFs to name only one of them.

The ADC Andreas is using is an excellent ADC, I've used it and some of its 'brothers' but while it has a resolution of 24 bits, it accuracy is closer to 20 bits (1 PPM) just like other 24 bit ADCs,  resolution cannot be confused with accuracy, they are two distinct, different entities.  Math cannot wholly compensate for error, statistics have limitations (proven every day) and frankly, I do not rely on math to iron out inconsistencies, at best you might be able to gain one more digit of accuracy, probably closer to a half of a digit, the rest of it is all noise and error sources being averaged out.  Unless the readings can be compared to a known accurate source WITH accuracy, then the readings cannot be considered valid, no matter how much they may repeat.

In a calibration lab, the rule is that a minimum of 5 times the accuracy of the unit being measured (under controlled conditions) and 10 times the accuracy is often required, particularly for very accurate measurements.  This includes the uncertainty of the measurement and all of the instruments and standards have uncertainties in their measurements which must be taken into consideration.  That is precisely why your SR-104 resistor has an uncertainty specified in its measurement and is part of the accuracy of the SR-104.  For an accuracy of say 1 PPM, the comparing standard and instrumentation must have a minimum of 0.2 PPM total accuracy and uncertainty verified by a primary lab which is usually something comparable to our NIST.

I am not attacking Andreas' test setup per se, considering cost it is a pretty decent setup, what I was trying to point out was that in my opinion the setup is not capable of producing the results Andreas was hoping for with the accuracy needed.  There was absolutely nothing personal in my comments.  I have many years of experience in calibration (my first position out of college) as well so I know what I am talking about.  One of my jobs, among others, was to design and build test equipment capable of measuring such quantities and it is not easy to eliminate or account for all of the error sources that can creep into such measurements.

I stand by all of my statements concerning the flaws in resistors, both PWWs and foils, there has been no evidence presented to the contrary by any of the other resistor manufacturers, nothing more than airy claims, "our resistors are welded", no they are not and the evidence proves they are not welded, both in physical examination and in actual performance data.  Until proven otherwise, my claim that my resistors are the only welded PWWs available (and the performance data backs that up) stands.  It is very easy to prove welding on my resistors.  There are manufacturing details about my resistors which I am not willing to publicize as those details are proprietary.  The PWW industry has a long history of 'borrowing' manufacturing ideas from one another, even manufacturers from the EU had done it and I suspect the Asians have also done it.  The evidence is right there in their resistor construction of the wide spread 'borrowing' of each others bobbin designs and that is further proof that they all share in the same flawed designs.

If you have any questions about resistors of either kind, I will be happy to answer them, I do try quite hard to keep my statements factual and if I sound like I'm beating my own drum, at least I have evidence to support it.

Andreas, I would be most willing to give suggestions about your test setup if you would like, you are perfectly free to use them or not.  I do encourage you to read my comments on the TCRs concerning the LTZ1000/A.  The Z hermetic foil resistors are quite unnecessary and will only result in excessive cost.  This has nothing to do with the Z's performance, claimed or otherwise, it is a question of where to apply the correct specifications.

I am still willing to send you some samples to try out if you will tell me a range of resistance that you can use, if I have any stock in that range I will send them and I guarantee that there will be no 'hand picking' involved, just randomly picked, standard line resistors with no special treatment that meets my specifications as given in the data sheet I posted earlier. I will even post the resistor values here as well.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 18, 2014, 11:01:21 pm
Hello,

although I wanted to wait with responses until I have my first results ready I think its time to read some of the posts.

For the first: it is good to have someone with so long experience on this area. So we can all learn from your input.
And thanks for the links to your articles. You saved me the time for finding them.

There is no 'sweet spot' in resistors,

Is this valid for wire wound resistors only or also for metal foil?
I can imagine that the WW resistors if the base material always has a (more or less) positive TC will have no sweet spot except for divider ratios.

On metal foil vishay claims the expansion coefficient of the base material and the foil TC cancel out.
Of course the quality of matching could have something to do with the foil thicknes (resistance value).
The thicker the foil (low resistance values) the more positive the resulting TC could be.
The 10K metal foils of Dr. Frank are between -0.3 ... -1 ppm. My first measured 1K is around 0.9 ppm at 25 degrees.
So what is in between? Of cause not all metal foil alloys will have the same TC so the values might vary from batch to batch.
But these are only my speculations.

One more thing, the 'lumpy' TCR curve of foil resistors is inherent in the design, you should have seen the original TCR curves 30 or 40 years ago, it would make you seasick. 

would be really interesting do you have such a curve?
Up to now I found only one in the www (see attachment).

You didn't seem to have any problem with testing their sub-standard parts so why are you questioning mine?

I also have some reservations about your ADC test setup as well, from your statements, it sounds like you have to use a lot of math on those 'measurements' due to various influences such as noise, I am very leery of measurement results that use statistics to iron out garbage.  It calls into question the results accuracy.

I am not trying to throw cold water on these projects, on the contrary I think they are quite commendable and a valuable teacher but I still question a hobbyist's need for such component performance.

If you want to know more about my resistors you can ask here or I can tell you how to contact me more directly.

I hope you do not take it personally. In the mean time I have understood that your resistors could also be a serious alternative to other sources even for hobbyists.

And yes you are right. I have to live with the inaccuricies of my setup. ADC noise is one of them.
The measurements are meant only on a comparison basis. So in this case the repeatability and stability is of most concern.
I do not claim absolute accuracies. And the measurements are only on few samples and should not be generalized.
If someone other has better equipment we all would appreciate further comparison results.

By the way: the largest problem that I see at the moment is not the ADC but the temperature measurement.
With 3 ppm/K of the resistor and 1 K temperature difference between sensor and resistor the error is already 3ppm.
And I have at the moment large temperature differences between the 3 sensors of my UPW50 measurement.
So at the moment I have to work at the mechanical setup.
I know I should use a temperature controlled stirring oil bath. But I want to use the resistors afterwards for a reference.

To the hobbyist needs: You have to understand that its a little bit of "yes I can" and "mine is better than yours".
If you are building only 1 or 2 references they should be the "golden selection".

It would be a good idea if one of the resistor manufacturers would sell complete sets for e.g. LTZ1000(A) references.
If I would specify such a set:

For the 120 Ohms and 70K the absolute tolerance could be in the 1%(or even 5%) range.
Only long term stability and T.C. are of concern.
Standard T.C. would be 3ppm/K.  The golden selection set would have perhaps 1ppm/K.

For the temperature setpoint we would need a matched ratio set. 12K / 1K for LTZ1000 and 12,5K / 1K for LTZ1000A
If I could wish me the perfect resistor it would be made from the same wire (same lot) on the same bobbin within the same case if this is possible.
Ratio tolerance I would specify no more than 2 mV error (1 degree setpoint) since otherwise I would have to put the setpoint unnecesarily higher. 2mV / 600mV = 0.3% maximum ratio error. I hope that the T.C. mismatch is also below 0.3 ppm / K. (factor 10 improvement due to matching from same wire).
Do you agree with the specs?

The large question is:
So what would 2 sets with standard T.C.  3ppm/K or "golden selection" T.C. (1ppm/K) cost including shipping worldwide?

The other questions: do you have pictures from your resistors (sizes , forms). Is it more the 0816 axial style or radial style. I am a bit confused from your descriptions.

So long for today. (its already late here).

With best regards

Andreas

Edit: just to illustrate what I mean I have attached a curve with NTC temperature differences over temperature of the 2 NTCs attached to the UPW50. One near bottom and one near top. There is no concern that they differ absolute and over temperature. The problem is that there is a difference between cooling down and heating up of nearly up to 1K at the same temperature. So which one is the temperature of the resistor? NTC1 NTC2 or none? If I do not meet the exact temperature of the resistor I will see a hysteresis on T.C. curve which is not from the resistor but from the temperature displacement.
Title: Re: T.C. measurements on precision resistors
Post by: quarks on June 19, 2014, 10:34:49 am

The foregoing is standard calibration procedure, you may want to check a resistor's characteristics at something higher than minimum power levels. 
...
The 801 detector is about average in performance, you can substitute a better null detector such as a Fluke 845A...
...
The Fluke 8508A is an excellent DMM, in many instances I have been able to calibrate Flukes to much better accuracy than the specifications...
...
The ESI 240C and RS925A forms the most important parts of a Kelvin bridge and it is there that your accuracy in resistance readings resides.  The power supply is not critical but a good analog null detector is, you are 85% of the way to where you need to be with the two Kelvin bridge components you already have, in conjunction with your Fluke for the higher resistance readings, you are very well set for good accurate readings.

Hello Edwin,

thank you for your reply and your hints.

Because I do have all gear for a "242 like setup" (inkl. Fluke Null Detector 845), the Fluke 8508A was only meant to compare to the "ESI 242 system" results. 

Can you share / describe how you setup your own ESI 242 system to do your TCR measurements?

bye
quarks
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 19, 2014, 10:54:23 am
Hello Andreas,

first of all, I really appreciate your T.C. measurements.

Those ppm/K measurements are really very delicate.
Although you are using relatively crude measurement setup  (i.e. this nominal 24bit A/D), you can clearly demonstrate the T.C. over T curve of the Z201.

Before you continue your measurements, I'd like to propose some simple improvements in physical aspects..

In your R(T) curve, a hysteresis is visible.

As the temperature range is relatively small, i.e. below +/- 20°C, I assume that this effect originates mainly from the bad thermal coupling between DUT and temperature sensor.
You should improve your setup to distinguish such an effect from the known thermal hysteresis of the metal foil technology.

In such physical temperature experiments, an 'isothermal block' usually is used.

This is realized by a copper block with drill holes, inside which the DUT and the temperature sensor are mounted, both assembled with thermal conducting grease.
The copper serves two purposes, first it lags outer temperature changes by its heat capacity, and 2nd, it  tightly thermally couples DUT and sensor by its high heat conductivity.
Then you are left only with the heat transfer resistance inside the resistor, in my case, the oil inside the VHP package.

To summarize, your current setup suffers greatly from too high heat transfer resistivity, and you should simply add an isothermal aluminium block.
 
 
Then, instead of a forced gas flow, it's better to use temperature reservoirs, as for example cryogenic liquids  (He4, N2) at the bottom, below the isothermal block, i.e. no direct contact, but using the corresponding contact gas to slowly cool (or heat) the isothermal block. That mitigates hysteresis  effects from thermal imbalances greatly.

For the measurement on my VHP202Z, I have used an aluminium block (16x16x30 mm³), see photos.
The sensor (black leads) is a precision epcos NTC.
The outer aluminium box serves as an additional thermal shield, to further minimize air draught and thermal imbalances.

A cooling pad (-18°C, out of the freezer), or a container with hot water (90°C) inside a cool box served as the heat sink / source, and the natural air convection served as the thermal contact to the experiment.

The slower the experiment, the lower the possible temperature differences between resistive element and the temperature sensor, and the smaller possible hysteresis effects from temperature differences inside the experimental setup.

I'll show raw measurement data later.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 19, 2014, 11:25:43 am
Hello Edwin,

I also appreciate your interesting narrations about wirewound resistor technologies, and fully share your opinion about the Vishay metal foil technology.

I've encountered the same experience in my experiments on the LTZ1000 reference, and on my 5 VHP202Z reference resistors.

I disagree in one aspect: The Vishay hermetically sealed, oil filled resistors really are longterm stable, as Vishay claims, i.e << 1ppm/year.
My 4 year long monitoring of agroup of 5 VHP resistors show a drift of less than 1 ppm.

In contrast to that, I did not find (yet) wirewound resistors, which are promoted to have a similar good long term stability.

Only the ESI SR104 assembly is specified for a stability in a similar order of magnitude.

I also appreciate your  flaming appraisal  ;) of the wirewound resistor technology, as I agree, that this is mostly on par with the metal foil technology, but lacking hysteresis effects.

And I own a nice collection of old ww resistors and - sets, from Fluke, HP, Cohu, KINTEL, Alma, Burster,.. really nice handmade constructions.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on June 19, 2014, 12:40:28 pm

For the measurement on my VHP202Z, I have used an aluminium block (16x16x30 mm³), see photos.
The sensor (black leads) is a precision epcos NTC.
The outer aluminium box serves as an additional thermal shield, to further minimize air draught and thermal imbalances.


I know this little bugger!  :P

Frank, I got the cheapest available SR1010 from e-gay, cleaned it thouroughly and gave it the missing case screws and even cloned the optional  shorting bars (thanks SAH-Präzision for the great work), that are 12 pieces of very old 10 Ohm WW resistors... want to take a look ? :)
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 19, 2014, 03:18:18 pm

Those ppm/K measurements are really very delicate.

To summarize, your current setup suffers greatly from too high heat transfer resistivity, and you should simply add an isothermal aluminium block.
 
 For the measurement on my VHP202Z, I have used an aluminium block (16x16x30 mm³), see photos.
The outer aluminium box serves as an additional thermal shield, to further minimize air draught and thermal imbalances.


Hello Frank,
 
you clearly point out the largest error in my measurement setup.
And much thanks for your photos of your measurement setup.
This will help a lot, I will see how I can come close to that.
This is the best input on my problem up to now.

But I will not handle with cyrogenic liquids.
On the other side the thermal block will increase the effort for measuring different sizes of resistors.

I already asked myself how you did your 0.3 ppm/K measurement.
I hope you will deliver some details soon:
- temperature range
- temperature gradient

It could be possible that the bond of the metal foil to the ceramic substrate has a time constant regarding hysteresis.
So I think the ramp speed could also have a effect on hysteresis amount even if the temperature sensor is mounted correct.
I have seen such a effect on the LT1236AILS8-5 reference. But on the other side it could be also the temperature sensor mounting.

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on June 19, 2014, 05:00:26 pm
Hello Frank,

Would you consider a combination of technologies too much of an issue with measurement accuracy? For example, if Andreas used frozen blocks similar to this :

http://www.thermosafe.com/model/FPP56? (http://www.thermosafe.com/model/FPP56?)

and used it with his thermoelectric cooler to slow down the process, perhaps disabling the TEC device before measurements. This would mitigate the need for any potentially hazardous substance in the home. The packs are flat and retain shape as they cool. I was given one of these the other day and thought it might come in handy for a similar test.

Thanks,

ManateeMafia
Title: Re: T.C. measurements on precision resistors
Post by: Conrad Hoffman on June 19, 2014, 05:27:20 pm
Edwin,

I'm curious what you think of the various Julie Research resistors, the bobbin wound units in plastic, the similar ones used in their Kelvin Varley dividers, and the ones sealed in oil? I've also got a collection of old L&N standard resistors, that I assume are manganin. They don't have a good reputation, but seem OK if the temperature can be controlled. Finally, I've got a roll of H.P. Reid (now Kanthal) Rediohm 800 wire. It seems quite good, but can't be soldered. Are there any welding tips you can give me?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 19, 2014, 07:47:30 pm
Wow, lots of questions, it may take me awhile to respond to all of them but I will get to them.

To Andreas (post #33), previous mis-understandings we will just chalk it up to mis-communication and forget about them.  Thank you for the compliments and I certainly do encourage your efforts, it is a very good learning experience.  Now for some of your questions......

As to the 'sweet spots', all resistors, no matter what type, yields both plus and minus TCRs, if a particular resistor specification only indicates a negative or positive TCR, the resistors are being selected, nothing wrong with that per se.  If such TCR specifications leads to customer misconceptions, then the manufacturer is guilty of a cover up as such.  All resistor specifications should have a TCR spec such as 0 +/- 1 PPM/°C for example, the distribution of TCRs always have both polarities.

A customer can specify a particular TCR of either polarity if needed, that does reduce the yield of that TCR from the batch and increases the price due to additional labor and time to 'pick out' a given TCR.  Also note that the customer has to accept a given tolerance for that particular TCR, asking for an absolute TCR is really going to cost.  Most designers would like a zero TCR, which in a given batch of resistors, there may be some resistors that come very close to that but yields are likely small and the price is inversely related to yield, i.e. smaller yield, bigger price.

In the various threads concerning the voltage divider required for the LTZ1000/A (actually two dividers would help....can anybody guess where the other divider goes?), the hunt for the 'perfect' resistor is very consistent.  Along with what I've said earlier about the voltage divider, another technique which is potentially less expensive is to try and match opposite polarity TCRs reasonably closely which will actually cancel each other out to possibly a high degree.  This approach could cost somewhat less than trying to find resistors (such as the Z foil) with near zero TCRs that match.  With this method, any two TCRs of opposite polarities will cancel each other out and the chances of finding those kinds of pairs in any given batch is substantially higher, any two resistors within a standard TCR specification could be used, even 3 PPM/°C TCRs as long as they were opposite in polarity.  Of course, matching two TCRs of the same polarity can work just as well as the opposite polarity match can if you have the right resistors.

I think I am talking myself out of some nice fat resistor charges...but the customer should be given the best possible solution to their needs, the supplier should not just think of themselves.

Vishay has been working on the physical problems of the film/foil resistors since they were invented   over 50 years ago, they actually started out as strain gauges, which they are still used as today, the 'precision' resistor part of it came later, Felix Zandman claimed the first patent on the precision film resistor (the patent(s) were later broken in court by the French company Sfernice) I believe around 1962.  The early TCRs were hyperbolic (like the one you posted, which by the way is a much more recent resistor) with the peak of the curve sitting at 25°C and the curve going in a negative direction on either side, some of their resistor still exhibit the same curve albeit with somewhat lower TCR/°C values.  As they worked on 'ironing out' this pesky curve they managed to find materials and binders which reduced the physically caused 'lumps' in the resulting curve to a 'wavy' sine wave like TCR, the early versions (both types of curves) had TCRs generally higher than 10 PPM/°C ranging up to as high as 50 PPM/°C.  For the lower TCRs that customers demanded, Vishay 'cherry picked' (as we call it over here) lower TCRs out of the batches, with this technique, they could supply a limited amount of TCRs under 5 PPM/°C.

As the years went Vishay continued to slowly improve the TCRs of their best resistors and the alloy suppliers were able to improve their TCR results as well, benefiting both wire and foil resistors.  The 'Z' foil line is the result of many years of work to improve the foil resistor, if you look carefully at the 'Z' foil TCR's box in their spec sheet, you will see the same 'wavy sine wave' TCR curve as the earlier resistors had, only they have managed to reduce it to a very small sub-PPM value within a limited temperature range, it is still not flat by definition.  The shape of the curves have remained pretty constant over the years only the TCR values have decreased.

I should interject here that while the earlier precision wire wound (PWW) resistors had flaws as I related in my EDN article, their TCR curves tended to be flatter than the Vishay resistors over most of their temperature operating range, approaching a much more linear curve, a fact which Vishay did their best in advertising to discredit along with presenting their resistors in the best possible (albeit somewhat distorted) light.  The flaws in the early PWW resistors did not help their case but for some curious reason, the PWW side of the industry did not fight back in the advertising arena
and allowed Vishay's effective attacks to further discredit the PWWs.  PWWs have been effectively written out of many projects due to that ad campaign even to this day.
 
I believe someone, maybe it was Andreas, asked about my TCR curve, well technically, it isn't a curve, it is a flat line across its temperature range with small changes in the TCR at the extreme temperature limits (-55°C to +125°C).  No curve balls, no spit balls, no screw balls, maybe just a fast ball.

Now, one of the major (among several) problems in measuring minute quantities is controlling temperature and (very importantly) controlling air flow around the D.U.T.  In a calibration lab, the air temperature is usually controlled as closely to 25°C as possible, but we aren't in a calibration lab so we improvise.  As Andreas mentioned, he is not trying to measure absolute quantities, which would be very difficult in his circumstances.  So we work with less absolute accuracy but stable (over the necessary period of the test) and repeatable results.  This can give a good indication of a measurement that the system is not directly capable of.  If the measurements are stable and repeatable and some other sources of error are observed, reasonably accurate results can be expected.  As we say, the devil is in the details.  TCR measurements have a lot of devil in the details.

A small chamber made out of insulation or Styrofoam big enough to hold your thermal cooler/heater assembly and the D.U.T. with the holes for wiring, temperature sensor, ect., plugged up will work nicely and the chamber is comparatively cheap to make, metal isn't as good since it doesn't insulate very well.  Unfortunately you don't want to put the ADC circuitry in there since you need its temperature to remain stable throughout a test run.  Put a Styrofoam cup over the ADC circuitry to keep air currents from messing with it.  While it would be nice to be able to run a TCR sweep over a 100°C range, it is not practical with a home brew very easily.  A 50°C range from hot to cold is sufficient to give a decent indication of average TCR  (remember Z foil curves are not linear), it will actually be a better TCR than is actual over the range because of the shape of the curve being nonlinear but it is impractical to have the needed equipment on an experimenter's budget, that takes a standards lab grade to get the job done.

A fan inside the D.U.T. container is okay as long as it is sealed, if it has any significant leaks, it will cause temperature errors.  I would mount the fan near the 'cold' plate to circulate the air around.  TCR measurements at resistor houses are usually done in large temperature chambers using CO2 tanks and heaters, there are table top chambers but they are expensive, even the used ones.

The next bug-a-boo is thermal EMFs in your connections to the measurement circuits and D.U.T.  This can be difficult to fix, as a microvolt of EMF is very significant, make sure the solder joints are good and the number of connections are kept to a minimum.  The easiest way to minimize thermals is to keep everything at the same temperature inside the containers.

Noise is another source of error and often is very difficult to get rid of it, it is bet to get rid of it at the source, but is often no possible.  In Andreas' case, it can be tricky filtering out noise at the input, partially due to the input impedance of the ADC which tends to vary with input.  Filtering with active circuits can be effective but can also introduce some errors of their own, offsets and their own additional noise.  To some degree, if you can quantify how much noise you have and it is relatively stable, it can be subtracted out of the measurement with fair accuracy.  However, the relative stability of many noise sources tends to drift with time and attempting to use long term averaging techniques will not result in better accuracy.

I think the majority of your measurement problems is controlling the air currents followed by noise and the thermal EMFs, try the setup I suggested, it may surprise you and the best part of all, it is relatively cheap!  Sorry, I haven't had the time to look closely at any of the photos of test setups as yet, I'll try to catch those as well.

I will answer your questions about the resistors for the LTZ1000/A circuits in another post, sorry the turn around time here is so long.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 20, 2014, 01:29:02 am
Andreas,

I took a good look at the LTZ1000/A data sheet and curiously, many of their data sheets will specify resistor characteristics, even down to TCRs, this one is not the case.  Aside from the voltage divider resistors and the 120 ohm resistor whose TCRs are easier to determine, the other resistors, depending on just what circuit you are using (such as the positive (or negative) reference in the application section needs further examination.  Some of them have no significant effect on performance while others can.  I will post when I have completed my examination.  Quite a few variations are possible to enhance performance.....interesting.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 20, 2014, 01:55:20 am
To quarks,

(reply #34)  Since something along the lines of a full scale temperature chamber is not at my disposal at this time, I designed and built a small chamber out of high temperature insulation using a good glue that with standards higher temperatures than I intend on using.  I made a double thickness door out of two sheets of the insulation glued together, one that fits inside the opening and the other that fits across the entire side of the chamber.  At the top, I installed a fairly large TEC with a fan mounted to circulate air within the chamber, an accurate temperature sensor (platinum RTD) is connected to a custom designed temperature controller (by me) which should keep the temperature within less than a 0.25°C variance within the chamber and control the nominal temperature setting within 0.01°C.

The circuit board material is at least an FR4 grade or better, solder joints are kept to a minimum and clips are used to connect the resistor in four terminal mode through a bunch of teflon coated wiring through a small hole in the side to a switching console which, in-turn is connected to my ESI 242D.

I usually try for at least a 50°C temperature range or better between the hot and cold temperatures that the TEC is capable of.

That's it in a nut shell, if you are curious about any details, please ask.  Sorry, a bit short on time at the moment.

Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 20, 2014, 03:41:58 am



But I will not handle with cyrogenic liquids.
On the other side the thermal block will increase the effort for measuring different sizes of resistors.

I already asked myself how you did your 0.3 ppm/K measurement.
I hope you will deliver some details soon:
- temperature range
- temperature gradient

It could be possible that the bond of the metal foil to the ceramic substrate has a time constant regarding hysteresis.
So I think the ramp speed could also have a effect on hysteresis amount even if the temperature sensor is mounted correct.
I have seen such a effect on the LT1236AILS8-5 reference. But on the other side it could be also the temperature sensor mounting.

with best regards

Andreas

Hello Andreas,

the story about liquid Helium or Nitrogen was an example only.
At the physics institute, I used to measure that way, and also calibrated our thermometers in the temperature range of  2K ... 300K.

 
For this purpose here, simply use a  cooling pad, or even your heater instead.

And I really meant that using an aluminium block is a simple thing..

Aluminium bars, 1m long and 16x16 or 25x25 are available in your next DIY store (Baumarkt).

Saw off a piece of appropriate length, and drill several holes completely through that block, so that you may attach different resistors in that block.

For tubular resistors and the NTC it's very easy, add 1/10..2/10 mm for the diameter.

For rectangular resistors, you have to drill and file a slot, well right, that's a little bit more effort.

Build one additional block for your reference resistor.


I use an HP3458A in 4W Ohm mode, with offset compensation and 0.1ppm resolution.
Consecutive readings on the same resistor are also stable to <= +/- 0.2ppm, at 100 NPLC (4sec measuring time).
 
Measuring my 5 VHP202Z one  after each other several times within 10..20 minutes,  I always get repeatable readings  which differ not more than 0.2ppm for the same resistor.

The ambient and internal temperature is stable to <= 0.1°C in that time period.

Therefore, the transfer stability is about 0.2ppm.

Even for much longer times, keeping the temperature stable, gives stabilities in that order of magnitude.


The T.C. measurement on the V334 took about 8 hours, see diagrams.
The vertical graticule is 0.2ppm wide.

Each measurement point is simply read once from the instrument, without further averaging than the NPLC 100 measuring time. I also apply a calibration factor for the HP3458A (derived from the group of these 5 VHP202Z), but that's secondary for the T.C.

There is a jitter from point to point about 0.2ppm wide.
Those Ohm measurements are relatively sensitive, and the DUT was connected with four 1m long, unshielded cables only.
I simply let the linear regression function average out all this jitter for determination of the T.C.

In the beginning, heating was done too quickly, so the curve deviates from the average line by a bigger amount. Therefore, about the first 100 points were skipped for the T.C. calculation.

For the rest of the temperature cycle, over 6h, the measurements follow nicely the linear regression line, without a noteworthy hysteresis.
If you compare the starting measurements around 25°C (#185) to those at the same temperature after 6h (#378), they agree within +/- 0.2ppm.
That's quite stable, I think.
The internal temperature changed not more than +/-0.2°C.

There are several glitches visible in the measurement, caused by myself, when I moved the heat reservoirs inside and outside of the cooler box.


This resistor itself obviously shows no hysteresis.

If you would repeat that measurement with a much faster temperature gradient, you might see a hysteresis loop, which would be caused by the temperature lag between DUT and T-sensor only.
By the shape of the hysteresis loop, you could judge, how good your temperature coupling is.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on June 20, 2014, 06:22:59 am
@Andreas

Would you accept if I donate a metal slab (might be brass or whichever fitting slab I find) with drills (say 2 mm interval) and a tube of thermal grease? My own observations with medical thermistor catheters shows a big difference in thermal coupling between putting reference sensor, heater and DUT only in close proximity inside a small space surrounded thermal insulator to a drilled metal block surrounded by isolation.

BR

Hendrik


Title: Re: T.C. measurements on precision resistors
Post by: TiN on June 20, 2014, 11:41:08 am
I can play with liquid nitrogen :)
Just got some foils today, and will build a test setup for TC and resistance measurements using TEC module.
Hope TEC will give me enough stability.

Ill post in my KX thread or create article on my site after all done.
don't want to hijack this nice thread.  :-DMM
Title: Re: T.C. measurements on precision resistors
Post by: Galaxyrise on June 20, 2014, 05:30:24 pm
In the various threads concerning the voltage divider required for the LTZ1000/A (actually two dividers would help....can anybody guess where the other divider goes?)

First: Welcome to the forums, sir! I very much appreciate having you here.

Now to your "riddle": Are you referring to the technique (I first saw explained by Dr Frank  https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg240405/#msg240405 (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg240405/#msg240405)) of using one divider to trim the zener output to exactly 7, then using 10 "identical" resistors to implement the gain divider to get to 10V?
Title: Re: T.C. measurements on precision resistors
Post by: branadic on June 20, 2014, 09:42:40 pm
Quote
Would you accept if I donate a metal slab (might be brass or whichever fitting slab I find) with drills (say 2 mm interval) and a tube of thermal grease?

I had the same idea and have send some brass to Andreas today. Hopefully the mail is fast enough to arrive tomorrow.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 21, 2014, 02:02:37 am
To Dr. Frank (reply #36),

I was almost done with this post, this morning, had to stop and this afternoon, Chrome decided it needed to crash...argh!

First, thank you very much for the compliments.  now if I just remember what I'd written before.....

I should probably clarify some of my statements on stability, I work mostly with what would normally be called 'commodity' resistors, even if I don't consider mine in such a category.  So sometimes I kind of forget the boundary, as such, between resistor standards and regular resistors, even though, in my case, these is quite a bit of cross over between them.

Generally, stability is defined as the lack of change in a resistance with time and/or use, the less the change, the happier everyone is.  Stability is specified under two conditions, one for standards, one for other resistor types.  For a primary standard, such as the SR-104, stability is defined in terms of time, under the standard definition of use of a standard, i.e. very low power, constant temperature, no physical shock and kid-glove care.  This is very similar to the drift specification on a shelf non-powered of non-standard class resistors.  This drift can be anywhere from PPM to hundreds of PPM per year, it all depends on the type of resistor and its manufacturing processes.  The second stability test, which isn't applied to standards, is the powered long term drift specification.  Depending on the resistor type this can be anything from <10 PPM / year to hundreds of PPM / year.  This powered stability test can eliminate the boys from the men as such.

The real heavy duty stability test involves thermal shock MIL-STD-202 which has been around since the dinosaur age, at a minimum of 5 cycles between -55°C to +125°C and then checked for drift and failures.  To my knowledge, no Vishay Z foil resistors have come out of this test smelling like roses.  On the other hand, mine have gone through this wringer with no more than 12 PPM change and an average of only 8 PPM change plus the real show stopper, no failures which no other resistor has achieved.  One laboratory ran my samples through 50 cycles of thermal shock (why, I don't know), the stability shifts were still within the same specs and still no failures.  Frankly, I challenge any resistor manufacturer to put their resistors through that test and come out anywhere near as good.

I do know that the shelf and power stability specs on my resistors are nearly identical, we specify shelf as 0 +/- 5 PPM/year and full power @ 125°C 0 +/- 10 PPM/year but I've never seen one of the resistors come close to those limits.  To be honest, I do not have any specific data on the measured drift of these resistors, no past customer has ever come back and complained, it seems self-evident by the thermal shock tests, that these resistors have very good inherent stability and reliability designed in.

As to sub-ppm figures in TCR, I know that a significant percentage of any given batch TCR is within 0 +/- 1 PPM/°C with no special treatment.  Just how many of them are close to zero TCR I do not know and that would take some time to find out.  Tain't easy to measure folks!

Some of the old resistors did have some craftsmanship in them and even a bit of art to them.  The only problem with the old resistors are that they have probably drifted out of the initial spec some time ago which may not be an issue.  Unless they have opened they likely have some good like left in them.  The earliest resistors were pretty crude dating back to the 1910s and 1920s when mass production became more common, more 'automated' ways of making resistors were thought of, partly because even then, miniaturization was beginning to knock on the door.  The 1930s brought in ceramic bobbins of various designs for both regular (not really precision yet) and powers, these bobbins stayed with us, mostly unchanged until the era of plastics arrived in the 1950s and miniaturization was knocking even harder at the door, bringing troubles with it (ask Murphy).

I have a small collection of various bobbin resistors including some early ceramic precisions which were about half an inch in diameter and at least an inch long, some are encapsulated in bakelite, with a mounting hole in the middle.  Then there are some smaller ceramic precisions a little bit smaller than a half inch in diameter and only about 5/8th of an inch long but very similar construction.

The construction of the 'flat' mica bobbin resistor was originated by General Radio back in the 1940s, maybe a little earlier and that design has stuck with the standard resistor boys ever since.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 21, 2014, 04:36:30 pm
Hello Andreas,  (reply #38)

I don't know if any of you have seen the insides of a foil resistor so I've attached one, sorry it is a little fuzzy, the original lacked definition.  The main thermal resistivity is from the inside to the outside world, although the foil is bonded to the ceramic substrate with a fairly low thermal resistivity, the resistivity from the ceramic to the outside through the molded case is substantially higher being made from plastic.

Obviously, the rubber pad protects the 'strain gauge' from outside stress, without this rubber pad (which also has very low thermal conductivity), the foil's resistance value would vary quite a bit.  Putting this 'strain gauge' inside a hermetic can virtually isolates it from stress except from temperature effects, both from self-heating and ambient.

The construction of the Z series resistors are even more complex inside to further isolate the 'strain gauge' from stress.  They have been quite successful after working on the problem for about 55 years.  Granted, it took my colleague and myself a bit less than 25 years to solve the welding problem and other improvements to the stagnated PWW.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 21, 2014, 05:17:58 pm
To Conrad,  (reply #41)

The Julie resistor design varied over time since his original patent (which really wasn't all that good of a resistor but good for its time), he did fiddle with the resistor design over time and tweaked some characteristics into better performance.  My colleague, Mike Chesselet, had more experience with Julie's resistors directly than I have, Mike was not too impressed with the designs, on the whole, partly because Loebe was fussy about details, they typically outperformed most of the wire wounds on the common market.  I would say they did not compare with the military grade resistors we were making at Ultronix during the 1970s but close.

One market Julie went into which the other commodity makers didn't go into was standards, mainly working/transfer types but Julie also tried to outdo the SR-104 and Thomas one ohm standards which he failed at.  At one time he even claimed that his laboratory was more accurate than NIST which made him the butt of many jokes.  In the case of his decade boxes and other transfer standards, they performed quite well but there were quality control issues which at times, bit him in the butt.  Most of them were mainly traced to poor solder joints inside the units which could be repaired.

On the whole, Julie's resistors tended to be better than average but not the best except in a few limited cases.  Loebe was a really terrific engineer and had many good designs to his name but he wasn't the best resistor design engineer.  As I told another colleague who asked about Julie and mentioned the patents, I replied that a patent does not guarantee a good device, it only guarantees that it may be unique.

The old L&N tubular secondary standards can be quite good and very stable over time but, like other standards, they must be carefully handled and unfortunately, many of these old standards were mistreated over the years decreasing their stability and permanently changing the value.  There are still good ones out there but there really isn't any way to tell if it is good without checking them.  They are made with Manganin (still are by IET) which is a somewhat fragile alloy, it can exhibit excellent stability if treated right but it is easy to abuse them and ruin them.  The 'good' ones still deserve an excellent reputation, finding one is the problem.  I have several of them myself and nearly half of them show signs of misuse.

I presume you have a spool of bare Evanohm wire, it must be welded to another 'hard' metal to get a good weld joint and at the right pressure and energy which varies with the wire size and whatever you are welding it to.  Unfortunately, it can be welded to a hard brass but you will also encounter soldering difficulties with the brass.  You can weld it to a nickel alloy but that doesn't get you much closer to a solderable joint.  Stainless Steel alloy 305 can also be used to terminate Evanohm but it also has some solder issues.   Most of the industry uses alloy 180 ribbon terminated by an arc (or butt) weld to make the joint, alloy 180 is solderable but has a relatively high TCR of its own.  Unfortunately arc welders aren't cheap, even used ones.  A really good crimp joint might work but depending on the metal, it could create nasty thermal EMFs and very small wire is hard to crimp.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 21, 2014, 08:03:51 pm
I took a good look at the LTZ1000/A data sheet and curiously, many of their data sheets will specify resistor characteristics, even down to TCRs, this one is not the case.

Hello Edwin,

the "spec" for the resistors is not in the datasheet but in the famous AN86 (APPENDIX I).
"R1 THROUGH R5: VISHAY VHP-100 0.1%"
It seems they have cut out only a part from the full cirquit in the data sheet.

I also think I have to do a review of the setup when I install the isothermal blocks.
Perhaps I can put the solder junctions closer together (especially on the UPW50),
to further reduce possible thermocouples.
On one of my measurements I have seen a
"oscillation" on the NTC difference curve (only if the cooler is active)
from which I think it cannot be physically possible.
I think I have to install a EMI-Filter on the Multiplexer of the NTCs too (as I have already done on the resistor inputs from the beginning) to reduce the effect.

In the mean time I also think that the time constants are so large that I have to do other strategy for the hysteresis measurement. I will only have 5 setpoint temperatures 25, 10, 25, 40, 25 degrees an a appropriate settle time.

Another question is: why do I see always a non-linear (convex) T.C. curve?
I think I have to check NTC linearity influence. (and possibly correct the curves).

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 22, 2014, 08:49:53 am
..

Another question is: why do I see always a non-linear (convex) T.C. curve?
I think I have to check NTC linearity influence. (and possibly correct the curves).

With best regards

Andreas


Andreas,

for the NTC, you probably have a calibration table in your measurement program, so that should not be the root cause at all.

This convex shape - it is probably real - is where your measurements are really interesting under physical aspects.

Well, all pure metals as platinum (in the Pt100), or alloys as Manganin, NiCr, etc. have a strictly linear R(T) behavior. Also the precision wire wound types should show that.

But for the metal foil technologies (C-, K-, Z- foil) it is claimed, that this linear R(T) is eliminated by the thermal expansion of the ceramic where the metal foil is bonded to.

So in the ideal case, there should only be left effects of higher order, like R(T²), that is the superimposed curvature you probably see.
Therefore, only in the case of nearly perfect match between metal foil and ceramic, those ultra low average T.C. of "typical 0.05ppm/K" will result.

See the appended scientific paper of Vishay.

The incomplete compensation of the linear behavior increases the T.C. greatly, and is covered in the datasheets by the additional "+2ppm/K max. spread".

If they cannot guarantee better, compared to those incredible low 0.05ppm/K, then this means, that Vishay still is not able to control this technology (the matching) good enough in series production.

I also have one VHP202Z resistor, where you can separate the huge linear and a much smaller quadratic term. I'll show that later, perhaps.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 22, 2014, 09:12:44 am
Hello Frank,

I also see that curve on the UPW50 wire wound resistor. With -0.022 * dT² part.
See updated UPW50 section on page 1.
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462300/#msg462300 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462300/#msg462300)

As all here are on the opinion that WW resistors should behave linear I have to examine that.

P.S. someone dropped that paperweight yesterday afternoon in my front garden.
I will see how I can use it.
Thanks @ branadic.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: branadic on June 22, 2014, 09:30:05 am
Hi Andreas,

do you have another ADC board available? I could sent you 2x leaded PT1000 tomorrow for your measurements, if you like to.
I would also suggest to check the NTC calibration, as they have nonlinear behavior and the calibration is a linear approximation.

branadic
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 22, 2014, 11:56:21 am
Hello branadic,

To ADC: I still have 2 with VRE3050A reference (but i dont like them because they are rather noisy).
I only run out of multiplexers (and probably good pull up resistors) so I can do no 4 wire measurement.
With a Z201 resistor as pull up to reference I will get a sensitivity of around 2.8 mV / K.
(against 55mV/K average on a 10-40 degree span with the NTCs).
That should probably work.

Which diameter shall I plan in for the hole?
The NTCs use around 3.3-3.8 mm. I have a 3.2 and a 4mm drill in that range.
Also for the UPW50 the question is if its better to buy 6.5-6.7 mm drills or use the 6.8 mm that I have.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: branadic on June 22, 2014, 12:03:56 pm
Quote
Which diameter shall I plan in for the hole?

Okay, I will send them tomorrow, so they should arrive on tuesday. Please refer to the following link for the required dimensions:

http://de.farnell.com/ist-innovative-sensor-technology/p1k0-232-6w-b-010/sensor-pt1000-600-c-klasse-b/dp/1266940?Ntt=1266940 (http://de.farnell.com/ist-innovative-sensor-technology/p1k0-232-6w-b-010/sensor-pt1000-600-c-klasse-b/dp/1266940?Ntt=1266940)
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 22, 2014, 05:38:20 pm
To Galaxyrise,  (reply #48)

Thank you very much for the welcome and thank you for the compliment.

You are in the right neighborhood, Dr. Frank's setup is quite interesting but a bit complex and all those Vishays are going to cost.  It does indeed involve the LTZ1000/A's zener diode and its TC.  A very good guess for a start, my compliments, keep thinking you are close.  The 12.5K / 1K divider could also benefit as well.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 22, 2014, 06:37:29 pm
It was so darn sticky outside, that I moved back to our cool and comfortable basement.

See what I've tinkered there, quick 'n dirty:

A thermal block for those epcos precision NTC S862, for a precision Lake Shore PT103, and for a G.R. 10k PWW.

Drill holes being 0.2mm broader in diameter than the DUT is just fine; for the 10k resistor with 6.3mm I used a 6.5mm drill.

The comparison / calibration measurement of the NTC vs. the PT100 sensor is currently running.
Note the cool football box (from 2010)!

The 10k resistor will be done later in the evening, I think.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 22, 2014, 10:05:25 pm
Fine,

so we will also have a 8E16 resistor in comparison :-)

Today (before I change the setup) I did a measurement with a 3 hrs rest at 25 degrees and the UPW50.
The UPW50 on a SK09 heat sink with thermal grease for the NTCs.
It seems that the hysteresis is still there ...
I will update the UPW50 post on page 1 the next days ..

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 23, 2014, 01:07:38 am
To Andreas (reply #53)

Thank you for pointing me to those resistor specs, I remember the VHP100 series, as I recall they came out in the late 1980s or 1990.  I was rather amused that in the app note, the address given for Vishay/Ultronix was still Grand Junction, even the phone number.  Vishay had already moved Ultronix out of Grand Junction around 1984 and that address/phone number still appeared on documents for several more years after 1991 when the Linear Tech AN86 was first published.

Careful examination of the VHP100 series data sheet shows that Vishay has been busy updating it, nothing wrong in that, but they have also did their best to pull the older data sheets off line.  At any rate, in 1990 when the Linear Tech boys were working on this app note, the claims by Vishay about these resistors being the best available technology was partially true.  None of the 'regular' PWW houses specs could come very close and unfortunately,  the Ultrohm Plus resistors were not available through regular channels.  We had operated as what is termed a 'captive operation', we could only make resistors for one customer and no one else.   Although some word-of-mouth got around, we were pretty much unknown.  It didn't matter at the time as we could not sell to anyone else anyway.

I see Dr. Frank answered your question about the convex curve, he is mostly right in his answer.  Vishays precision foil resistors have two types of curves, sine wave and hyperbolic.  The C, K, and Z all have hyperbolic curves, if Vishay actually published readable curves instead of purposely obfuscated curves, you would see the hyperbolic curve.  These are the sum of all of the various stress sources working on the foil and Vishay has done a very good job getting them down to a very low level.  The design of the internal resistor is more complex than the earlier drawing I showed in another reply.  Vishay has basically done what was done to the Manganin resistors over the decades, the alloy was 'tweaked' so that temperatures around the cardinal point resulted in the lowest TCRs but, like Manganin, the foils are still susceptible to permanent change from stresses inside and outside.  It has been minimized but not eliminated.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 23, 2014, 07:09:45 am
To Andreas (reply #53)

Thank you for pointing me to those resistor specs, I remember the VHP100 series, as I recall they came out in the late 1980s or 1990.  I

...

I see Dr. Frank answered your question about the convex curve, he is mostly right in his answer.  Vishays precision foil resistors have two types of curves, sine wave and hyperbolic.  The C, K, and Z all have hyperbolic curves, if Vishay actually published readable curves instead of purposely obfuscated curves, you would see the hyperbolic curve.  These are the sum of all of the various stress sources working on the foil and Vishay has done a very good job getting them down to a very low level.  The design of the internal resistor is more complex than the earlier drawing I showed in another reply. 



Dear Edwin,

I have uploaded that Vishay scientific paper here, and within that, all those "real" curves for C, K, Z foil are shown, and explained. They all feature a quadratic shape of T.C., either negative or positive orientation, which should be a typical characteristic of the foil technology used.

There is also a hint about a combination of C and K resistors in one housing, which is complicated to match, but gives the best T.C. performance by additional compensation by the contrary running T.Cs.
This type also has that strange convoluted T.C. shape.

As I were told from Vishay, the actual VHP100 series are exactly those, i.e they contain two resistor chips (of C,K foil) inside.
Vishay told me, that this type currently is the best they could offer.

But I really wonder, why Z foil technology is still not outperforming this hybrid type.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: mzzj on June 23, 2014, 07:39:29 am

The 10K metal foils of Dr. Frank are between -0.3 ... -1 ppm. My first measured 1K is around 0.9 ppm at 25 degrees.
So what is in between? Of cause not all metal foil alloys will have the same TC so the values might vary from batch to batch.

My luck with  VHP202z's
5pcs of 100 ohm resistors: TC <0,3ppm/K  :-+
5pcs of 25 ohm resistors TC 1,5....2.2ppm/K  :--

I Have also full set of VHA516-4z resistors but I haven't had time or interest to measure those yet. 
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 23, 2014, 08:28:54 am
@ Andreas,

yesterday I made  two quick 'n dirt experiments on this new thermal block.
Mainly, I wanted to check the thermal coupling of the different components, and also if I could create hysteresis loops by fast temperature changes.

The results are  a mixed bag, and interesting for your set up, also, I think.

Starting at room temperature, I used a hair dryer to heat the alu block to about 45°C within a few minutes.
I let it slowly cool down back to RT, and then inserted a cooling pad, which cooled down the block to about 13..14°C in about 1/2 an hour. Then, again, I removed the pad so that the block slowly warmed up to RT again.

The curve for the NTC vs PT100 shows a small kind of hysteresis loop, the temperature deviation is 1°C maximum.

This loop is not a real hysteresis / difference between both sensors.
They indeed follow each other very exactly and with no real delay.
The maximum real deviation is about 0.2°C, that means, that the NTC is really very precise.

The visible loop is caused by the measurement program only:

The NTC is measured by the HP34401A, with about 200ms sampling time.
The HP3458A measures the PT100, but only after the 34401A has been read, and it needs a much longer sampling time of 2sec or so, because of the Offset compensation, and because I have chosen a too long NPLC for that purpose.
Therefore, the 3458A measures the PT100 at a considerably higher temperature, than the foregoing NTC measurement.

If I would trigger both instruments simultaneously, with the same sampling time, I would for sure see absolutely no loop at all.
That also means, that both sensors are sensitive to heat changes by their bodies, not by their leads.
 

The 2nd experiment was done the same way, but on the 10k PWW vs the NTC.
I also had to use a long sampling time for the 3458A, about 4sec, due to the necessary ppm resolution.

But this was obviously not the problem which caused those gruesome hysteresis loops you see in the 2nd diagram, as I would have expected a maximum temperature difference of < 1°C between PWW and NTC, instead of over 10°C.

I think, that the PWW mainly transfers external heat via its leads, (which I did not want to cut short), and not over its body.

In such cases, another trick is to thermally "catch"/absorb the wires also.
That means, to solder the ends of the wires to a pad on the alu block, which is electrically isolated, but thermally coupled to the block.

Ok, to mitigate that problem otherwise, I would surround the whole assembly by another aluminium box, so that also the wires would be protected from air draught.

If you just use the linear parts of the curve, i.e. when the sample cools slowly down from 40°C to RT, and warms up from 14°C to RT, the T.C. is calculated to about 2.2ppm/K.

Specification was 3ppm/K typ.

 Frank


Edit:
From the inner construction of the 8E16 PWW, it's clear that mainly the leads transfer the heat:
The outside of the bobbin is thermally isolated by a rubber protection, following epoxy, following a plastic tube.

The copper wires go deeply inside the bobbin core, and an additional resistance tape is assembled.
See drawing, taken from the specification.
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on June 23, 2014, 08:47:38 am
My setup at work is a brass cylinder with a center drill that takes the DUT or my reference thermometer. A small Heraeus Pt100 is glued on the block too, on the same height as the center drill. A layer of teflon tape from your friendly GaWaSch is wound around it with the heating coil (enamelled cooper wire) on top of it. A HP6632B feeds thet heater, reading back the heater resistance used to limit heat and a 34401A reads the Pt 100 to control a simple Regulator programmed in EZGPIB - usually fixed at 38°C, but sometimes heated to 46°C and slowly cooling down. (Heat flow is always something I want to prevent, but..)

All the heated part is embedded in a big blox of polystyrol foam - the hard yellow kind, recovered from a trash bin, with only very little clearance to the heater block and just a small diameter tunnel to insert the DUT or Thermometer.



Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 23, 2014, 04:16:42 pm
To Andreas (reply #55) and Dr. Frank

I've compared the data sheets of RhoPoint Econistors and the Neohm UPW series, from the descriptions, they both use essentially the exact same bobbin designs, absolutely nothing proprietary there, materials and encapsulation.  Also, they both specify 0 +/- 5 PPM/°C TCRs, then say 0 +/-3 PPM/°C is typical.  Depending on where you look, the 5 PPM/°C spec may be left out, naughty.  The power ratings are a bit different between the two considering they both are the same, I think the UPW50 is a bit over rated in power for its size and is going to run rather hot above 0.33W.  The derating curves are also different, the RhoPoint curves are more realistic.

Dr. Frank, you are mostly right about the thermal conductivity, because the epoxy bobbins have very low thermal conductivity, as does the silicone rubber coating and shell, some of heat is 'pulled' out of the resistor by the leads and whatever they are soldered into.  If they are using an unfilled epoxy the thermal conductivity is ~0.00047 cal/sec/cm/°C, for a filled epoxy, ~.00123 cal/sec/cm/°C, quite low indeed.  Depending on just what kind of epoxy these guys are using, the resistor's characteristics will be different, mostly evident in the linearity of the TCR line.

Given enough time, the thermal 'lag' of the resistor will catch up to the change in temperature and will stabilize at that point, until that point is reached, any readings will be in error and could show up as a 'hysteresis' error.  Once stabilized, the resistance readings should be consistent, if they are not, some other form of 'error' is evident, possibly within the resistor itself if all else remains the same.  Unfortunately, you cannot directly read the internal temperature of the resistor without a temperature probe (such as a NTC) in close contact with the resistor's windings, quite impractical.  At best, you can only come in close contact with the resistor's shell and try to compensate for the thermal lag by waiting a sufficient time.  Indeed, TCR measurements have many ways of cobbing things up.

However, compared to the other type of PWW resistors which are molded in DAP or alkyd, the thermal conductivity is much higher but the molding process imparts a severe strain on the resistor's windings which cannot be completely relieved by any stability bake, the resistor will continue to drift at an unknown rate over its lifetime.

Obviously, oil filled resistors are probably going to have a shorter heat lag as mineral oil has a higher thermal conductivity than epoxy but the same situation still applies here, sufficient time must be given for temperature stabilization or the readings are inaccurate.

While I'm thinking of it, in the postings on stability curves, I still have not seen any references to reading uncertainties, while I've seen some very well done stability measurements, the readings are consistently inferred as accurate, such as 0.2 PPM/°C, while that number may be consistent, the actual value could be anything within +0.5 PPM/°C to -0.1 PPM/°C (the uncertainty I chose is a ballpark figure, it could be a bit less or even a bit more).  No calibration lab would ever give an measurement number without an uncertainty figure of the measurement.  The measurements I have seen here do indicate a good stability figure but do not represent an accurate one and there are uncertainties in regression calculations as they are only as accurate as the data being used at best.  Another source of error (and this one is fairly big) is that, at a minimum, the instruments and references must be calibrated/accurate to a known figure that is at least 5 times better, preferably 10 times better than the measurements being attempted.  None of the equipment nor references used here have specifications good enough to even come close to accurate readings; linearities, resolution (big one), ect., are no where near good enough, the uncertainties are very significant.

My own instruments, the ESI 242D, in a direct comparison to my calibrated SR-104 would be on the ragged edge of sub-PPM measurements and the uncertainties makes the absolute measurement of those quantities rather suspect.  If I can make repeatable measurements within a given time period, that only means that the uncertainties are also stable with time but that does not improve the accuracy of the measurement.  This is quite acceptable if one is attempting to match resistor TCRs, the uncertainty is still there but the matchup of the resistors remains very accurate, only the absolute accuracy is suspect.

If you are working with a resistor that has linear TCR qualities, the best way to improve the accuracy of the TCR measurement is to measure it at several temperatures over as wide a temperature range as possible, a 100°C range is typical if the resistor can stand those temperatures.  If the TCR curve is nonlinear, then it becomes much more difficult without very high grade laboratory equipment and if accuracy is important, a 4-wire Kelvin bridge is really not up to the job.  A much more complex setup is required, primary calibration labs usually employs such a bridge for very precise measurements or a DCC bridge if the resistance in question is below 10K ohms.

Dr. Frank's setup has shown very good stability over a given time period, he has done an impressive job in that respect but I still caution readers here to not accept any readings as anything more than a figure of stability which may, after all, be the only characteristic intended.
Title: Re: T.C. measurements on precision resistors
Post by: Conrad Hoffman on June 23, 2014, 05:10:57 pm
Some time back there was an Audio Precision Powerpoint presentation, http://www.ap.com/download/file/747 (http://www.ap.com/download/file/747)  where they talked about the common resistor technologies. One thing they said was that the low frequency modulation distortion was surprisingly worse than expected for metal foil resistors, and that metal film were better. I toss this out because not everybody is working with DC, and it might be a useful bit of trivia.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 23, 2014, 07:04:07 pm
To Dr. Frank (reply #63)

Interesting item about the VHP100 series, Vishay rarely revealed any details about the inner workings of their resistors but I am not surprised that they used two opposing resistor TCRs for compensation.  The practice of selection to get an end result is still common today.  I know that Vishay claims that some of the Z foil resistors are 'designed' to produce sub-PPM performance, but I am still a bit skeptical about that.  I've never seen a alloy process that could consistently produce such results without any selection.  That could be little more than Vishay buttering the specs again.  After all, who could challenge such a claim?  The only ones privy to the actual processes are under a no disclosure contract.  While I have some knowledge of the general processes used in the various C, K, and Z foils, I do not have really specific details that I could say they are telling the truth or lying.

With the Z foils, I think that Vishay is pretty close to as good as it is going to get without some new advances in materials.  The resistors I produce are pretty close to as good as they can get until improvements in materials come along.  In some respects, the Evanohm alloy is nearing its limits as well in wire form.  It is very difficult to control minute mixtures and heat treating processes either in wire or foils.

I am looking into new improvements in my resistors if it is possible, which could make significant changes to some characteristics but that is still a ways off.  I don't have the fat R&D budget of Vishay to work with.

I suppose if I use the marketing method that Vishay and others use, I could validly claim that my typical TCR is 0 +/-1 PPM/°C since the majority of my resistors fall into that spec and I wouldn't be lying.
Title: Re: T.C. measurements on precision resistors
Post by: branadic on June 23, 2014, 07:33:30 pm
Quote
I suppose if I use the marketing method that Vishay and others use, I could validly claim that my typical TCR is 0 +/-1 PPM/°C since the majority of my resistors fall into that spec and I wouldn't be lying.

My suggestion is, that if you come along with a real datasheet this would be a good point to start at and to compete with other resistors. How do your resistors behave under same condition to PWW by competitor?
It's hard for me just to "Bolieve" in the specs you ensure without showing any graph or measurement. No doubt that you have long experience in PWW and that the resistors you manufacture are damn good, but it's one thing to complain about specs given in datasheets by competitor but to not have an own datasheet of resistors that are fabricated by you for long time. That is somewhat strange, won't you agree?
However, the details and historical facts you have given are for sure very interesting, but for me there is something left.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 23, 2014, 08:28:55 pm

In such cases, another trick is to thermally "catch"/absorb the wires also.
That means, to solder the ends of the wires to a pad on the alu block, which is electrically isolated, but thermally coupled to the block.


Hello Frank,
good Idea.
I will try to make an experiment with some "chip" heat sinks + thermal tape connected to the wires.
And also I will have to look for some "software" effects. But since I measure similar values with
a 0.3K/minute and a 0.12K/minute ramp I do not believe that there is such a effect.

Edwin: thanks for giving the numbers on the thermal conductivity. That confirms the theory of Dr. Frank.
And yes you are right: my measurements are intended only as comparison between different types of resistors and intended to pair a set of resistors for a stable voltage divider. (Thats what I wrote in the very first post of this thread). And if I meet the figures between 0.9 and 2.1 ppm / K with about 10% relative error this will be "good enough" for me to get a factor 10 improvement for the divider.
Of course I will also have do some estimation on the error of my setup. For the T.C. I am confident that it will be ok.

Hysteresis is the much more difficult to measure.
And I fear that there is still some hysteresis at the UPW50 resistor which has nothing to do with the temperature measurement error.
Edit: See also updated measurements on page 1 for UPW50 from 22.06.2014.
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462300/#msg462300 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462300/#msg462300)
But since the hysteresis of about 2 ppm for my temperature range is much less than a 18-32 degree shift with 0.9-2.1 ppm/K it will be negligible.

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Galaxyrise on June 23, 2014, 09:30:31 pm
Quote
I suppose if I use the marketing method that Vishay and others use, I could validly claim that my typical TCR is 0 +/-1 PPM/°C since the majority of my resistors fall into that spec and I wouldn't be lying.

But it's one thing to complain about specs given in datasheets by competitor but to not have an own datasheet of resistors that are fabricated by you for long time. That is somewhat strange, won't you agree?

He posted his specifications sheet (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg463479/#msg463479), dated 2001, which states "60% of units will be 0+/-1 PPM".  That's actually more specific than just the word "typical".
Title: Re: T.C. measurements on precision resistors
Post by: branadic on June 24, 2014, 06:51:06 am
Quote
He posted his specifications sheet, dated 2001, which states "60% of units will be 0+/-1 PPM".  That's actually more specific than just the word "typical".

Please allow that I'm another opinion. This is just a "specification", some words printed on white paper, nothing more. Is it now task of the end user to verify if his ensured specs are fullfilled?
All I'm asking for is a database made by measurements on his own resistors to prove his written words, that's it. As I said, I can't claim others datasheet if I haven't one myself.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 24, 2014, 03:30:27 pm
To Branadic (reply #73)

I would argue that Vishay has not posted actual test results as such only graphs with said claimed performance on them, usually with no 'window' of minimum and maximum except as a brief blurb somewhere on the data sheet.  Vishay, in my opinion, does need graphs (even if they are 'fudged' a bit) because none of their TCR curves are linear, never have been and never will be.  To my knowledge, I rarely, if ever, have seen any 'official' test data put out by Vishay in the manner I believe you are asking for.  Their data sheets are pretty much it, there are no measurement "data bases" in any other data sheets.  It makes me wonder why you are being so fussy about my resistors?  Would you be so fussy if I was only claiming similar performance to the other resistor manufacturers?

I/we have never bothered to put a graph in the data sheet because it is simply a straight line of the TCR with very slight deviations at the far ends and 25°C as the reference point.  It is linear, in that respect a graph is redundant, words describe it accurately.  The other specifications are simply called out as they are, simplicity as needed.  A graph would be a bit redundant here as well.

To accede to Branadic's request for data, here is a test report for our standard parts, no special preconditioning and it includes the 'warts' as well.  A mistake was made with the wire size in one of the bobbins which caused an aberration in the test results, a tear-down was done and the cause of the aberration found.
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on June 24, 2014, 06:53:01 pm
I consider Edwins Information very accomodating compared to Vishay (the publicly available stuff.) Vishay might - at least I guess so - just say go away or have you signing a NDA if you want the answer to such questions.

Isn't it right that you usually get either the exact measurement data of the part you bought or a collective datasheet with anything, where statistics is used to tell you typical or guaranteed values?

@Edwin: How many revisions are in your recent resistor design compared to the first sold, either forced due to supplier problems or due to new insight ? At least how many digits does this count have ?

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 24, 2014, 07:45:49 pm
Hello Babysitter,

Since we first went into full production back in the mid-1980s, there has been no 'revisions' to the basic specifications of the standard line parts since the design was up and 'running'.  During the design phase, there was some testing of different materials and different manufacturing procedures of course.  The specific materials and procedures, from start to finish, is what produced this resistor which has proven to be a very robust design.  We did end up having to change one material supplier whose pricing became excessive, apparently they didn't want to deal with this kind of product anymore and just priced it skyhigh.  We were able to find another material that worked just like the original so we did not do a revision to the resistor specifications, only the supplier list and material type.  There has been minor revisions to manufacturing procedures when we found an unexpected omission in a procedure.

For instance; an operator was welding 1 Meg resistors which uses a very small wire size (they were expensive back then too), at one point in welding the batch of resistors, the wire was being 'burned' off instead of welded like it should be.  Since the resistors were expensive, the operator tried to 'save' them; there was just enough wire left to make a second weld joint right next to the first joint.  In this manner she finished the batch, 'saving' a number of resistors from being junked.

At this time, we were no longer TCing entire batches but just doing a small sampling for TCR, none of the 'saved' resistors managed to get into the small sample and the finished resistors were sent to the production line.  A few days later, we received the entire batch of resistors back and were told they were out of tolerance on TCR which was 0 +/- 1 PPM/°C.  We immediately ran the entire batch for TCR and found that the 'saved' resistors had a TCR of 3 to 4 PPM/°C.  Mike tore down one of the 'faulty' resistors and the cause was immediately clear.  The 'double' weld had caused a line contact in the weld area instead of a point contact, throwing the TCR off.  The procedure handbook was amended to correct the welding procedure and the problem never appeared again.

I am working on a new design which would require a major revision of the original, which would be the first time but until that happens, no revisions.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 27, 2014, 09:42:58 pm
Hello,

I checked some of the error sources:

For the convex curves of the resistors I checked NTC linearity.

Usually I use my NTC-configuration 33K NTC with 27K pull up in a 10-40 degree range.
I optimized the pull up for linearity in this range (0.2 - 0.3 K maximum nominal deviation).
the equation for the temperature is: 75.95614516 deg C  -0.018517152 * ADC value in mV (5000mV ratiometric end value)

In this setup before I used the FAN I had problems to reach the upper temperature limit.
So I increased the setpoint without checking linearity. With fatal result: up to 1 K error on 46 degree C.
see 20140625_NTC_linearity.PNG

I generated a correction curve to correct the nominal error:
So non linearity from Steinhart + Hart equations is reduced to 0.02 deg.
see 20140625_NTC_linearity_corr.PNG

I recalculated one of my T.C. curves of UPW50 to see the difference:
20140621_TC_UPW50_1K_1_raw_temp.PNG shows the uncorrected diagram
20140621_TC_UPW50_1K_1_raw_temp_corr.PNG is the NTC linearity corrected diagram

The main difference is that the corrected x-axis reaches 1 degree higher temperature and 0.3 degree lower minimum temperature.
so the average tempco after box method decreases by 3.5% from 2.05 ppm/K to 1.98 ppm/K
The temperature gradient at 25 degrees changes from 2.06ppm/K to 2.10ppm/K. (2%)

And: The NTC linearity is not the reason for the non-linear T.C. curve.

corrected LMS curve for UPW50 #1 T.C.

T.C. Curve = A0 + A1 * t + A2 * t * t + A3 * t * t * t   with: t = (Temp - 25 deg)

A 0 =  7.3038543243537982E-0001
A 1 =  2.0971377460001767E+0000
A 2 = -2.2197393589624226E-0002
A 3 = -1.0659158556926091E-0004

max. deviation to regression curve 1.6871989574339004E+0000 ppm

Edit: the story reminds me of another story of Josella Playton:
http://www.josella-simone-playton.com/solarpow.html (http://www.josella-simone-playton.com/solarpow.html)
Sorry only in german.
The quintessence is: never leave proven limits.

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 04, 2014, 07:56:06 pm
Assuming (that almost always gets you into trouble) that all external sources of error have been accurately accounted for (careful how many digits of resolution you're using in the calculations, most of those digits are really useless) that leaves the resistor's sources of error.

A precision wire wound resistor should have a nearly ruler flat TCR 'curve' (that's a misnomer! as it is flat, no curve) throughout its operating range with the possibility of slight deviations from flat at the temperature extremes.  The cause of stresses which account for a non-linear TCR curve comes from multiple sources.  The interaction of the bobbin material with the windings, incorrect choice of wire size for the given resistance, insufficient rubber coating of the windings and quality of the welds to name a few of the possible sources.

A TCR curve which shows hysteresis with temperature changes is a guarantee that there are problems with the resistor's design and construction.  This further leads to more instability with time, it is the mistaken assumption that baking these unstable resistors gets rid of or minimizes the long term drift factors, it doesn't, an incorrectly designed resistor is just naturally more unstable with time than a properly designed and built resistor.

Rhopoint says that they age their resistors for a full week to completely stabilise their resistors before final calibration, what a load of rubbish, if they were "completely stabilised" (no resistor is completely stabilised!) their drift specs would be about five times better and running them for a full week in an oven is a huge waste of time and energy, those resistors should reach a stabilised condition in less than a day if they were built correctly.

Neohm (UPW series) is a bit more tight lipped about their manufacturing process except to extol the RTV coating on the windings but their long term drift is even worse than Rhopoint's, so much for  exceptional resistor claims, pretty much another 'me too' resistor shop.  What little else they mention about their resistors is just average, me too manufacturing.

Just in case our friend Branadic is peering in over my shoulder, yes I am whacking on all these me too resistor operations.  They have not bothered to take the time to ferret out the flaws in their manufacturing and fix them, they have just copied everything they can find out about resistors from their competitors who have the same flaws.  It took a lot of time and effort to design a damn good resistor and from that standpoint, I have earned the right to whack on my inferior competitors with a big stick.

Title: Re: T.C. measurements on precision resistors
Post by: branadic on July 06, 2014, 08:46:30 pm
What are the thoughts on resistors made by Burster?

Precision and High Precision Resistors Series 1140, 1150, 1160, Model 1178 (http://burster.de/en/products/calibration-instruments/c/standard-and-precision-resistors/p/precision-and-high-precision-resistorsseries-1140-1150-1160-model-1178/v/)

We haven't talked about them yet. They say that they can achieve <2ppm/K with selecting materials and Zeranin, but also do some "meticulous artificial aging procedure".

I've been told that in a quantity of 10 a single resistor (1142) is about 23,13€.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 06, 2014, 09:48:28 pm
What are the thoughts on resistors made by Burster?

We haven't talked about them yet. They say that they can achieve <2ppm/K with selecting materials and Zeranin, but also do some "meticulous artificial aging procedure".

I've been told that in a quantity of 10 a single resistor (1142) is about 23,13€.

Hello branadic,

I also thougth of them. Another option could be Powertron.

My personal opinion is: If you can get a matched set of resistors for a voltage divider
with even narrower specs or a special resistor value this could be fine.
For a single resistor with low TC I see no advantage against a Z201 with similar price for a single unit.

On the other side I stumbled over the Riedon T.C. curves:

http://www.riedon.com/media/pdf-tech/TemperatureandTC.pdf (http://www.riedon.com/media/pdf-tech/TemperatureandTC.pdf)

what the hell does the statement "The TC adjustment is per charge at the reference temperature of 25°C (T0)." really mean?
I fear my english is too bad to fully understand what they want to express here.
And what does this mean together with the "picture 2" with the mean value on -1.8ppm/deg?

Ok I know that T.C. curves of resistor manufacturers are never true since they test only at exact 3 temperatures.
25 deg, -55 deg and +125 deg. All other values inbetween are interpolated.

But for me the interesting part is the behaviour around 25 degrees.
I do not want to do precision measurements around -55 deg on north pole or in the sauna at 125 deg C.

By the way I´m still working on the theme but have no spectacular results at the moment.
In the mean time I found a systematical error in my calculations.

But since the (non-linear) error of 0.008 ppm cannot be seen in the curves it´s not worth to mention.

On the other side today I tested a buffer amplifier between multiplexer and ADC.
There is also no significant change against unbuffered mode.
But the buffer will allow me to use much larger resistor values (than the 1K) together with the ADC
without running into trouble with non-linearities.

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: branadic on July 06, 2014, 09:57:25 pm
Quote
Another option could be Powertron.
Powertron - A VPG Brand (http://www.vishaypg.com/powertron/high-precision/)

Do you mean a special series?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 07, 2014, 01:49:47 am
To Branadic,

The Burster resistors you linked to are made out of three versions of Manganin, the two newer versions sports some improved characteristics, mainly TCR,  over the much older Manganin but also retains a lot of the same characteristics.  Given their small size and power rating, I'd wager there is a ceramic bobbin inside as these power ratings with a 'plastic' bobbin would make the resistor run on the hot side which these alloys don't like.

Properly treated and handled, Manganin can make a very good and stable resistor, the main limitations are only lower values ~<100K are available because of the limitations on the size these alloys can be drawn down to.  As I've mentioned elsewhere here, these alloys do not like being manhandled very much so care should be taken to observe the ratings and conditions of use.

Manganin is an old and proven alloy, it is still used in low ohm resistors, particularly in shunts and to some extent low ohm standards, but in most respects Evanohm and its multiple versions have superseded Manganin in standards, both working and primary.  Only the quantum based resistance standard is superior if you have a few hundred thousand dollars laying around for one and don't forget the liquid nitrogen to keep it nice and cool.

I will leave to the buyer whether or not these resistors are worth the cost.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 07, 2014, 02:45:24 am
Hello Andreas,

Can't say I've heard of Powertron either, I will look them up.

As to Riedon, I just came back from a visit with them in May, I have seen the statement you are asking about and frankly, I'm not sure what it means either.

On your point about TCR curves and manufacturers always 'lies' about them....I must pose an objection here, at least in my case.  You are generally correct, the standard procedure is to measure the TCR at the three cardinal points and this can (and in some cases does) result in covering up aberations in the TCR curve.  I won't go as far as to say it is intentional but I'll leave that conclusion up to the reader.

I do know of some resistor manufacturers who do not 'intentionally' hide TCR warts, it is more a case of ignorance of the warts, which one is worse depends on the circumstances.  I would not use a universal stroke of the pen to say "all manufacturers never tell the truth" as it is invariably not true.

When I say my TCR is flat, that is exactly what it means,  it is difficult to obtain such a 'curve' without all of the correct pieces of the puzzle.  Many PWW resistors do have fairly flat TCRs around the 25°C cardinal point, that is relatively easy to do, it is the rest of the temperature range that causes the problems.  The 'room' temperature is what everybody tends to use as the reference and after the so-called stabilization bakes have been done, that particular cardinal point is where things tends to settle down and look good.

While the three cardinal points are the usual ones requested, other points can and are specified, I know of at least one military specification that called for seven points on the TCR range.

Depending on the resistor type, TCR may only be sampled in the beginning to establish maximum and minimum values for TCR, commercial resistors generally are not TCR'd unless requested at additional cost.  The same thing is generally true for industrial grade resistors, military and aerospace are usually the only ones who put TCR measurements into the specification and those must be done all the time.
 
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on July 07, 2014, 05:18:48 am
@Edwin G. Pettis:

The Burster resistors are either made of the standard Manganin, i.e. CuMn12Ni2 and T.C. 10ppm/K, or for their precision types, they use a different alloy, called Zeranin (brand name from Isabellenhuette for this alloy and their wires), i.e. CuMn7Sn2.3, which is selected by Isabellenhuette already to have T.C. <3ppm/K, and a "flat", parabola shaped T.C. curve between 20..60°C.

Obviously, Burster then makes some further selection or treatment, I do not remember that any more correctly, I fear.
I bought many custom specific Zeranin resistors from Burster 25 years ago, for my precision current sources.


Which alloy (with < 1ppm/K ?) do you use for your resistors?
I assume that you also have to buy resistor wires, with a special alloy, but also from wire manufacturers like Isabellenhuette, or whoever delivers such specialties in U.S.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 07, 2014, 01:52:13 pm
I examined the data sheet from Burster last night and all the claims appear to be reasonable with the Manganin alloys.  I could not find any mention of the conditions under which the claimed long term stability were tested.  There should be specifications as to the temperature and power conditions under which the long term stability is tested.  Usually, the only long term stability specification I've seen for any Manganin resistor is at the calibration temperature and low power levels.   It would be of some help if Burster specified its long term stability test conditions.
Title: Re: T.C. measurements on precision resistors
Post by: robrenz on July 07, 2014, 02:03:41 pm
Hello Mr. Pettis,  Thanks for all your excellent input on this thread and others. It seems to me you may have some valid input on the topic of Tellurium Copper.
 
I am of the opinion that Tellurium copper is used purely because of its machinability with a side benefit that it has slightly better corrosion resistance. Since the conductivity of Tellurium copper is only 90% of ETP copper it could be argued the thermal EMFs will be greater in a Tellurium Copper to ETP Copper connection than a ETP Copper to ETP Copper connection.  I think this is indicated by ETP copper being used on low EMF spade connections and Tellurium copper used only when the part has to be machined as opposed to stamped.
 
Some think that there is something special in the electrical properties of Tellurium Copper that make it exceptionally low thermal EMF compared to pure Copper. This is helped by all the high end test gear touting “Tellurium Copper” input connections as a sales buzzword.  Can you debunk this myth or tell me why I am wrong?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 07, 2014, 02:33:26 pm
Dr Frank,

With improvements in alloys come big improvements in price, I can currently pay as much as $33,850 per pound for the smallest wire size, 0.0004" diameter that meets my specifications.  Larger wire sizes, of course, cost less but still are not cheap, a 0.0063" diameter wire costs $239.71 a pound.  Material prices have skyrocketed in the last 5 years.

While Evanohm has been with us since the early 1940s, it has also undergone changes over the years, most have come in the last 25 years with about 5 improved versions, each having its own advantages, including higher costs.  I use three of the alloys depending on which particular specifications are required.  The newest alloy of the group has some terrific characteristics but it also more difficult to handle.  This is the one I use if the customer requires particularly tight TCRs, tight tracking TCRs and long term stability, it also costs the most.

I cannot use cheaper materials or I would not get the same results.  If the best performing resistors are required in a design, it is very foolish to buy cheaper, inferior resistors, that is why my PWW and Vishay's VHP series resistors cost more, performance comes at a cost.  Cheap only gets you trouble.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 07, 2014, 02:51:28 pm
To Robrenz,

Thank you for the compliment.  While I am not 'technically' a machinist, the use of tellurium copper alloy is generally used because copper, being a somewhat soft metal, does not machine nicely and tellurium gives it just enough 'stiffness' to machine smoothly.

Interestingly, Tellurium copper is rarely ever seen without gold plating on all of the binding posts and other terminals for obvious reasons.  Frankly, I agree with you that Tellurium is not used because it has 'special' electrical properties but because of its machining properties, a gold to gold connection would produce little EMF, the Tellurium copper becomes irrelevant.  Manufacturers have taken advantage of this 'myth' to charge rather high prices for items like Tellurium binding posts, sometimes into three figures (>$100)  each.....a pure ripoff.

Tellurium does have its uses but not for the usual reason.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 07, 2014, 06:22:06 pm
Powertron - A VPG Brand (http://www.vishaypg.com/powertron/high-precision/)

Do you mean a special series?

Hello,

I have to admit that it is some years ago since I had a look on their components.
Against formerly you now cannot distinguish the locally produced parts (mostly w.w.) and the vishay global parts.

Perhaps I will find something about the parts in my notes.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 07, 2014, 06:39:59 pm

On your point about TCR curves and manufacturers always 'lies' about them....I must pose an objection here, at least in my case. 


Hello,

Ok you are right it is a standardized procedure. And since all manufacturers do the same stuff there is a certain comparability.
On the other side I cannot understand how they generate the 0..60 degree coefficients out of the 3 cardinal measurement points.
(The 0..60 degree value would be that what comes closer to precision applications).
I have found no document about this up to now.

So they do not "lie" in a direct manner. But I feel that they dont tell you the whole thing.
It´s a bit like the "miles per gallon" on your car from manufacturer (or the exhaust gas values)
which is only true for the standardized test but not valid on the "Autobahn".

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 07, 2014, 06:43:53 pm
I was just looking at the Powertron website, Vishay again and it has the hallmarks of Vishay here and there too, they just can't resist putting their own 'spin' on things even though it probably didn't need it.  It is a relatively 'young' shop, founded in 1990, it may have been a local resistor house, bought out by Vishay or Vishay just founded a new one for the region.

The resistor offerings look similar to most other resistor house lines and the data sheets appear to have the hand of Vishay in them as important details tend to be put in some out of the way place.  For instance, the 100/SM/PC series looks an awful lot like Ultronix parts.  At the head of the data sheet, it plainly claims TCRs of +/-2 PPM/°K, looks impressive.  Further down the sheet on page 2, they finally spell out the 'standard' part specifications and the TCR is +/- 10 PPM/°K, the 2 PPM/°K is a special order.....not quite so impressive.  The entire data sheet would pass for an Ultronix data sheet if the header was changed, although Ultronix used to manufacture a much wider range of wire wound parts before Vishay got their sticky paws on them.  But, since Ultronix was owned by Vishay, they could do whatever they wanted with it.

The rest of the film/foil resistors look just like Vishay commodity parts, I didn't see any thing like a VHP in the lineup.  All in all, I don't see any compelling reason to buy from Powertron over the other shops.

I also found this line in their temperature dependence and TC paper:  "The TC adjustment is per charge at the reference temperature of +25°C (T0)."  I'm still not clear about the usage of the word 'charge' here and elsewhere in the document.  It sounds like they are referring to possibly a change in temperature but the usage of the word does not seem to be consistent, any more so than the Riedon usage.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 07, 2014, 06:55:01 pm
Hello Mr. Pettis

A few years ago I got some 0.04" enamelled manganin wire. It is probably 50 years old, 40 at least. I think building a current shunt would be a good way to use it.

But the problem is how to properly solder manganin wire to copper wire/plate. I thought it would be simple, but after reading this thread I am not convinced at all. All advise very welcome because the limited amount of wire doesn't allow much experimenting.

First, how to clean the wire ends to be soldered. Sand paper? Scraping with a knife?  When I got the wire, I was told that the manganin oxidises very fast and needs to be soldered immediately.

Is it possible to use a standard 60/40 solder? I am a little worried if the flux meant for copper (for example Multicore Crystal 400) works with manganin. The person who gave me the wire used some special flux but doesn't remember the details anymore. That was very long time ago and long before the soldering technology we have today.

Would it be a good idea to heat sink the rest of the wire with metal clamps to limit the annealing to maybe a half inch? Which is only a small part of the total resistance and TC.

Thanks in advance.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 07, 2014, 07:39:47 pm
To Andreas,

You are correct in that the much more limited temperature range of Manganin, et al, does require a different TCR procedure than the other wider range resistors.  The cardinal point still remains 23°C (or 25°C as the case may be) as the reference point.  Because Manganin, et al, has a hyperbolic curve for TCR (the newer alloys, particularly Zeranin, does flatten out the curve some around the cardinal point but it is still primarily a hyperbolic curve), this requires that the TCR be measured at points within its specified operating range.  Generally, the initial TCR curves are measured at 5°C intervals above and below the cardinal point to establish the basic curve characteristics, once that has been established in a repeatable manner, the batches are then only checked at three points, usually these are the points where the TCR curve intersects a specified reference line.  This line may or may not be at a zero TCR point depending on the 'average' TCR curve established for that particular alloy type.

Yes, this does require careful direct measurements (no statistical lying here) since the temperature range is restricted but the TCR measurements should be done carefully in all circumstances to insure accuracy no matter what the measurement range is.  I also agree that in the case of resistors which have a non-linear TCR characteristic, the three cardinal points are very inadequate to convey the necessary information about the curve, in this manner, warts may be hidden from the customer.

Well made resistors will stay close to the established TCR curve(s) within a normal variance, any that deviate outside an established norm should be considered faulty.  Unfortunately, the customer usually does not have such information and must rely on the manufacturer to do it right.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 07, 2014, 10:49:38 pm
To Itz2000,

If your Manganin is 40 or 50 years old, you may have an enamel which comes off pretty easily during soldering, those were fairly low temperature coatings unlike today's high temperature coatings.  The old coating evaporated during normal soldering and did not require a separate operation to remove it before soldering.  Soldering must be accomplished using standard rosin core 60/40 solder and the soldering must be done as quickly as possible as the temperature is well above normal operating temperature range for Manganin.  It certainly wouldn't hurt to use a heatsink such as forceps. 

Manganin does oxidize but you should be able to wet the wire and get it soldered before oxidation takes place, if you do encounter oxidation problems, use a small amount of liquid rosin on the wire before soldering.  Using a fine abrasive or knife edge to remove the coating/oxide does work, just try not to nick the wire with the knife but this should only be necessary on modern coatings, Manganin is fairly soft.

If you are soldering the wire to something bigger, such as a copper plate, large terminal, ect., first prepare the larger material and get its surface soldered, then you can quickly solder coat the Manganin wire, reheat the larger solder joint, insert the wire, remove the soldering tip and cool off the solder joint quickly, this should result in a nice clean joint and minimal heating of the Manganin.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 08, 2014, 11:27:57 am
Thank you very much for your answer and sharing your expertise.

I think I could try removing the oxidation and the enamel with fine sand paper dipped in the liquid rosin ("wet sand paper"). If I understood correctly the rosin protects the wire from oxidising (at least for a short period of time)? Do you think that would work and do you have any specific liquid rosin that you would recommend for manganin wire?


And finally to the original subject of this thread:

Almost 20 years ago I measured the temperature coefficients of 10 or maybe 20 bulk metal foil resistors. If I remember correctly the resistors were the old yellow/orange Sfernice. I still haven't been able to find the documents, but I keep on searching...
Title: Re: T.C. measurements on precision resistors
Post by: branadic on July 08, 2014, 07:23:39 pm
Quote
I think I could try removing the oxidation

We use something that is called "micro acid", a delute acid, to remove only the oxide from copper traces before end finish is processed. Maybe something similar (natrium persulfate) could be used?
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 08, 2014, 08:29:41 pm
Hello,

I have updated page 1 with UPW25#1 data.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: branadic on July 08, 2014, 08:59:14 pm
Interesting, obviously this part is out of spec, even if it is one third of UPW50 in price.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 08, 2014, 09:29:03 pm
Interesting, obviously this part is out of spec, even if it is one third of UPW50 in price.

I don´t think that you can say that.
the T.C. in datasheet is +/-3 ppm/K "typical" and +/-5ppm/K max.
And the measurement conditions are different. 8-45 degrees (continuous)
against -55, +25 and +125 degrees cardinal values.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 09, 2014, 12:58:57 am
To Itz2000,

The liquid flux I've been using is from Radio Shack, #64-021, a 1 oz plastic tube, officially called, "rosin soldering paste flux".  It isn't a paste, it is liquid, I don't know who the dummy was on that one.  I've had this tube for some years, I'm sure that other electronic outlets such as Mouser, Allied and Newark still carries this stuff, should be listed right with the solder.  I usually just dip the tip of the wire into the flux about 0.2" (50.8mm) and then directly solder it to the connection.

I'm sure, if this Manganin is that old, the enamel will just melt off during soldering, if it doesn't, then use some fine sand paper (crocus cloth) on it, it doesn't have to be wet, just a little quick rubbing and it should come right off.

By the way, it was Sfernice that broke Felix Zandman's patents on the film resistors, unfortunately for them.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 10, 2014, 09:46:52 pm
Hello,

updated UPW25 #2 on page 1.
T.C. again around -4.3 ppm/K like UPW25 #1.

There is one remarkable thing:
UPW25 #1 has a rather high resistance value (around +620 ppm against Z201 #1)
UPW25 #2 is at +10 ppm
UPW50 #1 is around -100 ppm from Z201#1

So if the T.C. values of the 2 UPW25 were not so tight together I would have thought of some kind of production error on UPW25#1.

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 10, 2014, 11:04:11 pm
Hi Andreas,

You likely received two resistors from the same production batch, although chances that you would get the same TCR from two resistors is a bit unusual if by chance.  Remember that the claim of 0 ±3 PPM/°C is for 0°C - 85°C temperature range (according to their data sheet) and as I recall you were told that the TCR 'curls' beyond those lower temperature limits to 0 ±5 PPM/°C.  These TCR measurements  of -4.3 PPM/°C within your 8°C to 37°C clearly places the range of your measured TCR outside the stated 0 ±3 PPM/°C limits.  While that TCR is within the 0 ±5 PPM/°C, it isn't within the lower limits, I'd say that makes these resistors defective in either case, but since they hedged their bets and said the 0 ± 3PPM/°C is 'typical', that lets them off the hook by a technicality.

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 11, 2014, 04:14:49 am
You likely received two resistors from the same production batch,

Hello Edwin,

both UPW25 carry the same date code 1244.
So its very likely that they are from the same production batch.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 11, 2014, 04:27:27 pm
Hello Andreas,

After looking at your data for the UPWx resistors, in my opinion the flaws of the resistor design are revealed in the measurements to some degree.  The characteristics are not terribly flawed, at least within your measurement range nor are you subjecting them to any great stress, still the flaws are evident.

I would like to point out a couple of things here, first, they have a rather quaint way of specifying TCR, a 'typical' of 0 ±3 PPM/°C over a 0°C - 85°C embedded within the overall TCR spec of 0 ± 5 PPM/°C, by doing this they lead the customer into thinking that they will get the tighter TCR spec over the limited temperature range, which they very well may get if the definition of typical is adhered to (at least a majority of resistors fall within the spec), however a resistor can actually have a TCR anywhere within the 0 ±5 PPM/°C range and still be 'within spec', a bit tricky.

My second point concerns the tolerance of a resistor, in this case, we're looking at a ± 0.1% limit, technically, a resistor can fall anywhere within this range and still be considered good (note this measurement is considered valid at the time of manufacture only), usually the manufacturer will make use of a 'guard' band, this means they will 'pull in' the actual tolerance limits a little bit to compensate for whatever measurement errors exist on the production floor.  This 'guard' band varies with the equipment in use in production and usually can be as large as ±0.01%, so the manufactured limits may be ±0.09% for instance.  The measuring equipment at final QC should always be the most accurate on the production floor to insure the resistors going out the door are within the specified tolerance (±0.1% in this in instance).

Depending on the manufacturer, the resistors going out the door should be within the required tolerance and can be anywhere within that tolerance, even within the same batch.  Due to inherent variations in the manufacturing processes, whether the resistors are made by an automated machine or by hand calibration.  The distribution of values tend to be a bit different between machine and hand calibration, machine tends to be distributed around whatever the nominal setpoint of the machine is and the distribution tends to be more of a clump (note machine calibration in the case of PWW is only possible with larger wire sizes as the machines tend to be a bit more heavy handed than a human hand) while a human calibration tends to be more distributed within the given tolerance band.

Tight tolerances ?0.01% are always by hand and depending on the operator will have more of a scattered distribution of values within the tolerance band, some operators can be very good at 'hitting' the nominal value quite consistently but these people tend to be fairly rare.

TCR tolerance works somewhat similar to the value distribution, depending on the manufacturing process, of course the TCR should always be within the stated range, but the distribution of TCR varies somewhat with the manufacturing procedures, most manufacturers produce a wider variance within the tolerance band than others do, that is inherent to the process and design of the resistor.

In conclusion, very few manufacturers give any kind of numbers on how many resistors fall within those 'typical' (undefined) specs if they even mention a 'typical' specification at all.  Frankly, I think this is a bit deceptive, if you are going to give a 'typical' spec, then you should clearly state (even approximately) what percentage of resistors tend fall within this range.  Now to my favorite punching bag, Vishay, while these folks often do give a 'typical' spec on their TCRs, there is absolutely no statement as to how many of these resistors actually fall within the 'typical' range given.  According to many engineers I've talked to (direct or indirect), they have found few if any of these resistors even come close to the 'typical' TCR range.  That is not to say that these resistors are not within the stated overall TCR range, they are and that is perfectly fine, they are within the stated TCR spec but few are within the 'typical' spec and I find that misleading.....how many of these resistors are actually within your 'typical' spec?  Vishay should know exactly since they have to perform the measurements on these parts, time to come clean.  Oh, wait, Vishay doesn't intentionally do that.  Just to make sure I'm clear on this point, these VHP, et al, resistors are within the general TCR spec, I'm not saying they aren't, they just don't tend to hit the claimed 'typical' mark very often and they don't say how many do, it appears that their 'typical' is just a calculated statistic.  If Vishay has the actual numbers, they aren't sharing them, why is the question.

I do not accept any kind of statistical manipulation of the data, exact numbers are available which should give a good figure of what 'typical' is by simple averaging of batch results, of course there is going to be some variance to that number, that sort of thing is inherent, nothing is absolute.  Typical does not guarantee that you will receive more than the 'typical' yield in your resistor order, you just might get more than the 'typical' number of resistors within the 'typical' band, you might receive less than the 'typical' too, by nature, typical is an average figure and a good manufacturer tends to produce more consistent results, that means more customers should receive 'typical' more often than not, it just doesn't guarantee that.  No silly statistical manipulation, just simple actual results from actual measurements.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 11, 2014, 09:03:12 pm
Hello Edwin,

if we remain at statistics:

What I as "customer" would expect is that typical means:
At least measured at room temperature one sigma (63%) of the units are within the spec.
(so from 2 measured devices at least one should fit; Ok 2 is not a statistical quantity).

For the "max" specs I would expect that 3 sigma (99.7%) of the parts are within the spec.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 11, 2014, 11:01:47 pm
Hi Andreas,

Granted, what you state is possible, but not always probable, statistics also states that any given random sample does not necessarily conform to the 'norm'.  Your definition may hold true for some samples while others don't fit at all.

Basically, what I am saying is that if you don't use statistics, just using hard measurements, the numerical playing around pretty much goes away.  When I said about 60% of my resistors are within 0 ±1 PPM/°C, the yield stays close to that number all of the time, no matter what batch you might pick, the yield is close to 60% (in sufficient numbers, see below).  Now if I use statistics to come up with that yield, I could possibly come up with a number higher than that, 75% or 80% depending on the batch(es) that were used for sampling.  The only constant truth about statistics is the old joke; there are lies, damn lies and statistics.

I'll be the first one to agree that statistics can be a very useful branch of math, the problem is that it is much too easy for the data to become skewed or just plain nonsense unless the person doing it is very careful.  I've seen many instances of statistics used to skew data to make it look better than it was, sometimes it was unintentional, other times it was intentional.  If you don't have to use statistics, so much the better.

Even without statistics, the math may not work, for instance, if I make ten resistors are six of those resistors going to be within 0 ±1 PPM/°C, odds are, probably not.  They all might be within ±1 PPM or only 1 or 2 or 3 might be, but that doesn't mean the 60% statement is in error.  If I make 100 parts, it likely that ~60% are within ±1 PPM probably or some number close to it or possibly some number not very close to it (although the likely hood is that it will be close to 60% simply because the vast majority of them are because those figures were actually measured, not sampled). 

These preceding figures are not from selected parts, otherwise the specification might be different, if I was selecting parts for a given TCR result, say the 0 ±1 PPM/°C TCR, the 'typical' spec, if one was going to be used, might be something like ~45% is within ±0.75 PPM/°C, ~35% is within ±0.5 and ~20% is within ±0.25PPM/°C.  Statistically, those figures might be true or maybe not, not really much use to the customer.

Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 12, 2014, 10:52:01 am
Almost 20 years ago I measured the temperature coefficients of 10 or maybe 20 bulk metal foil resistors. If I remember correctly the resistors were the old yellow/orange Sfernice. I still haven't been able to find the documents, but I keep on searching...

Found something...

Numbers 1-20 are Sfernice RS92 10kohm 0.1% orange case. No selection made, just ordered 20 pcs the cheapest version the local distributor had in stock. Number 21 is a free sample from Vishay but I don't remember the type. It had the same datasheet specs but the price was higher.

As you can see the temperature sweep turned out to be way too fast. In the second test run the setup was moved away from the disturbing oil bath on/off thermostat, the sweep time was increased from 3 to 24 hours and the resistor pack protected by thin plastic foil was placed in a cup of mineral oil with the thermometer. The hysteresis practically disappeared. Except the Vishay and one Sfernice (which had hysteresis larger than others in the first measurement) still showed some hysteresis. Despite the obvious problems of the first test run the very rough TC slope approximations turned out to be surprisingly correct.  Unfortunately I haven't been able to locate the data of the second test run yet. The temperature cycle in both cases was 25C -> 15C -> 35C -> 20C.

The graph gives an idea of the absolute accurary too. The reference was a Guildline standard resistor in an oil bath. The data was corrected so that 0ppm in the graph represents 10kohm plus the Guildline calibration uncertainty (which was probably less than 5ppm). The resistor chain was fed with a constant current source and the approximate 1V across each resistor was measured with a 3458A.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 12, 2014, 03:39:46 pm
Hello LTZ2000,

thanks for sharing.

so the Sfernice RS92 have roughly from +0.0 ppm/K to +1.5 ppm/K if I see it right.
Whereas the Vishay has around -0.6 ppm/K.

By the way: does anybody know how to get off the nasty thermal
grease from the resistors without destroying/ageing them.
It does not solve in alcohol. And also "Kontakt LR" PCB cleaner does not help.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 12, 2014, 03:51:13 pm
Hi Itz2000,

Interesting graphs, by the way, the RS92 was discontinued by Vishay/Sfernice last year, they said demand was low and production costs were high.  It was interesting to see that there were some drift graphs on the data sheet showing the approximate distribution percentages, that at least gives an idea of some important performance parameters.  The data sheet did not have much in the way of specifics on TCR though, page one gives a list of TCR over specified temperature ranges but does not specifically say that these values are typical or maximum, it is only inferred.  On page three, there is a graph which shows TCR with a typical, minimum and maximum set of lines, note they are not linear. 

The apparent typical TCRS;  + 0.6ppm/°C (0°C to + 25°C, - 0.6ppm/°C (+ 25°C to 60°C), + 2.2ppm/°C (- 55°C to + 20°C), - 1.8ppm/°C (+ 20°C to + 125°C), notice there are curious overlapping of the outer TCR ranges which completely overlaps the ±0.6 PPM/°C ranges.  This in effect says that any resistor that is within +2.2PPM/°C to -1.8 PPM/°C is still within spec, isn't that cute?  Still, despite being rather non-linear, the TCR specs are quite respectable.

The plastic wrap around your resistors in the oil bath is responsible for the long temperature tails, there are no problems directly putting the resistors into the oil bath with their connections, that is a standard procedure when using an oil bath, insulating the resistors causes problems.  If your wires are soldered to the resistor leads, that is an oil tight connection, no problem.  In TCR testing that required the use of an oil bath, we used spring loaded clips to make the connections to the resistors and the entire board was submerged into the oil.  This did require cleaning the spring clips between each use with an approved cleaning agent, a standard electronics grade cleaning spray.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 12, 2014, 04:14:02 pm
Hello Andreas,

I don't know if this is available in the EU but I'm sure you probably have an equivalent:

http://www.miller-stephenson.com/contact_re-nu/ (http://www.miller-stephenson.com/contact_re-nu/)

I have used it in the past to clean up thermal grease, a small soft brush (old toothbrush) works nicely if the grease has become gelled.

I checked, Miller-Stephenson is available in the EU.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 12, 2014, 08:59:41 pm
Hello Edwin,

which one of the 3 solvents do you use from the link?
I guess its the MS-730

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 12, 2014, 10:20:48 pm
Hi Andreas,

Either the MS-570 or the MS-730  should work, I've used both of them.

Cheers,

Edwin
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on July 12, 2014, 10:53:57 pm
Another product that works well is CRC Brake Cleaner.  The stuff for car brakes.  Not sure if there is something like that in Europe. It is not an official safe solvent for electronics, but if all else fails that's what we use in the lab.  In combination with a small brush it is good degreaser and flux remover, and we've never had it ruin a PC board.  You will want to test on a small spot first, and use it outside with common sense.  It might work for you, maybe not.  For us it has worked well in a pinch too loosen up and flush mineral oil and vacuum grease.  Using the official stuff is better but sometimes you have to get work done.

If the parts are wash-safe, usually we follow up with a good soap wash & brush as required,,  rinse in De-I water, run thru dry nitrogen air knife at several angles to get all the corners blown out  and then bake for an hour or two to drive off any remaining moisture.  The boards / resistors will be very close to 100% clean at that point.  You can check with a Gig-ohm meter to look for any remaining board residue, and usually its fine - otherwise it may need more attention.
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on July 12, 2014, 11:35:36 pm
Edwin,

Good to see you here!

I'm not sure if this was in another post, but I loved the piece in EDN - and I'm sure others would enjoy it also if they haven't seen it:

http://edn.com/design/analog/4427151/The-last-half-century--Wirewound-resistors-Part-one (http://edn.com/design/analog/4427151/The-last-half-century--Wirewound-resistors-Part-one)

http://edn.com/design/analog/4427940/The-last-half-century--Wirewound-resistors-Part-two (http://edn.com/design/analog/4427940/The-last-half-century--Wirewound-resistors-Part-two)

I would like to point out again to all the guys building LTZ1000(a) refs and chasing after resistor TC:  At least in my experience and dealing with voltage refs used in a semiconductor process clean-room:  Its not the resistors that cause any measurable drift, at least not in the hundreds I've built in production.  I always used the basic App Note circuit for resistor values, and we used 13k/1k for the heater circuit without issue. We built some with 5ppm WW, 3ppm WW and some with Vishay Foil.  We never used any slots or any special construction, just cover up the circuit and keep the wind away.  We tried some with copper tape wrapped around the resistors and some we potted in Tfe + Foam.  It all worked, but the basic circuit construction guidelines and doing an initial burn-in helped the most.  The things that had absolutely zero effect on the final outcome were expensive resistors, slots, weird copper-fill patterns, putting the board in at crazy angles etc.

The beauty of that circuit is that the resistor change in PPM is attenuated at least 100 times.  The circuits we built with expensive Vishays were performing no better after 10 or 20 years than with good wirewounds - in fact on a cost / benefit ratio the wirewounds are MUCH more cost effective over time.   Even the units that we built with 5ppm WW and even 5ppm PTF56's were running fine after years of service.

I'm not saying the Vishays were bad - but at least in our experience they were never, ever worth the price for use in an LTZ1000a circuit.  I would say that from what I've seen over longer time frames is the Vishay datasheets have proven to be a bit on the optimistic side, especially when it comes to the more esoteric varieties.  Especially the oil-filled magical varieties.

The point being is that if the LTZ1000 ref is drifting, that means the strain in the crystal lattice of the Zener die is still changing and it hasn't settled out yet - and you need to burn it in a few more weeks / months and recheck.  They will settle down.   Some Vrefs take longer than others to settle down and get stable.  Be patient.

If you're looking at resistor stability on a unit that's only a few months old, you want to also look at stability at WW that are decades old.  I have a Dial-A-Vider 4107C that is is a work of art - still in calibration, and it hasn't been opened since I think late 1990's to clean up one switch contact.  Nothing but WW's inside, and good switches.  Very useful even today.

Also, on a production line, nobody cares if the Vref used in the quality control room last a year between calibrations - nobody is going to take a chance on that, nor does anyone need that on a production environment.  Typically the production line test gear is swapped out and goes to recal every 30 or 90 days anyway.  The good voltage standards stay in the temperature-controlled calib lab, and then the small boards with LTZ1000's go out onto the production line, where the temperature might vary a couple degrees at most.

I get it that the hobby Volt-Nut types want to chase down the drift.  Its kind of fun and addictive.  But really, outside of a cal - lab I have never seen a "huge" need for low drift over 365 days.  Usually on the factory floor the standard is to swap in fresh re-cal'ed units much more often. 




Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 13, 2014, 12:57:07 am
Hi MisterDiodes,

Thank you, yes someone posted the History article links in the Ultra Precision Reference LTZ1000 a couple of months back.  Unfortunately, a few people misunderstood the point of the article; how difficult it can be to fix basic design flaws in a resistor design, particularly when they are being masked by other flaws (even admitting that there are flaws!).  The trials and tribulations involved were frustrating but in the end, they were fixed and a new design cleared up the long lasting flaws.  It wasn't about how bad precision wire wound resistors were compared to Vishay's imperfect resistors at all.  Vishay's resistors went through very similar design flaws and fixes over a period of 55 years, despite what some idiots may think, Felix Zandman did not pull a perfect resistor out of his butt 55 years ago.  Truth be told, Felix had a lot of help, a very lot of help, over the years tweaking the Vishay designs, he sure didn't do it all on his own by any length of the imagination.  Yes, Vishay does make some really good resistors, I'm not saying that (again) but Vishay has always buttered the specifications over the years and they haven't stopped yet.

By the way, EDN also published an article (that's a laugh) after mine from Vishay on their resistor 'history', if you'd like to read that I can post the link here as well.  Be forewarned though, the published article was somewhat modified from the original before it was published.  The original was put together by a Wall Street PR firm and was freely handed out among employees and at shows.  Frankly it was full of cock and bull hot air and very little fact, it was exceptionally thin on the history of how the Vishay resistors came to be and the long trek of re-engineering them (time and again) to the point that they are at today.  I know, I was around for most of it; I can post a copy of the original for comparison if anyone would like to see it.  It is good for a laugh or two and is also as effective as ipecac syrup.

Welcome aboard, MisterDiodes, the postings are often informative and sometimes controversial and sometimes just nonsense like other forums, enjoy.  I haven't been here all that long myself, having been pointed here by somebody that posts here.
Title: Re: T.C. measurements on precision resistors
Post by: Galaxyrise on July 13, 2014, 01:53:36 am
The beauty of that circuit is that the resistor change in PPM is attenuated at least 100 times.
For the ~7V portion, yes.  But if you're looking for 10V, there's still a problem to solve that isn't answered in the datasheet.
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on July 13, 2014, 02:37:13 am
A precision boost from 7V to 10V?  There are several proven, precision, virtually "no-drift" ways around that one.  I can't show you, because the design I use is still owned by my customers.

Someday over a beer...




Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on July 13, 2014, 03:04:57 am

....
By the way, EDN also published an article (that's a laugh) after mine from Vishay on their resistor 'history', if you'd like to read that I can post the link here as well.  Be forewarned though, the published article was somewhat modified from the original before it was published.  The original was put together by a Wall Street PR firm and was freely handed out among employees and at shows.  Frankly it was full of cock and bull hot air and very little fact, it was exceptionally thin on the history of how the Vishay resistors came to be and the long trek of re-engineering them (time and again) to the point that they are at today.  I know, I was around for most of it; I can post a copy of the original for comparison if anyone would like to see it.  It is good for a laugh or two and is also as effective as ipecac syrup.
...

I am quite familiar with the follow-up "article" which was more a thinly disguised Vishay ad. I used it to light that evening's fire in the wood stove.  It has no place here, it was mostly smoke & oakum.

As far as I can tell, the more expensive the Vishay resistor is, and the more "magic" there is inside, the more goofy tricks are on the datasheet.  Again, I'm not saying they are bad - its just I've never seen a need for it that couldn't be solved with a better, more robust design that doesn't need such a high precision resistor.  The datasheets are (diplomatically) just plain optimistic.

What Vishay (/ or Bournes) does make that is fairly useful is a good 20ppm low-noise trimmer pot at say 10ohms - if used properly with a 3ppm Wirewound to trim out the last few ohms of a 1k /  3ppm WW for instance, sometimes you don't have to buy one of the custom laser-trimmed foil units they sell (Look at the stability of one of those laser trimmed units after 4~5 years - or after a small pulse surge - and tell me what happened).  You will still have a cost-effective, fairly low-noise 3ppm Tempco resistance even when an "old school" - but good - pot is attached.

Sometimes a pot isn't wanted or seems old-fashioned (or can be replaced with a DAC if you're careful), but on occasion they are just the ticket.  The good benchtop test voltage refs / dividers were made this way, and in general they work fine after decades - at least mine do.  Unlike a lot of "modern" equipment.  Was it a perfect design?  No, but it was very clever, and I always admired those designers.  Some of the old stuff just keeps on working long after the new stuff is tossed out.

A bad design using bad components will still be bad no matter what you do.



Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 13, 2014, 03:32:54 am
MisterDiodes,

Oh you didn't see anything with that published article, you really should read what the original pap was!
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 13, 2014, 08:38:57 pm
Hello,

I updated first values for S102JT on page 1 with surprising results.

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303)

Seems to be similar to the RS92 values: low T.C. but high hysteresis.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 14, 2014, 01:13:06 pm
low T.C. but high hysteresis.

Even if the thermal resistance from the metal block to the resistor housing is low,  it still can be quite high from the housing to the chip. Seems possible that most of the hysteresis comes from the long thermal time constant and not the resistor itself. In the second test run the hysteresis practically disappeared when I increased the sweep time from 3 to 24 hours. You can estimate the time constant by introducing a temperature step and monitoring the resistance value until it stabilises.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 14, 2014, 01:59:16 pm
there are no problems directly putting the resistors into the oil bath with their connections, that is a standard procedure when using an oil bath, insulating the resistors causes problems.

I decided to use the kitchen plastic foil because I was worried that the epoxy housing could absorb oil. For example some integrated circuits are quite sensitive to humidity due to the mechanical expansion of the epoxy housing. It seemed logical that the metal foil originally designed for straing gauges would suffer the same problem.

I actually sacrified one of the resistors and tested my assumption. First measured it 24 hours in air just above the oil bath surface (very stable temperature) and after that another 24 hours in the oil. The first 24 hours were more or less a straight line, but when in oil I noticed a few stepwise changes.

The process took some time to start, so it is probably safe to dip the resistors in oil for a short period of time. Unfortunately I had other things to do and not enough time to find out if the resistor would eventually find balance in the oil. Probably, when the housing has absorbed all the oil it can.

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 14, 2014, 03:15:06 pm
Hello Itz2000,

Yes, IC manufacturers found out a long time ago that water did indeed wick up the leads past the mold compound into the IC and caused problems.  The main problem was with the aluminum wires used to connect the outside pins to the IC, aluminum corroded fairly quickly and that was the end of the connections.  There was two solutions, keep it powered up and the heat kept the aluminum dry or use much more expensive methods to seal the insides from the outsides.  Over the years the molded types were improved to the point where wicking was considerably reduced (but not eliminated).

You are also correct in that early strain gauge / 'precision' film/foil resistors also had some sensitivity to water wicking.  Precision wire wounds (PWW) also had some sensitivity to water wicking, mainly because of the non-welded crimped joints between the wire and the lead terminations, this was not a severe problem however, it was more of a gremlin that caused short-term variations in the resistor's resistance (looked like TCR) when the part had been unpowered for some time and then powered up.  The effect was small enough that parts routinely passed the military humidity tests.

Generally, precision film/foil resistors are not particularly affected by water wicking as the interconnections between the leads and internal resistor are welded and the materials used are also not particularly sensitive to water molecules.  That being said, the lower cost types are not as well protected against wicking as the more expensive types.  If water does manage to get inside film/foil types, they are rather sensitive to corrosion as everything is very thin.

In PWW, their main weakness has always been the lack of welded terminations, the enameled wire is coated the entire length and unless there are tiny pin holes (they do occur) the water has no way of getting at the wire alloy, water in the crimped joint was and still is a weak point but it tends to be a long term exposure weakness as the terminations usually are not particularly sensitive to corrosion, somewhat similar to the better film/foil types.

Welded terminations eliminate, for all intents and purposes, the problem of water wicking, what tiny amounts of water may be absorbed has an insignificant effect on the resistance, I've never seen any measurements higher than sub-PPM in mine.

Oil, a very good grade of heavy mineral oil, is what is used in oil baths and hermetically sealed resistors (yup, those magical Vishay hermetics are full of oil).  It should not have any effect on the resistors, if it does, that resistor is bad.  One note here, in oil baths, the oil is changed periodically, it gradually becomes contaminated from the air, resistors and anything else it is in contact with.  In hermetically sealed resistors, as long as things are kept clean when making them, there is no long term problems with the oil.  So you should not see any changes in your resistors by putting them in the oil, the epoxies used for resistors should not absorb the oil.

By the way, oil does vaporize, just like water, over time, particularly when in a stirred oil bath and even more so if it is heated so the oil can get into everything around the bath.  I have a story about that I'll have to relate sometime, caused a lot of problems on a military contract at Ultronix.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 14, 2014, 07:17:14 pm

Even if the thermal resistance from the metal block to the resistor housing is low,  it still can be quite high from the housing to the chip. Seems possible that most of the hysteresis comes from the long thermal time constant and not the resistor itself.

Hello,

if I look closer at the first picture then I see that it cannot be a thermal time constant.
For a thermal time constant the restistor change would lag all the time behind the temperature.
In the diagram the resistor change leads a long time (minute 160 - 300) and then reverses direction and then lags from minute 500 to 800.

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303)

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=101983;image (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=101983;image)

For me no thermal insulator can achieve such long thermal isolation (cycle time is ~11 hours).
So this effect would never be a thermal time constant. -> this effect has mechanical root cause.

The bad thing with such effects is that you never know from which point you are starting off. So the resistance value is unknown by a value of +/- 5-6 ppm even if you stay within the temperature range.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 15, 2014, 09:39:07 am
You are correct.

Increasing the sweep time from 3 to 24 hours remarkably reduced the hysteresis of the RS92 resistors, but didn't much affect the Vishay. The speed of the temperature change seems to be important. Interesting...

Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 15, 2014, 10:37:50 am
I took a few pictures of the internals of an ESI SR1 10kohm standard resistor. This specific unit has measured TC less than 1 ppm/C in room temperature and predictable drift approximately +0.3 ppm/year. Evanohm wire on mica card, but it looks like the wire was not welded but soldered...
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 15, 2014, 02:27:42 pm
Hi Itz2000,

No , it isn't Evanohm, I reiterate, Evanohm cannot be soldered, it must be welded.  That looks like 0.001" diameter Manganin, 290 ohms/foot, so there is approximately 34.483 feet of wire on there.  An equivalent Evanohm size wire would only need approximately 12.5 feet.  Any of the ESI series standards with resistors like the one pictured here is made with Manganin.

By the way, Manganin requires years to achieve 'predictable' aging, even with the newer versions such as Zeranin, it does not 'completely stabilize for at least 2 or 3 years under careful use.  Evanohm does tend to stabilize quicker and remain very stable for many years, primary standards have been made with Evanohm for some years now and have replaced nearly all of the Manganin standards in primary labs.

Cheers,

Edwin
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 15, 2014, 04:07:00 pm
The ESI SR1 datasheet from the 90s says "modern alloy with excellent stability, an extremely low temperature coefficient over a wide range of temperatures, and very low thermal EMF to copper".

The same decription can still be found on the IET Labs website:
http://www.ietlabs.com/esi-sr1-calibration-resistor.html (http://www.ietlabs.com/esi-sr1-calibration-resistor.html)

Manganin ... modern alloy? Well, at least ESI had a good adman.

By the way, the exact same type resistors were used in the ESI 242D bridge (240C, RS825D).
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 15, 2014, 05:25:45 pm
Hi Itz2000,

The 'modern' alloy IET refers to would be Zeranin, it is a slightly updated version of Manganin, the main difference is that Zeranin has a slightly 'flatter' curve around the reference temperature range than Manganin does.  There was some small improvement in long term drift and arguably TCR but the main Manganin characteristics are still there.  The TCR curve is still hyperbolic and that cannot be changed significantly.

Yes, the 242D system has always used General Radio style Manganin resistors, ESI used them and now IET uses them, they were originally developed by GR many years ago and the general design of these resistors has changed very little over the decades.  As I said, Evanohm has replaced Manganin (and Zeranin for that matter) as primary standards around the world.  By the way, the specified TCR of those ESI/IET resistors are 0±3 PPM/°C for >100 ohms in the RS925D (earlier versions were 0±5 PPM/°C), basically my standard line resistor specification as far as TCR is concerned.  In the SR1010 series of working standards, the TCR is 0±5 PPM/°C, TCR varies with value and grade of standard but the older ones were 5 PPM/°C or higher.

In the ESI 242D system, most of the resistor decades were trimmable in the 240C and RS925D to high accuracy but according to the manual the system only held 0±20 PPM/year (usually better than that).  Manganin (and its relatives) require kid glove care to maintain their values, TCR and long term drift.  They do not tolerate mishandling very well at all, there are many Manganin standards selling on eBay which are out of tolerance and drifting badly because they were mishandled.  Unfortunately it is easy to cause permanent changes in these alloys by mishandling them.  I've worked with ESI 242Ds since 1973, I am very familiar with their characteristics, probably more so than many of the people at IET.  Very good resistors indeed, but there are better ones available now.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 15, 2014, 06:19:08 pm
Hi Ken,

It would be possible to wind the finer gauge wires on these mica cards but you probably could not terminate the wire to the terminals on either side of the card, they are too soft and since they were made compatible to Manganin for soldering I'm sure they would not weld to Evanohm.  Since Evanohm is a much 'harder' alloy, it would not bend around the edge of the mica cards very well particularly with large gauges, probably <0.0063" diameter at best and that might be pushing it a bit.  Otherwise there is a little bit better heat dissipation given the open winding surface but I'd say the difficulty of winding on mica and the slow winding speed would likely negate any advantage over a bobbin style.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 15, 2014, 07:22:43 pm
Is it possible to wind Evanohm wire on an old-school mica card?  Would there be any advantages or disadvantages to this?

I would vote for advantages, because that is how the ESI SR104 is made. The thermal expansion coefficient of thin mica card is low and close enough to that of Evanohm. The final unit has several resistors of opposite TC cancelling out each other.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 15, 2014, 07:55:04 pm
Hello Itz2000,

The SR104 and its two brothers, the 1k and 100 ohm versions are very special cases, they cannot be made in production numbers by any stretch of the imagination.  They are slow to make and go through many cycles of processing, it is far more than winding a wire on a mica card (which, by the way, has absolutely nothing to do with matching thermal expansions, Evanohm is far stronger in tensile strength than mica, mica can't 'stretch' Evanohm if it tried) for long term stability and low TCR.  The SR-1010 and SR1 series are of similar make, wire on a mica card, but the manufacturing processes are not the same.  These resistors are not production line resistors, they cannot be made in large quantities, not even moderate quantities.  If you have ever checked out the quotes for these resistors, they make Vishay's hermetic resistors look like a bargain. 

I can easily beat the specifications of the SR1010 and SR1 series resistors and with 'special' processing, I can come pretty darn close to primary standard specifications too.  Mica card build resistors are obsolete now in most cases, they have been surpassed by newer technology.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 15, 2014, 09:29:47 pm
You are correct.

Increasing the sweep time from 3 to 24 hours remarkably reduced the hysteresis of the RS92 resistors, but didn't much affect the Vishay. The speed of the temperature change seems to be important. Interesting...

Hello,

not totally. Some effect is also different with respect of perspective or scaling of the diagrams:

The "leading" part only looks leading when regarding unzoomed.
When you zoom in within the linear part (and rescale) it will look like 2 lines in parallel.

I made a set point jump measurement yesterday within the linear part of the rising T.C. edge.
(From around 21 deg C to 30 deg C).
The front edge is the NTC on the heat spreader of the heater.
The dotted line the resistance of S102#1
The following line the NTC near the S102.

The T.C. 6ppm resistor change for the 9 deg C temp rise is around 0.75 ppm/deg C for fast temperature changes.

I see nearly no thermal time constant at the rising edge within thermal block.
There seems to be a much slower time constant after the top edge.
The resistor value seems to fall sligthly whereas the temperature within thermal block still is rising.
I guess this time constant is some kind of plastic / bond creeping which needs time.
Would be interesting what the difference between S102 and Z201 is regarding the construction.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 15, 2014, 09:38:32 pm
Do you have any photos of the insides of an SR-104 that you are allowed to post?  Also-- how many resistors are in an SR-104?  Are they in series, or parallel, or series/parallel?  I always wondered exactly how they made such a good resistor...

They are slow to make and go through many cycles of processing, it is far more than winding a wire on a mica card (which, by the way, has absolutely nothing to do with matching thermal expansions, Evanohm is far stronger in tensile strength than mica, mica can't 'stretch' Evanohm if it tried) for long term stability and low TCR.

I have never seen the internals of an SR104 and I am not an expert of this subject in any way. Long time ago I was in a seminar where an ESI engineer (or a university researcher involded in the project?) told us about the rocky road of development of these resistors. I remember seeing a diagram with four resistors in series-parallel configuration but that could have been just an example simple enough for mathematically representing the idea of the opposite temperature coefficients. The key of the whole manufacturing process was the special temperature treatment which was used to "null" the TC of the individual resistors. Like Edwin said it was not easy to control and large percent of the resistors were not usable. But if I understood correctly, it was possible to do the treatment again and again until the result was good enough. I got an impression that the thermal expansion properties of the mica card were very important and made possible that the SR104 withstands large temperature variations that would ruin all other same level standards.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 15, 2014, 10:56:48 pm
Hi Itz2000,

This is a little speculation on my part plus some theory concerning the innards of a SR-104.  While it is possible that they are using four resistors in series/parallel configuration, I think that may be more trouble than it is worth to build and match.  Theoretically, since Vishay has used it in at least one of their wiz-bang resistor types, it is possible to use two resistors in series (in this case, two 5K resistors) with matched opposing TCRs to 'cancel' out to near zero.  Out of a given batch, say a number larger than 50 resistors, there is a fair probability of finding a reasonable number of equal but opposite TCRs to build near zero TCR sets out of.  At least with Evanohm, there are no hyperbolic TCR curves to mess with.

This may or may not be how the SR-104 is built, I've never seen the insides of one either and because they are very expensive I doubt that anyone outside of the production line has seen the insides of an SR-104.  The most recent descriptions I've read of the SR-104 resistor sounds more like there is only one 10K resistor inside there as the primary, the other 10K resistor is strictly for internal temperature measurement.  It is certainly not out of the question that there is only one resistor in there instead of two.

Fortunately, since an SR-104 deals with very small temperature excursions, this makes the matching game a bit easier and if the TCRs are reasonably linear around the cardinal point, you get a very nice, near zero TCR as a result.  That is the relatively easy part of it, the other part is the conditioning of the alloy/bobbin (mica in this case) to play nice with each other.  While it is possible to get wire with zero (or near zero) TCR, when it is wound onto a bobbin (whatever the kind) things do change and only if the remaining processing does the right things to the wire/bobbin assembly does the resistors turn out with the desired end TCR.  The variables involved are many and most of them cannot be controlled real tightly so there are always variations in the end product no matter how good the total processing is.  Just about every process done to a resistor, right up until the final calibration, imparts stress or removes stress, the end product, hopefully, has had most, if not all, of the stresses removed or compensated for.

Yes, a metallurgist told me that alloys can generally be recycled as long as they are not contaminated so wire that misses the TCR mark can be cooled down and put back through the heat treatment process again to try and hit the mark.  The tighter the 'mark', the harder it is to hit.

That impression about the mica card making it possible to withstand large temperature variations was more smoke and mirrors, perhaps in their minds mica did that but I don't use mica and my resistors withstand even wider temperature swings which would likely wreck havoc on even an Evanohm standard, but yes, an Evanohm standard will certainly take more abuse than a Manganin standard with much less damage to it.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 16, 2014, 08:44:43 pm
Hello,

added results for S102JT#2 on page 1:

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303)

similar hysteresis +/- 6.3 ppm like #1 (+/- 5.5 ppm) but additional T.C. of +0.55 ppm/K overlayed.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 17, 2014, 08:03:46 am
similar hysteresis

It would be interesting to test the audiophoolery VAR.

http://www.vishaypg.com/docs/63140/var.pdf (http://www.vishaypg.com/docs/63140/var.pdf)

According to some sources it is just a naked Z201 (probably lacquered). That could give some idea how much the mechanical connection from the rigid housing to the chip affects the hysteresis, compared to a "free" chip and wire connections.

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 17, 2014, 03:02:31 pm
Hello Itz2000,

An interesting data sheet from Vishay, the supposed photo of the VAR is too small to reveal very much detail.  I doubt very much that it would be coated in something like lacquer (just as an example) but if it is coated with something, it would have to impart no significant strain to the resistor element and yet be able to endure the total temperature range.  I think that a detailed photo of the real finished VAR product would possibly be more revealing than what is shown in the data sheet.  Vishay still likes to play with its customers in the dark.

The data sheet has a fair amount of true, partially true and downright BS information in it.  Obviously it attempts to appeal to the mostly less informed crowd in the audio sector with all the non-sense piled on for audio's benefit (if you can call that a benefit).  Here again, all Vishay needed to do was tell the facts and that should be enough but no, the data sheet is drowning in buttered BS.

You are right, Itz2000, with the Z201 outside of its hermetic can, it does lay the resistor element open to external stresses that it is normally protected from.  The only question is how good is the coating in protecting the element?  It definitely is fragile being out in the open, almost any accidental contact with it is going to likely cause damage.  I wonder how much these are going for?  Perhaps Digi Key lists them.

http://media.digikey.com/Photos/Vishay%20Foil%20Resistors/Y0706.jpg (http://media.digikey.com/Photos/Vishay%20Foil%20Resistors/Y0706.jpg)

The VAR should respond pretty quickly to temperature so the temperature tails should be short, noticeably shorter than the hermetic version but these resistors will have to be physically protected  or else these could be some expensive accidents.

Nice, the actual photo of an VAR is the back side which doesn't tell much at all.  My guess is that Vishay is coating the VAR with polyester-imide rated at 180°C.  The actual connections to the element appears to be something else, almost looks like the old black dopant we used to put on high voltage connections but I know it isn't.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 17, 2014, 03:36:18 pm
I wonder how much these are going for?  Perhaps Digi Key lists them.

Digikey:

100 ohm
http://www.digikey.com/product-search/en?mpart=Y0706100R000T9L&vendor=804 (http://www.digikey.com/product-search/en?mpart=Y0706100R000T9L&vendor=804)

1 kohm
http://www.digikey.com/product-search/en?mpart=Y07061K00000T9L&vendor=804 (http://www.digikey.com/product-search/en?mpart=Y07061K00000T9L&vendor=804)

10 kohm
http://www.digikey.com/product-search/en?mpart=Y070610K0000T9L&vendor=804 (http://www.digikey.com/product-search/en?mpart=Y070610K0000T9L&vendor=804)

Mouser:

71-VART100R000TB (100 ohm)
71-VART1K00000TB (1 kohm)
71-VART10K0000TB (10 kohm)

EDIT: Photo added.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 17, 2014, 04:40:10 pm
Hi Itz2000,

Nice photo, where did you dig it up at?  There are a lot of trim points in the element, these resistors are always made at a lower resistance and trimmed 'up' in value, the photo does show some refinement in the trimming process over the years.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 17, 2014, 06:52:12 pm
similar hysteresis

It would be interesting to test the audiophoolery VAR.

http://www.vishaypg.com/docs/63140/var.pdf (http://www.vishaypg.com/docs/63140/var.pdf)

According to some sources it is just a naked Z201 (probably lacquered). That could give some idea how much the mechanical connection from the rigid housing to the chip affects the hysteresis, compared to a "free" chip and wire connections.

Hello,

feel free to contribute your measurements. :-)

I on my part do not think they are really devices with "long term stability".
If they are laquered: where is the benefit for audio? This will give a bad dampening factor and "nonlinearities".
The PCB (pin-stabilizer) between the pins (according to datasheet) will swell when humidity is rising.
This will also give some tension to the chip.
So I personally think that a hermetically device will be the better choice.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: acbern on July 18, 2014, 06:29:56 am
real long term stability (sub ppm pa) like the sr104 can only be achieved be a fully hermetic housing. conformal coating like it is used on mil and space hardware helps avoid humidity/condensation issues by passivation but does not by any means subsititute a hermetic housing. (so btw: when we are talking about high precision resistors here, to me the aging drift issue is more important than the temp. coefficient which can be controlled by measuring the temperature whenever the resistor is used, you cannot do this with the aging factor, you have to rely on the unit being stable by design and verify from time to time. what help is a <1ppm/K tempco resistor if the annual aging is 10ppm and beyond).

the only way to get hermeticity is by metal/glass/ceramic.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 18, 2014, 12:53:00 pm
real long term stability (sub ppm pa) like the sr104 can only be achieved be a fully hermetic housing.

Sorry, but I have to disagree. It depends on the technology and contruction, if the hermetic sealing is necessary or not. It helps only if remarkably reduces some of the main drift mechanisms.

For example the 1 ohm standard designed by the Australian National Measurement Laboratory, a sort of  "modern Thomas", can be as stable as 0.01 ppm/year. The construction is open, basically just a perforated metal can.

Another example could be the classic 1 ohm Thomas. A common problem with the very early units is that the hermetic seal starts leaking. That increases the pressure coefficient but the long term drift is necessarily not affected.

And don't forget my ESI SR1 mentioned earlier. It has predictable drift of approximately +0.3 ppm/year which is quite good even compared to the SR104 (0.1ppm typical and 0.5 ppm/year specified). A bare mica card in a bended aluminium box which not even dust proof.

I have never used the Evanohm resistors manufactured by Edwin. But if carefully selected I wouldn't be surprised to find sub-ppm units.

Title: Re: T.C. measurements on precision resistors
Post by: Conrad Hoffman on July 18, 2014, 08:57:18 pm
With the talk of what's inside the ESI boxes, I have to wonder what's inside the high end Fluke calibrators? I would assume very few inherently stable standards, with everything else being compared and compensated in software. Is that how it's done, or do they use banks of near-perfect resistors for both resistance and dividers?
Title: Re: T.C. measurements on precision resistors
Post by: acbern on July 18, 2014, 10:03:47 pm
real long term stability (sub ppm pa) like the sr104 can only be achieved be a fully hermetic housing.
Sorry, but I have to disagree. It depends on the technology and contruction, if the hermetic sealing is necessary or not. It helps only if remarkably reduces some of the main drift mechanisms.

For example the 1 ohm standard designed by the Australian National Measurement Laboratory, a sort of  "modern Thomas", can be as stable as 0.01 ppm/year. The construction is open, basically just a perforated metal can.

Another example could be the classic 1 ohm Thomas. A common problem with the very early units is that the hermetic seal starts leaking. That increases the pressure coefficient but the long term drift is necessarily not affected.

the fact that there are resitors out there that were measured stable and re not hermetic (the NML is operated in oil and thats the way to keep humidity away, this is in no way comparable with a resin encapsulated resistor) does not mean that by design they are stable. you wil also find ref01 references that are much more stabe that the data sheet. that does not mean anything. otherwise they would have been specified that way.

for critical resistors earlier fluke units used oil filled resistors, as far as I know current fluke units use hermetic metal can resistors. the 34401 network e.g. is ceramic.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 19, 2014, 12:58:46 am
Hello Acbern,

The reasons for hermetically sealing resistors are not necessarily the ones assumed.  In the case of Manganin (et al), it is very sensitive to oxide (rusting if you please), barometric pressure and also water, i.e. humidity.  In the case of a primary standard, Manganin must be sealed or its stability will significantly degrade over time, its nominal value will change as well.  In the case of a working standard where the absolute stability requirements are not so severe, enameled wire will keep humidity problems to a minimum, barometric pressure is not such an issue and keeps oxide at bay.

Evanohm (et al) is not sensitive to humidity like Manganin, is not sensitive to barometric pressure and oxide is not a significant issue if built correctly, even bare.  Evanohm has none of the sensitivities that Manganin has.  Therefore, Evanohm need only be coated in enamel and the job is done for a standard.

The main purpose of an oil bath is not to keep humidity out of the resistor but to keep the resistor at a constant temperature, it has no other function.

All primary standards require utmost processing of the wire (even multiple times through the heat treating process) to achieve the best possible stability and even then, not all of a given batch ends up as standards, the wire may be sent back for another trip through processing again.

My resistors do not need to be hermetically sealed to achieve superior stability compared to other non-hermetic resistors and can compare favorably even to hermetics under similar conditions.  My main claim to fame is that my resistors can take more of a thermal shock licking than anybody else's with much less change in resistance or long term drift under such conditions.  I do not claim that my resistors are primary standard grade, that process is too complex and demands equipment that few can afford, not to mention developing the process in the first place.  My resistors do exceed working (transfer) standard specs under military working conditions.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 19, 2014, 11:22:36 am
for critical resistors earlier fluke units used oil filled resistors, as far as I know current fluke units use hermetic metal can resistors. the 34401 network e.g. is ceramic.

Reminds me of the Datron thermal slots i.e. replicating things made by some "high authority" without thinking why. They make the best instruments you can buy, so they can't be wrong. And they aren't. If it works well and doesn't cost too much, don't change. If you own a hermetic resistor production line or stock them in large quantity, it can be economically wise to use them in all products even if not techically required. Or just in case, if the hermetic adds only a tiny fraction of the total manufacturing costs. It can even save money if you get rid of expensive and time consuming environmental testing.

Hermetic sealing obviously improves stability, but it depends on the resistor techology and construction used, how much. And if the improvement is meaningful at all in the big picture. With bulk metal foil, based on the Vishay stability figures, it seems to be important. But not with wire wound, if designed properly. But if not, hermetic sealing can help to hide the design flaws or material limitations.

the NML is operated in oil and thats the way to keep humidity away, this is in no way comparable with a resin encapsulated resistor

Oil doesn't stop humidity or oxygen. Unfortunately.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 19, 2014, 11:28:00 am
An SR-104 that is more than a few years old will only drift [typically] about -0.07ppm/yr.  This is too small of a drift to measure without some very sophisticated gear [probably, a QHR and CCC].  The drift value in the data sheet only applies to the first few years [typically the first year].A *very* old SR-104 that has an intact hermetic enclosure can drift even less than the -0.07ppm/year.

Please note that the numbers you can find in literature were determined before 1990 against the US legal ohm which also drifted to the same direction and almost the same annual rate.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 19, 2014, 02:06:09 pm
Historically, there were three reasons for hermetic sealing resistors.

1. They were going to be used in harsh environments which the resistor itself could not easily tolerate, mainly military or aerospace and resistor technology had no other solution at the time.

2. They were going to be used as primary standards and while the environment was 'friendly', the inherent resistor characteristics were not good enough to meet long term stability without hermetic sealing and resistor technology had no other solutions at the time.  Because hermetic sealing was in use for decades and was a 'proven' technology, its use remained mostly unchallenged even today.  It is also very expensive having NIST qualify a new resistor as a primary standard, it takes years and truck loads of cash, another reason to not buck the system.

3. Hermetic sealing was found to be a very effective method of covering up resistor design flaws, keeping all that unfriendly environment at bay made the resistor look better than it actually was.  This technique is still in use today and while it is not cheap for the customer, it is an excellent selling point.

Hermetic sealing is not cheap, never has been and it is labor intensive to boot.  Because hermetics have had such an illustrious and long history, its unfettered use today is mostly unchallenged and unquestioned.  The one really serious weak point of a hermetic seal is the seal itself, unless carefully tested it can and will leak, after all, it is nothing more than a soldered joint and some of them are quite large.

Not all high quality resistor standards are hermetically sealed, many are not in oil but in air and perform very well.  Primaries still tend to be in oil for all of the historical reasons and perhaps because the hermetic primary standard has not been challenged, the old adage, "if it works, don't fix it" is a favorite of the resistor industry even when it is broken and can be fixed, they still don't want to change things.

Getting the resistor industry to change the way they do things is more difficult than going to the Moon (and even that is still questioned.....see!).
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 19, 2014, 02:35:56 pm
To Conrad Hoffman (reply #148),

The resistors inside the Fluke 5420A / 30A and 50A units are just good quality working standard grade resistors, no hermetics and the resistor specs are not all that special, either TCR or tolerance.  When these units are calibrated by the cal lab, the actual measured values are stored in non-volatile memory, it is these values that are displayed on the unit.  They are reasonably stable resistors but again, they do not have to be exceptional.  If best accuracy is needed, a cal cycle is done just before use, otherwise, it is roughly a working grade standard.
Title: Re: T.C. measurements on precision resistors
Post by: quarks on July 19, 2014, 03:56:35 pm
With the talk of what's inside the ESI boxes, I have to wonder what's inside the high end Fluke calibrators? I would assume very few inherently stable standards, with everything else being compared and compensated in software. Is that how it's done, or do they use banks of near-perfect resistors for both resistance and dividers?

Hello Conrad,

unfortunately I do not have a Fluke Multifunction Calibrator, but from my Wavetek calibrators I know, hermetically sealed Vishay Z-Foils (mainly 0.005% and 1ppm/K TCR) and Caddock (0.1% 15ppm/K TCR for 100MOhm) are used as internal references.  Additionally some trimming resistors are there in every range.

My guess is, Fluke is very similar, but maybe with their own Fluke Resistors.

bye
quarks
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on July 19, 2014, 03:59:52 pm
If we throw out the data on the SRX which has nothing to do with the SR-104, and throw out John Lion's self-calibrated value, then we are left with an average of +0.07ppm/year drift.

Yes, I noticed the sign mistake, non stressed Evanohm drifts upwards. But that was not my point.

First, the drift of the US legal ohm prior to 1990 makes the simple calculated drift look much better than the actual drift.

Second, the calibration uncertainties are not low enough for determining the drift of an SR104 from just a few data points. Not even in the high level labs and especially not in the 70s or early 80s.

Title: Re: T.C. measurements on precision resistors
Post by: Conrad Hoffman on July 19, 2014, 08:15:55 pm
By a stroke of surplus luck, I actually have a 5450A. It gets "calibrated" simply by storing the measured values in memory.

Though I like the historical aspects of manganin, I've also read it get used as a pressure sensor for high force shock waves like nuclear blasts! It doesn't seem like something that can be used as a pressure sensor is ideal for something that should be immune to the external environment, though I try to keep the number of nuclear blasts in my lab at a minimum.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 20, 2014, 01:42:58 am
Hello folks,

By George, those certainly are hermetic resistors, fairly old ones but their specs for these resistors are not very impressive and even less so for hermetic resistors.  Those are all hand soldered, I've done many of those myself over the years.  Brass tubes and usually, but not always, oil filled (those were always fun to solder), all two terminal in there.  Can't tell who may have made them, Fluke plastered their own part numbers on a lot of components made by others.

It is possible I may have seen an early production model because I do not remember seeing hermetic resistors in it.  Fluke could have easily changed the resistors when they went to full production on them.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 27, 2014, 07:40:09 pm
Hello,

I have updated Z201#2 measurements in the Z201 post on page 1.

measured T.C. of #2 is  -0.27 ppm/K which is nearly the "typical" value  of +/- 0.2ppm/K
compared to
measured T.C. of #1 was 0.9 ppm/K

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 28, 2014, 07:10:54 pm

Humidity sensitivity:

On the other side a volt-nut friend of mine has reported a humidity sensitivity between 5-50ppm for 30% RH change of 8E16 resistors which are similar to the UPW50 wire wound resistors. I hope I will get further information on this topic from him when he is back from his trip.


I got now some measurement values for humidity values:
these values are reversible when humidity changes back and forth.
Time constant for humidity is in the range of days to weeks.

8E16 1K about 10-30ppm for a 25-30% change in rH. (around 25% to 50% absolute)
8E16 100K about 50ppm in the same range.
the trend for wire wound seems to be that for higher ohmic values the humidity sensitivity increases.

S102J 1K + 10K about 30 ppm for a 30% change in rH.
S102J 100R is about 5-10ppm in the same range
So also the trend seems to be lower for lower ohmic values.

So I am wondering wether the measured hysteresis on my S102J has to do something with humidity.
On the other side the time constant is relative fast for a humidity change.

Any ideas?

@Edwin: How do you explain the (reversible) rH sensitivity on the wire wound resistors?
Is the trend for higher ohmic values explainable?

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 28, 2014, 09:17:39 pm
Hi Andreas,

Two reasons, the main defect at work here is the so-called weld joints, water molecules are getting into the joint and causing trouble (caution, oxides at work) at the 'weld' joint, it has a similar, although usually smaller, effect than a crimped joint produces.  The secondary defect, which would be of no consequence if the joints were welded, is that the resistor is not sealed completely and allows humidity to sneak inside the resistor.  When the resistor is heated sufficiently, the water evaporates returning the parts value to the original value or close to it.  Repeated exposure to moisture and heat cycles will eventually cause the resistor to drift more and more, possibly out of tolerance if the original tolerance was tight.  A well crimped joint (like the resistors that went to the Moon) can actually stay intolerance for years under the right circumstances.

The enameled wire will have exceedingly little or no response to water as the enamel is very effective at keeping the water out and Evanohm (or its other family members) is not particularly sensitive to water either but a defective weld joint is an open invitation to trouble.  One thing which does help keep these resistors in better health is simply keeping them powered all the time.  That is less likely these days as not so many can afford to keep their equipment running 24/7.

In the case of foil resistors, they are usually welded, that is not the problem, in foil's case, it is the fact that the resistor element is so thin it is very susceptible to water effects as are the very tiny interconnect wires between the element and the outside world.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 29, 2014, 08:06:23 pm

As for foil resistors, the early designs [and even the newer Z-foil resistors] use high-temperature epoxy to glue the foil to the ceramic substrate. 

The new Z1-foil resistors use polyimide to glue the foil to the substrate and to seal the resistor on the top after they are done adjusting it. 


Hello Ken,

from where do you have the information with the epoxy glue (or polyimide).

The only document that I have found speaks from "cement" to fix the foil to the ceramic substrate:
http://www.vishaypg.com/docs/63171/TN104.pdf (http://www.vishaypg.com/docs/63171/TN104.pdf)
On page 5 they speak of about 20 ppm seasonal changes for humidity. (foil resistors).

On the other side: all metal (wire or foil) can be regarded (more or less) as strain gage.
So if the epoxy housing swells due to humidity it may have a influence on the resistor value.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: nadona on July 29, 2014, 08:51:00 pm
...

The solution for foil resistors is to get them in hermetic packages if this kind of shift is important to your design-- or, just buy less expensive PWW resistors from Edwin that minimize this problem.

So Edwin has less expensive PWW resistors than Vishay resistors?

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 29, 2014, 11:02:45 pm
Hi Andreas,

To quote, " On the other side: all metal (wire or foil) can be regarded (more or less) as strain gage.
So if the epoxy housing swells due to humidity it may have a influence on the resistor value."

Whoever (Vishay no doubt) wrote that sentence, doesn't know what they are talking about (and I think I know who it was).  PWW, properly processed, are NOT strain gauges, for that matter, they never were characterized as such, unlike the Vishay resistors.  Yes, there is some strain imparted to the bottom layer of turns on a PWW resistor, this is the quite normal result of winding but that stress (again, if properly made) is essentially removed during the resistor processing.  On the 'other' PWW resistors floating around out there, instability is more often caused by nonexistent weld joints, a not so good seal or maybe just an inept resistor design.  Winding stress in every manufacturer's resistors I've examined has been mostly relieved and is not generally the source of other instabilities.

In the case of foil resistors, they are the most sensitive to stress of all thick. thin and foil technologies, this stress cannot be removed simply by baking the part, in the case of foil, they have had to find materials which minimized the actual stress on the foil byway of its bedding on the substrate.  PWW and Vishay resistors are two entirely different resistors, their manufacturing processes share little if anything in common and mostly have different issues affecting the resistance.

Epoxy does absorb water like many materials, but it depends on just what epoxy and hardener was used, some epoxies will absorb only a couple of tenths of a percent of water during a humidity bath while others can absorb a magnitude more.  While that certainly is a problem for foil resistors as Ken put it, PWW resistors (again, properly made) will have little to no effect from humidity.

I believe it was asked if humidity may have a greater effect on higher resistance values than lower values, that is a qualified yes, in the case of foil, if humidity does get inside of it (and Vishay seems to indicate that it does for non-hermetic) it would tend to have a larger effect on higher values than lower values because of the much finer circuit traces making up the resistor, the water molecules will have less distance to travel between close circuit traces than farther circuit traces.

In PWW resistors (again, correctly made) humidity would not affect the resistance any differently be it high or low.

I do not know if Vishay is using a similar enamel like mine or not, I have not seen any information that gives that detail.  I would have to say, that in the case of PWW resistors, the enamel I use does not absorb enough water to be of any concern.

Since the inception of film resistors (of any kind) moisture has been a problem, carbon, metal, thick, thin or foil, they all have a sensitivity to moisture and that resulted in many different encasements, even glass to stop the humidity's effects.  Plastics, for foil at least, have turned out to be only a partial fix to the problem.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 30, 2014, 09:04:18 pm

I have a love-hate relationship with foil resistors.  Currently, I am hating them.  Tomorrow is another day...


Hello Ken,

I can understand this.
Same resistor similar game two days later but totally different results:

On 27th I had forgotten to switch off the heater during night.
So actually i had a fast cool down from 45 deg C to 25 deg C in the morning.
So most probably a "dry" resistor.

On 29th the temperature was "room temperature" during night.
The result is a strange behaviour for the first "cold" cycle of the T.C. measurement.

At the moment it is rather rainy outside.
So humidity is rather high (65-67% rH) on my hair hygrometer.

So on rainy days you can hear the resistors whine.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on August 03, 2014, 07:15:41 am
Hello,

same game two days later on 1st of Aug, similar humidity (room temperature since last measurement) but slow temperature ramp (0.12 K / minute)

So it seems that its not humidity but temperature ramp speed which makes trouble to the Z-foil resistor.

with best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on August 03, 2014, 03:32:56 pm
Hi Andreas,

Humidity's rate of absorption or egress tends to be a balanced process under the same conditions, note that is the same conditions, when the rate is changed in either direction, so changes the rate of absorption or egress.  As the mass of a foil resistor is small, it has a comparatively short thermal tail, even though the plastic it is encapsulated in has a low thermal conductivity.  In your second test, you changed the rate of flow of humidity out of the resistor by using a very slow temperature ramp, thereby changing the conditions and making the comparison between the two measurements somewhat inaccurate inadvertently.

If the foil resistor does not have an encapsulation defect, the absorption/egress of humidity should be relatively slow compared to the response to temperature.  Obviously, a faster temperature ramp causes accelerated responses in both humidity and foil.

If I am reading your graphs correctly, I believe you are seeing a case of internal condensation effect for the first cold cycle, the condensation being evaporated during the subsequent heating cycle (for the 29th).  The resistor did indeed start off 'dry' because of the heater being left on during the night (for the 27th), therefore you are indeed seeing the effects of humidity on the foil resistor.  It is not a terribly big effect but it certainly is a significant effect.  That is why, in electron tube days, the equipment was left on 24/7 because the carbon resistors sucked water like a sponge and keeping them nice and warm effectively eliminated the effect.

The effect because of a difference between 0.12°C and 0.30°C ramping rate is quite insignificant.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on August 03, 2014, 08:51:10 pm

If I am reading your graphs correctly, I believe you are seeing a case of internal condensation effect for the first cold cycle, the condensation being evaporated during the subsequent heating cycle (for the 29th). 


Hello Edwin,

I already had the same idea. But why do I see this only on the "fast" cycle and not on the "slow" cycles (before and after).

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on August 03, 2014, 08:59:16 pm
Hello,

I have updated the Riedon USR2 post on page 1 with first measurement data.

Since the USR2 has some "sticker" (for labeling) its case does not fit easily into the thermal block.
So I had to use some (gentle) force to put it in.
There is some ageing drift (which I already mentioned yesterday after soldering with room temperature).
First measurement indicates T.C. around -1.1ppm/K.
I will have to check wether the mechanical stress of the thermal block has a influence.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on August 03, 2014, 10:48:25 pm
Hi Andreas,

As to your question, "But why do I see this only on the "fast" cycle and not on the "slow" cycles (before and after)."

Let's see if I have the sequence correct, on the 27th, the resistor had been 'cooked' over night so any humidity inside was minimal for the first test.  Rate of temperature change 0.12°C/minute.

On the 29th, resistor had been sitting at room temperature for two days in moderate humidity before testing, resistor showed odd reading.  Rate of temperature change 0.3°C/minute.

On the 1st, the same resistor had been sitting at room temperature and similar humidity, the only 'difference' was the rate of temperature change, 0.12°C/minute.  Readings appeared about normal.

I have a question or two for you.  How many cycles of cold and hot do you do before the resistor rests?  Any particular reason why you chose a different rate of temperature change for the second test?

Throwing out an educated premise, if we assume the rate of humidity absorption by the resistor was roughly similar between testing periods, the faster  delta T may have caused a different rate of condensation inside the resistor, i.e. it got colder faster, therefore more condensation occurred inside before the heating cycle was able to remove it.  With the slower delta T rate, the condensation took longer to form, thereby lagging the temperature change and forming less condensation inside to remove during the subsequent heating cycle.  I am referring to condensation directly on the foil of course; not so much around it, although that can have a small role in the effect as well.

I might add that a resistor 'sitting on a shelf' in a humid environment is considered benign, the usual humidity tests are conducted at an elevated temperature >70°C and 85%-95% humidity for 100 hours or more depending on the test requirements.
Title: Re: T.C. measurements on precision resistors
Post by: branadic on August 04, 2014, 10:53:59 am
Question: For what I know IC package mold mass is filled with glas balls, to minimize humidity influence. We use similar materials when doing Foil Assisted [Transfer] Molding process to create IC packages with [optical] interfaces. Is the same true for molded resistors?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on August 04, 2014, 05:08:24 pm
Hello Branadic,

To my knowledge, Vishay does not use any glass balls in their resistors unless that is something recently added.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on August 04, 2014, 08:20:55 pm

Let's see if I have the sequence correct, on the 27th, the resistor had been 'cooked' over night so any humidity inside was minimal for the first test.  Rate of temperature change 0.12°C/minute.

On the 29th, resistor had been sitting at room temperature for two days in moderate humidity before testing, resistor showed odd reading.  Rate of temperature change 0.3°C/minute.

On the 1st, the same resistor had been sitting at room temperature and similar humidity, the only 'difference' was the rate of temperature change, 0.12°C/minute.  Readings appeared about normal.

I have a question or two for you.  How many cycles of cold and hot do you do before the resistor rests?  Any particular reason why you chose a different rate of temperature change for the second test?


Hello Edwin,

the number of cold and hot cycles depends a bit on how much time I have on the day when I am soldering.
I try to do a (fast) cold and warm cycle after soldering. But this does not happen always.

For Z201#2 the history is as follows: (It was a saturday so I had some time).

Soldering on 26th after this a fast heating to 45 deg C
Fast cool down to 25 deg.
Fast cold cycle to 10 deg C.
Fast heat up to 25 deg
Fast heat up to 45 deg (over night)
Where fast means in this case: as fast as my equipment can.
(picture attached)

On 27th in the morning a fast cool down to 25 deg C
from this the measurement beginning with a cold cycle 0.12 deg/minute.
on 28th no measurement just room temperature.

On 29th the measurement (3 cycles) with 0.3 deg / minute.
On 30th+31th no measurement just room temperature.

On 1st the measurement 1 cycle with 0.12 deg/minute.
(pictures already shown in previous posts)

The different rates have several reasons.
with several cycles within one measurement I want to see wether there are some ageing
or hysteresis effects which change over time.
(0.3 deg/K gives up to 3 cycles per day).

With the slow ramp I wanted to see if the hysteresis has something to do with thermal delay.
So if the ramp speed is factor 2.5 slower then a "hysteresis" due to thermal delay (between resistor
and themperature sensor) should also change by factor 2.5.

But since the factor observed does by far change not that much the root cause of the hysteresis
is not (primary) a temperature sensing problem.

I also had different effects on hysteresis with different ramp speeds of (different types of) voltage references,
so I wanted to see if it has also a influence on resistors.
So Z201#2 is the first candidate where I see something unusual.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on August 11, 2014, 01:46:18 pm
Hello,

I have updated the USR2 section on page 1 with measurement data

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462305/#msg462305 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462305/#msg462305)

USR2 #1 has a large ageing drift. (Or is it humidity because it came freshly out of a welded polyethylene bag)

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=105219;image (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=105219;image)

USR2 #2 the first measurement indicates a T.C. of about -1.4ppm/K
Ageing drift is much smaller than on #1.
But this candidate had 1 week to acclimate after opening the bag.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on August 28, 2014, 09:01:29 pm
Hello,

I have updated UPW50 section on page 1 with several measurements.

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462300/#msg462300 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462300/#msg462300)

UPW50#2 0.63 ppm/K @ 25 deg C
UPW50#3 1.85 ppm/K @ 25 deg C
UPW50#4 4.76 ppm/K @ 25 deg C

So #2 beats even one of my Z201 resistors.

To keep track I created a overview from the measurements up to now.

I would interpret the results as follows:
With high sophisticated applications it is worth to measure the T.C. (+hysteresis) within the application temperature range and with some thermal cycling also the ageing drift over some days to further select the resistors and to sort out the "stinkers".
Some datasheet specs seem to be a bit too optimistic when looking at the "typical" values.
Sometimes (often?) the hysteresis is larger when having a low T.C. within a resistor type.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on September 01, 2014, 08:40:13 am
Hello,

I have done the first measurement on a UPF50 (molded metal film resistor) from TE.
see metal film section on page 1:
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462301/#msg462301 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462301/#msg462301)

Interestingly this device has no visible hysteresis.
So it seems that the hysteresis has nothing to do with my measurement setup
but is within the wire wound and metal foil resistors.
Unfortunately there is no shelf life data supplied with the UPF50.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on September 02, 2014, 01:35:06 am
Hi Andreas,

I've been busy working on a resistor order so I've had little time to reply.  The UPF50 data sheet appears to be a 'preliminary' one, likely they do not have any long term figures available as yet but given the environmental test limits, even if this resistor type was twice as good as the maximum test limits, they are nothing particularly special.  With test limits of 50 PPM to 2000 PPM for resistance change, they are about average for metal film, about the only 'special' spec is the 0 ±5 PPM/°C TCR which is quite good for this type of resistor.  Their power range is average, restricted above 70°C, derated to 0 at 125°C.  The shelf life should not be significantly different from any other similar metal film resistor of this type.  I did not see anything in the specifications that would lead me to think otherwise.

I would not be too quick to jump to conclusions about the presence or nonpresence of hysteresis, I have never seen a metal film resistor that didn't have it and this resistor's construction is no different.  One sample is much too small for any conclusions, there may be a valid reason why you did not detect hysteresis.

Ciao,

Edwin
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on September 04, 2014, 03:28:25 pm
Hello Edwin,

so you think that shelf life is around 300 ppm / year similar to RC55Y device?

In the mean time I did further measurements on UPF50 #1. (see also on page 1).
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462301/#msg462301 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462301/#msg462301)

With fast ramp (0.3K/minute on 01.09.2014) I got some hysteresis on the rising edge after the first cold cycle.
The slow ramp (0.12K/minute on 02.09.2014) showed again low hysteresis.

The 2nd device UPF50 #2 showed some hysteresis on cold cycle (0.12K/minute on 03.09.2014).
Here I did not do a cold cycle after soldering against sample #1.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on September 04, 2014, 08:28:46 pm
Hello,

for those who did not mention it (like me).
Ham Fest in Weinheim (Germany) is a possibility to meet:

https://www.eevblog.com/forum/chat/eevbloggie-meeting-at-ukw-tagung-weinheim-ham-fest/msg503262/#msg503262 (https://www.eevblog.com/forum/chat/eevbloggie-meeting-at-ukw-tagung-weinheim-ham-fest/msg503262/#msg503262)

http://ukw-tagung.org/ (http://ukw-tagung.org/)

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on September 06, 2014, 09:57:07 pm
Hello,

I have updated the metal film section on page 1:

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462301/#msg462301 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462301/#msg462301)

UPF50 #2: the hysteresis gets smaller with each temperature cycle but is still larger than on #1.

Also did first measurement of a RC55Y with 15ppm/K in data sheet.
This turns out as nearly -9ppm/K in actual measurement.
But there is a large ageing drift of 4-5ppm within the measurement.

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Andreas on September 08, 2014, 09:24:48 pm
Hello,

I finished measurement of RC55Y #1.
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462301/#msg462301 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462301/#msg462301)

total drift in 3 days around 14 ppm.
So it makes no sense to test further RC55Y.

Attached: Overview of the measurements up to now.

with best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Andreas on September 28, 2014, 10:10:33 pm
Hello,

currently I am characterizing two new ADCs with capacitive (LTC1043) dividers.
I decided to compare a LTC1043 divider against a high precision resistor divider.
In this case it was a Vishay DSMZ 5K/5K (1:1) divider.

http://www.vishaypg.com/docs/63121/dsmz.pdf (http://www.vishaypg.com/docs/63121/dsmz.pdf)

from datasheet
typical ratio tracking T.C. 0.1ppm/K
maximum tracking     T.C  0.5 ppm/K (-55..125 deg C for a 1:1 divider)

Measurement was ratiometric: The VREF of the ADC was used as supply for the DSMZ; canceling out the VREF drift.
The DSMZ was soldered with thin copper wires to a DSUB-connector which I plugged directly to the ADC.

I got 28.3uV change in 31.1 K which is 0.91uV/K or 0.36ppm/K average with respect to the nominal 2.5V at the output of the divider.
There is also a large hysteresis or ageing drift in the measurement.

As comparison a LTC1043+LTC1050 capacitive divider with ratiometric measurement (dividing VREF of ADC).

In this case I got 1.0uV change in 32.2K which is 0.031uV/K or 0.0124 ppm/K @ 2.5V
no hysteresis.

The offset drift of the LTC1043  #1101_004 divider was 3.2uV/33.2K

And the ADC #16 itself measured a gain drift (5V Vref measurement) of -2.1uV/33.6K
and a offset drift of 0.6uV/32.9K

With best regards

Andreas

Edit: attached picture of DSMZ#1 with ADC#16
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on October 05, 2014, 05:24:15 pm
Hello,

and just the next DSMZ (#2) measured with new ADC #15 dividing the reference voltage ratiometrically.

18.5uV change over 33.9 K giving 0.55uV/K or 0.22ppm/K (so better than the previous DSMZ)

This competes with the LTC1043 #1101_003 which has:

3.2 uV (max.) change over 31.8K giving 0.1 uV/K or 0.04 ppm/K
(and would be much better when the switching on of the cooler did not produce that spike at 32 degrees as can be seen in the drift over time diagram).

Offset drift of LTC1043 #1101_003 is:
1.2 uV change over 32.4K giving 37nV/K or 7.4ppb/K referred to the 5V ADC input (= divider output).

ADC #15 alone measured 0.3uV/24K offset drift (17-42 deg C)  and 2.2uV/34.5K gain drift.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on October 10, 2014, 09:30:43 pm
Hello,

I have repeated the measurements of the LTC1043 #1101_003 and DSMZ#2 divider.
(the jump on LTC1043 was not plausible to me).

The gain drift of the LTC1043 of 07.10.2014 is now without any jumps.
So I guess that I had a problem with one of the connectors on the measurement of 30th of September.

Without regarding measurement noise I get  0.5uV/31.8 K = 16nV/K = 6.3ppb/K for the LTC1043 gain drift.

DSMZ#2 was repeated on 08.10.2014.
T.C. of the divider is similar to the first measurement
17.2uV / 32.3K = 533nV/K = 213ppb/K (2.5V)

The ageing drift/hysteresis ist much smaller than on the first measurement. 0.7uV = 0.28ppm
So either the drift was due to longer time after soldering or due to relaxation after the first thermal cycle.

With best regards

Andreas




Title: Re: T.C. measurements on precision resistors
Post by: branadic on October 15, 2014, 06:29:05 am
Hi Andreas,

still very interesting results but somewhat confusing. Can you give a brief summary what resistor is best in your measurements?

Regards.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on October 15, 2014, 08:40:52 pm
still very interesting results but somewhat confusing. Can you give a brief summary what resistor is best in your measurements?

I think it's still a Work In Progress [WIP], but I too am interested in the "Reader's Digest" version of which [so far] are the best resistors.  But then, we have to define what "best" means, but to me, it is excellent TCR and super-stable resistance over time-- but that is because I will not be running the resistors under high power or pulsed power conditions [at least in what I am doing now].

As with many things in the world of engineering, the definition of "best" depends on how you are using the resistor.

I would be interested in what Andreas found with Edwin Pettis' resistors, which appear to be the best of the Evanohm-wire based PWW resistors available.

Hello,

I can only speak for the resistors that I have measured.
And there is no clear winner up to now.
Each resistor even from the same series behaves more or less individually.
And yes its a WIP I have planned to do further measurements at least on some Z201 and if I get some samples also from 8G16. (But there we have one measurement from Frank with 2.2 ppm/K. So this would be only to see if hysteresis will be better with slower temperature change).

From Edwin I still have no idea where the pricing will be for small quantities. And I fear that the shipment costs from US to Germany will be tremedous high especially when using UPS or similar service who does the handling with customs. And I do also not know if I could pay per PAYPAL.

From trend I would give the following statements today:

Z201 will be my first choice for low T.C. + hysteressis without selection.
UPW50 is second choice. And hand picked devices may exceed average Z201.
UPF50 are the surprising cost/performance winner regarding T.C. + hysteresis.

My UPW25 samples although similar to UPW50 seem to have problems.
S102 and USR2 have also measured far below UPW50.

I have no clue how long term stability will be.
(And up to now I have no reference grade resistor to compare with).
And I have no hermetically devices in my samples.

For dividers I would not use individual resistors but something like DSMZ.
The open point is: how will they behave when being soldered on a PCB?
Perhaps it would be better to use them dead bug mounted.

Another point:
the cylinder style resistors seem to be more critical with thermal coupling. (and perhaps large temperature gradients).
I have the impression that thermal grease is necessary to keep them at same temperature as the isothermal block, whereas a rectangular style resistor showed lesser difference (shown as "hysteresis") between "dry" and "greased" state.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: MK on October 16, 2014, 09:28:28 am
Perhaps you could use medicinal paraffin to thin down some thermal grease to make it easier to keep the round resistors in contact with the aluminium block?
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on October 16, 2014, 11:03:27 am
Andreas, DiligentMinds, branadic

up to now, this thread was a very instructional and successful demonstration, how such delicate measurements like the determination of these low T.C. of resistors could be done with relative simple means, instead of using heavy equipment like an 8.5 digit multimeter, or ultra stable and precise resistance bridges..

But at that point,  it is no way of judging, which resistor technology or brand is better, or best in class..

Reason for that is the mere lack of statistics.. means, too few different samples per technology / brand have been used.

Elsewhere, I already have provided 5 T.C. values for the hermetically sealed, oil filled VHP202Z, and found out, that these values showed a big spread, and all were an order of magnitude above the promoted typical value.
(Andreas, if you like, I may add these measurements here as an additional item)


To evaluate, what the different technologies are really capable of, say in statistical terms of 'typical' values and max. spread, you need much more samples than 2, or even 5 ones.

Therefore, up to now I would not draw such a conclusion, as DiligentMinds did.

Maybe others also can add such measurements...

Andreas, if you don't have an 8E16 resistor yet, we may discuss, how to organize that.. I have a bunch of 50 Ohm, .., 10k, 30k, 100k in stock.
Either I do some much better measurements on them,  (I mean, than the quick *n dirty one that I already did  ::) ) using my equipment, or we may exchange some samples..


The basic problem with these tubular case resistors is that the wires obviously transport most of the heat into the resistor, compared to the mould component.
That effect can be reduced by attaching the leads thermally to the isothermal block also.. there are several possibilities to do so.

Anyhow, this subject is still very interesting, so keep on going!

Frank
Title: Re: T.C. measurements on precision resistors
Post by: texaspyro on November 16, 2014, 05:46:50 am
Speaking of the Fluke 5450...  does anybody know of a source for the DL02614 displays used in these?  One of mine has a bad segment.
Title: Re: T.C. measurements on precision resistors
Post by: MK on November 16, 2014, 08:11:36 am
Andreas, DiligentMinds, branadic

"...it is no way of judging, which resistor technology or brand is better, or best in class..."

<snip>

Therefore, up to now I would not draw such a conclusion, as DiligentMinds did.

<snip>

Hi Dr. Frank,

Sorry, when I posted that I guess I kind of implied that I was basing my conclusions on the work done here in this thread by Andreas.  I apologize for that.  That isn't what I meant to say.

My conclusions were [and still are] based not only on Andreas' work, but long years of experience and hundreds of hours scouring the available literature for relevant material.

In a private email from Edwin Pettis to me, Edwin complained that many engineers over-spec the resistors they are using in their circuits.  I think he is correct.  You do not need a super stable hermetically sealed, oil-filled, 0.001% resistor for an LED ballast resistor.  Part of the art of electronics is knowing when to specify better parts, and in order to do that you have to know *why* you need a better part.  Then, you can use your engineering skills to ask the question: "how good do I need this part to be?"  You also need to know what parts are available, and what the environmental effects on each resistor type are.  In some applications, you also need to know what the long term drift might be for each resistor type.  Without this information, you might get lost in all of the data, and end up specifying the wrong part.  In some applications, you would want to know how to accelerate the aging of the resistors and other components in your design so that your design is very stable coming off of the factory floor.  There are different techniques for doing this, and not all of them apply to all resistor types [and/or other components].

I too think that the work Andreas is doing is important, and interesting.  Andreas, please keep going!

I agree that overspecifying is expensive, the problm I believe is a lack of trust in the datasheets, like some of the current TO220 devices that claim to be able to dissipate 200-300Watts, the same general weasel words have crept into many other datasheets too.

But this thread grew out of the LTZ1000A thread where there are many other error sources that creep in. For an LTZ1000 my current thinking is a mix of econistors/squaristors and some Edwin resistors in the most critical positions, and then jellybean resitors for current limit, psu etc.
Title: Re: T.C. measurements on precision resistors
Post by: TiN on November 16, 2014, 01:57:53 pm
Just won a broken Keithley 2510 TEC SMU, will be a nice tool for little controlled temperature chamber for TC testing.

Thinking of building plastic insulated box with BNC inputs and RTD sensors around to make accurate -20....+100°C controlled temperature chamber.
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on November 16, 2014, 02:29:05 pm
TiN,

I picked up one of those earlier this year for the same purpose. Mine appears to be fully functional. If you need me to do anything to help with the repair, let me know.

I have a small aluminum enclosure milled for a small peltier assembly. I have also purchased one of these   http://www.ebay.com/itm/Liquid-Thermoelectric-Peltier-Enclosure-Dual-Fan-With-Heat-Sink-/140898947623 (http://www.ebay.com/itm/Liquid-Thermoelectric-Peltier-Enclosure-Dual-Fan-With-Heat-Sink-/140898947623).

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 25, 2015, 05:14:40 pm
Hello,

I have done a repeated measurement on S102#1 just to see what happened in the mean time.

The measurement setup is slightly changed just to make it easier to change the resistors.
I do not use thermal grease; the resistor is pressed against the isothermal block with a spring.
The measurement lines are not soldered but I use 4 clips for the 4 wire measurement.

Measurement on 18.01.2015 compared to the measurement of 13.07.2014 shows
a different hysteresis curve and a drift of -12.6 ppm @ 25 deg of either the S102#1 or the reference resistor Z201#1.

Measurement on 19.01.2015 is more similar to the old measurement of 13.07.2014 regarding hysteresis.
Drift is reduced to -11 ppm @ 25 deg.
So both effects together seem to be related to the S102#1 resistor.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 25, 2015, 07:18:53 pm
Hello,

another repeated measurement with Z201#2

Comparison is to measurement of 01.08.2014

On 20.01.2015 the drift is slightly negative: -0.3 ppm @ 25 deg C
after 3 further temperature cycles:
On 23.01.2015 the drift is slightly positive: +0.9 ppm @ 25 deg C
Slope varies also slightly (but may be also different by using sligthly different minimum and maximum temperatures).

All in all the two Z201 (reference #1 and DUT #2) have much lesser drift than Z201#1 against the S102#1.
So either the Z201 are both very stable or at least have the same ageing drift.

other coefficients:
01.08.2014
A 0 = -7.81060638217116E-0001
A 1 = -2.31032022287009E-0001
A 2 = -4.31093591373655E-0003
A 3 =  2.78270315402516E-0005

20.01.2015
A 0 = -1.1077309086686037E+0000 -> drift -0.3 ppm @ 25 Deg
A 1 = -1.9762440120142556E-0001  -> slope -0.03ppm/deg
A 2 = -3.9960281447573684E-0003
A 3 =  1.7247792207858591E-0004

23.01.2015
A 0 =  1.4344628522426358E-0001 -> drift + 0.92 @25 deg (01.08.2014)
A 1 = -2.0906922064354618E-0001 -> slope -0.02ppm/deg
A 2 = -3.0228719576275895E-0003
A 3 =  1.2306553962915563E-0004

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 25, 2015, 07:47:32 pm
Hello,

the measurement clips brought me to a idea:
What is the influence of the lead length on the resistor T.C. ?

When doing the test with soldering I used the maximum possible lead length in order to make minimum thermal stress to the devices.
But now with the clips I am more flexible. (And when soldering finally to a pcb I would never take the full lead length).

So from datasheet of a Z201.
lead length outside from case:  about 25.4 mm (one inch) with AWG#22 copper.
(54.7 mOhms/meter TCR: 3880ppm/K)

Within case the lead is additionally up to 9 mm each.
So minimum usable lead length is 20 mm in sum.
Maximum around 70 mm in sum.
Difference is 50mm resulting in 0-2.7mOhms variable resistor with 3880ppm/K
That sounds not much but for the 120 Ohms resistor of a LTZ1000
2.7mOhms are already 22.7ppm variable resistor with 3880ppm/K T.C.
Giving from 0 to +0.088ppm/K additionally T.C. on the 120R resistor.
So slight negative T.Cs could be trimmed by the resistor lead length or by the PCB trace length.
(Ok this is probably not the best idea since on pcbs the trace thickness has large tolerances during production).

For the 1K resistors the lead lenght short/long gives also a 0.01ppm/K change.
If one would use a 10 Ohms Z201 the leads give up to 1ppm/K change in T.C. which is far beyond the "typical" datasheet spec.

So at which lead length the T.C. of a Z201 is specified?

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: MK on January 25, 2015, 08:11:45 pm
The z series are supposed to be mounted touching the pcb, so if we assume single sided with the copper opposite the component, it will be used about 1 mm from the body.
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on January 25, 2015, 08:43:11 pm
The Econistors say in their datasheet that the connection for calibration is "made - 10mm along the lead-out wires from the body", and also give a figure of Resistance of termination leads depending on Type.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on January 25, 2015, 10:51:25 pm
Hello Andreas,

I'm really sorry to disagree to your statements and your setup.
It's not at all from electrical, but from pure physical aspects.

Your (really wild) hysteresis figures show, that all of your measurements were always done in a very big thermal disequilibrium between your measured temperature (sensor?) and the resistor element.
It's always a dynamic thermal measurement (everything is changing rapidly), instead of near static measurements.
It's also caused by the fact, that these type of resistors were mostly heated / cooled by their wires, not over the body. So the thermal mass, as you use it, does not help.

You either have to couple the wires also to your thermal block, or you have to do the measurements much, much  more slowly, in equilibrium, i.e. not always by forced heating / cooling, but by very slow thermal convection.
Or you may put temperature sensor and DUT in one closed metal box and slowly heat/cool the whole box.

As a consequence, your T.C. curves do not represent the actual behaviour R(T) of the DUTs, and also your statement, that T.C. has changed between the two dates, I think, is unsustainable.


For a volt-nuts from U.S., I've just recently calibrated  his NTC against a precision PT100, and then later determined the R(T) of his PWW by using this calibrated NTC.

I append the hysteresis curve for the NTC vs. PT100 measurement, where you can nicely see, how forced cooling / heating cause hysteresis, but during slow convection (taking several hours for one direction), there is no hysteresis at all.

In the last chart (raw data) you can see, how well you can supress hysteresis for the R(T) measurement (it's the PWW in a aluminium box), if you simply make the measurements very slow.
The forced cooling is visible as an offset, but even the slow, forced heating by an IR lamp causes no  hysteresis.

The loop starts at room temperature, cooling down quickly to 15°C, then relaxing to room temperature, slowly heating to 32°C, and relaxing again to r.t.

Frank


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 25, 2015, 10:53:51 pm
The z series are supposed to be mounted touching the pcb, so if we assume single sided with the copper opposite the component, it will be used about 1 mm from the body.

The datasheet is of opposite opinion ;-)
http://www.vishaypg.com/doc?63187 (http://www.vishaypg.com/doc?63187)

At least for the resistance value they spec on page 4:
Lead test point: 0.5" (12.7 mm) from resistor body

I doubt that they change their test fixture for measuring T.C.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 25, 2015, 11:17:35 pm
Hello Frank,

one temperature cycle already needs a whole day. 11-12 hours for the cycle + 1-2 hours for overlap until I do not have a "open" hysteresis curve.
So I am happy when getting a hysteresis below 1 ppm and take the interpolation in between as "truth".

Maybe you are right for the Z201.

But the S102 components are (with similar shape and test fixture) so much worse that I consider that the inner construction of the S102 is much worse (some interaction between molding compound and resistive element which gives something as a time constant).

Also Vishay states up to 20 ppm seasonal effects for molded resistors due to humidity.

http://www.vishaypg.com/doc?63171 (http://www.vishaypg.com/doc?63171)

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on January 25, 2015, 11:32:43 pm
Hello Andreas,
maybe I did not explain precisely:

The basic problem with all of measurements is, that you never get some repeatable curves or parts of curves in ALL of your measurements.

So you can't identify, which effect is really hysteresis (e.g. by the metal foil technology), and which is thermal difference between the resistor and its wires, or the thermometer.
Even if you measure slowly, the wires may always be on a different temperature, than the thermal mass and the thermometer.

I don't dig it from your measurements, but all these curves look quite strange, if I recall temperature measurements, and also hysteresis measurements  I have done.

Maybe the S102K has a very huge hysteresis, but the 201Z definitely has to have much lower hysteresis (and only for bigger temperature excursions as +/-15°C only)


I propose that I repeat your measurements with my Veltins thermal chambers  ;) and see what happens.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 26, 2015, 08:43:09 pm

The basic problem with all of measurements is, that you never get some repeatable curves or parts of curves in ALL of your measurements.

Even if you measure slowly, the wires may always be on a different temperature, than the thermal mass and the thermometer.

I don't dig it from your measurements, but all these curves look quite strange, if I recall temperature measurements, and also hysteresis measurements  I have done.

Maybe the S102K has a very huge hysteresis, but the 201Z definitely has to have much lower hysteresis (and only for bigger temperature excursions as +/-15°C only)

I propose that I repeat your measurements with my Veltins thermal chambers  ;) and see what happens.
Hello Frank,

For the repeatability I look at the UPF50 (metal film resistors) on page 1. Those have virtually no hysteresis.
Ok since they are cylindrical the setup is slightly differen. But they should even be worse because the thermal contact is worse.

All in all you are right: the results look strange.
I have tried many different setups but I did not find a setup with much better hysteresis up to now.

When looking at the Z201 chart at 25 degrees C. The hysteresis in x-direction is around 7 degrees C  (or +/- 3.5 degrees in each direction).
7 degrees at 0.12K / minute slope correspond to nearly 1 hour (or +/- 30 minutes in each direction).
I really doubt that either the temperature difference between temperature sensor and resistor element is 3.5 degress.
It should be more in the +/- 0.5 deg C range when the fan is active.
And I also doubt that the thermal time constant is around half an hour with active fan.

Perhaps my fault is that I rest too long at the minimum and maximum temperature so that the hysteresis can "charge".
Ok I will have to check the temperature sensors. Perhaps they have not survived the treatment with the thermal grease.

If you want to spend your time, I will send you the Z201#2 and S102#1 for measurements.
It would be very interesting how the setups compare.
And if repeatability is related to measurement setup or resistors.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on January 26, 2015, 09:18:38 pm
Hello Andreas,
I look forward to get your resistors!

What's really strange is the lack of a virgin path if you really observe the hysteresis, which is a companion of the metal bulk foil technology.

Then, the Z201 should nearly be free of hysteresis in the temperature range where you measure.

And I can really not believe, that the S102K should be that bad, because for a claimed T.C.  of 2ppm/K (even with Vishays butterfly method), or so, such a big hysteresis should make them useless.

What might cause such behaviour, is a foregoing history of extreme temperature excursions.. remember, how I received my VHP202Z back from Vishay Precisison/ Germany, he had heated them asymmetrically to 125°C, and they were stuck at +5ppm higher resistance, until I unloaded this memory / hysteresis effect.

Do you remember, what happened to your resistors?

Maybe, that you already have a prepared aluminium box with jacks for the resistor and the sensor?

Otherwise I have to build one.

That will be an exciting experiment, again.  8)

Frank

PS: I may probably measure these two comrades during that session, the left one is also a Z201.
Has yours the same case?
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on January 27, 2015, 06:05:15 am
Maybe closer inspection  of Andreas' isothermal box including heaters and sensors would gain some insight?

How big are the gaps between the block and the parts? I suppose you already reduced it as much as reasonable and that the parts are touching one part of the wall due to gravity, but how would you describe it? Thermal grease works well usually only in very, very thin layers. Also I would recommend some thermal isolation of the block, even if it is just that kind of sticky tape that is used for mirrors where you can put it without closing holes.

Andreas: More photos, please ! I am just aware of one.

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 27, 2015, 06:40:02 am
Do you remember, what happened to your resistors?

Maybe, that you already have a prepared aluminium box with jacks for the resistor and the sensor?

Otherwise I have to build one.

PS: I may probably measure these two comrades during that session, the left one is also a Z201.
Has yours the same case?

Hello Frank,

Treatment of my side for Z201#2 was as follows:

26.07.2014 in the evening
soldering 4 wire with crocodile clips on the leads for protection.
Thermal grease for trying to get better thermal contact to isothermal block
temperature setpoint 45 degrees (first hysteresis test).

27.07.2014
thermal cycle 8.5-46 degrees setpoint beginning from room temp 0.12K/minute

28.07.2014
3 thermal cycles 8.5-46 degrees setpoint beginning from room temp 0.3K/minute
Temperature measurement error due to failure in USB device -> no evaluation possible

29.07.2014
3 thermal cycles 8.5-46 degrees setpoint beginning from room temp 0.3K/minute

01.08.2014
thermal cycle 8.5-46 degrees setpoint beginning from room temp 0.12K/minute
in the evening:
desoldering with crocodile clip to cool the leads

some days later:
degreasing of the device with solvent (Kaltreiniger).

20.01.2015
first repetition measurement without thermal grease and with IC-grabbers instead of soldering.

At the moment I am doing measurements on a never greased device (Z201#3)
Hysteresis seems to be better. (so that I can blame it on my measurement setup).
So the question is if I better wait until these measurements have finished.
You would get a 3rd device.

And no: since I want to use the resistors afterwards for a LTZ1000 I have no test fixture.
At maximum I could send you a spare iso thermal block.

According to PCN you have a device before 2006
Mine are the black version of Y1453

http://www.vishaypg.com/doc?63200 (http://www.vishaypg.com/doc?63200)

By the way in 2014 they changed again the molding compound:
http://www.vishaypg.com/doc?63254 (http://www.vishaypg.com/doc?63254)

of course this has never a influence on the electrical specifications  :wtf:

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on January 27, 2015, 11:47:20 am
Hello Andreas,

well your Z201 has onviously seen no excursion from R.T. more than +/- 20°C.. therefore, the Z foil resistors should not show hysteresis, from what I learnt from the VHP203Z.

Especially not so huge ones.


Well, leave yourself time, and then send me more samples, idf you like.. I will prepare a box then.. careful soldering at the end of the wires, with a copper pincer is allowed, I hope?

I also would like to get more insight in your heater/cooler assembly.. One sentence drew my attention.. you mentioned, that you run the fan.. all the time?

Does that imply, that either the thermal heat or sink is always on, also the fan?

Then there might lay the reason..

As I indicated, it's better to leave the system alone w/o forced heating or cooling, and especially no air draught.

That's the main purpose of my beer thermal box.. it stores the heat in the box, so that you get a constant, very slowly varying air stack, cold at the bottom, warmer at the top (R.T.), or warm metal box, which slowly cools down by thermal air diffusion.

Well, it's total speculation at the moment, let's simply do the test at my premises.

 regards Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 27, 2015, 08:48:19 pm

I will prepare a box then.. careful soldering at the end of the wires, with a copper pincer is allowed, I hope?

I also would like to get more insight in your heater/cooler assembly.. One sentence drew my attention.. you mentioned, that you run the fan.. all the time?

Does that imply, that either the thermal heat or sink is always on, also the fan?

Then there might lay the reason..

As I indicated, it's better to leave the system alone w/o forced heating or cooling, and especially no air draught.


Hello Frank,

soldering is allowed (I did it before).
I hope you do not want to use the Pb-free solder wire with higher melting temperature.

Attention: According to PCN the size of black resistors is smaller than the green ones.

Yes the "inner" fan is running all the time (with about 9V voltage for a 12 V low noise fan).

Most of the pictures on page 1 are still valid for the setup.

The fan of the cooling box is only running during cooling phase which goes from +32 degrees C (downwards) to 8.5 degrees and from 8.5 degrees (upwards) to 15 degrees.
The heater works with 1 second on/off time to do the fine adjustment of the actual temperature against the setpoint ramp.

And when looking at my "failed" measurement of January 22nd where I forgot to switch on the inner fan. (I thougth it was useless to look at it). I fear we have found the reason. Above 32 degrees where the fan of the cooler is also off I see no hysteresis at all.
Damned that hurts: I have learned a lection. I will have to fix my setup.

The open question is: how can I cool down the resistors with a defined ramp with minimal air draught within the cooler?

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on January 27, 2015, 09:03:44 pm
Thank you for re-pointing to the first posts with the setup images.

My suggestion:

Put some thin foam, say 1 to 5 mm,  around every available spot on the isothermal block and try again.
Lack of foam on non-available spots intentional, the ramp shall be done before eternity is over. You might even consider putting holes in the foam where not necessary if it improves symmetry versus the necessary ones.

This should help a lot in my opinion.

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 01, 2015, 08:26:59 pm
My suggestion:

Put some thin foam, say 1 to 5 mm,  around every available spot on the isothermal block and try again.

Hello,

I did some tests. But with the results up to now I think that it is not sufficient to only put some foam around the isothermal block and let the contacts outside the shielded room.

What I did up to now:
First: put a additional temperature sensor right between the legs of the DUT.  (bound with isolated wire).

27.01.2015
The fan from the cooler box (outer fan) is a horror for stability.
The inner fan cannot compensate fully for this.
So I decided to use a carton box for the heat spreader and the isothermal block.

30.01.2015
Due to (large) temperature differences between the leg sensor and the isothermal block I decided to put a further heat spreader (TEKO A3 enclosure) around the DUT and isolate the resistor. Intention is to keep the resistor and the legs at the same temperature.

31.01.2015
Still not sufficient I put a copper bar (around 5-6mm thick) above the DUT and below the TEKO A3 enclosure to get further improved temperature distribution.

01.02.2015
I got the suspicion that there are Seebeck voltages which generate the hysteresis.

Edit: 1uV thermocouple voltage may change the measured resistance by 0.4ppm.
The copper-beryllium contact has about 0.8uV/K against copper.
The solder junction at the other end 3-4uV/K against copper.
So a 2 ppm hysteresis might be a +/-2.5uV thermocouple voltage.

So I started a measurement (still running) with reverse polarity of the reference voltage for the resistor divider.
Perhaps I will have to use the cirquit on page 7 of LT AN96:
http://cds.linear.com/docs/en/application-note/an96fa.pdf (http://cds.linear.com/docs/en/application-note/an96fa.pdf)

@Frank: does the HP3458A any measures against thermocouple voltage like current reversal or pulsed measurement

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 01, 2015, 10:18:21 pm
Hello Andreas,
Yes the 3458A has OFFSET COMPENSATION for Ohm measurements.

The reference current is switched on and off, and thermo voltages were subtracted to give the true resistance.

For voltage measurements, that's of course not available, You would have to reverse the DUT manually

Not totally clear, what You Want to measure in this instance.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 03, 2015, 11:02:00 pm
For voltage measurements, that's of course not available, You would have to reverse the DUT manually

Not totally clear, what You Want to measure in this instance.

Hello,

according to AN96 I could also reverse the "(half) bridge" voltage.
Or switch off the VREF alternately.
But this would increase the overall noise level since I have not so many values to average.

As you noticed : As you increase the thermal mass, you have lower the slope of temperature accordingly, or phase lag will appear. So : The smaller test system, the better.

Hello Emmanuel,

thank you for your comments. This gives me a idea how to improve the thermal coupling of the leads.
I think in my case with the fan from the cooler I will need some thermal mass (at least around the resistor and the junctions) to equalize temperature.
On a thin sheet plate you can easily measure temperature differences of some degrees.
I have ordered some thick walled Hammond enclosures for better heat spreading in case it is necessary.

And I have also some small aluminum bars 19x19 mm where I can put the resistors in between with some silicone rubber when I get some slot milled for the resistor.
So we will see.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 03, 2015, 11:22:51 pm
Hello,

in the mean time the measurement from 01.02.2015 with reversed polarity shows the strange effect that the hyseresis is halved but no change in direction as I would have expected for thermocouples. So there seems to be another fault.

02.02.2015: measuring offset by shorting both ends of the resistor line to AGND.
Shocking: the offset is relative large -15 .. -23 uV and depending on input impedance.
The low impedant pins have the -23 uV and the resistor taps have the -15 uV.
One resistor tap (red) shows a hysteresis of 1uV and nearly no other temperature dependancy.
So only around 1uV effective thermocouple voltage visible.

The ADC itself has usually below 5 uV Offset and usually in positive direction.

The -15..-23 uV "Offset" reminds me of my first measurements with the capacitive voltage divider:
The capacitive switching input of the ADC generated the offset by rectifying the noise at the protection diodes of the buffer OP-Amp output.
So in this case the multiplexer protection diodes are affected by the switching noise of the sigma delta ADC.

First measurements with a filtered buffer amplifier between MUX and ADC show a equal offset of all mux channels at room temperature.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 07, 2015, 02:25:24 pm
Hello,

I have made a offset measurement with a ADA4638 buffer.
Against unbuffered measurement all 4 channels show similar behaviour over temperature.
Only one channel (the voltage pin on the measured resistor) shows around 0.5uV hysteresis.
(which would give 0.2 ppm resistance change).
This is similar to previous measurement (the red curve of unbuffered measurement).

So I think that thermal EMFs are not the primary problem in my setup. Since all differences nearly cancel out.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 07, 2015, 02:30:15 pm
Hello,

I also did a comparison between unbuffered TC curve of Z201#3 and with the ADA4638 buffer:

So there are no significant differences in the diagrams.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 07, 2015, 02:51:17 pm
Hello,

I did also do a comparison with reversed supply of the resistor divider:

What I do not understand is that the hysteresis differs (is nearly halved) from previous measurements.
Between the 4 measurements I did no mechanical change to the setup within the cooler box.
Any ideas?  Are resistors with welded connections in reality some hidden diodes?

The only thing that I have is that there are around 0.4 deg C difference between different NTCs at the same temperature depending on rising or falling temperature.
But 0.4 deg C at <0.3ppm/K would not explain a 2 ppm hysteresis opening.
As calculated on the first page the self heating of the 1 K Z201 resistor is around 1 deg C.

I will follow the advice of Emmanuel for the next: using better thermal contact to the wires of the resistors.
In this case I cannot use the test clips and will have to solder again.

But all in all I think searching for the reason of the hysteresis is more a academic theme.
In all diagrams I have more or less 10 ppm difference between the endpoints for 33-34 deg C temperature difference.
So if I interpolate in between the hysteresis I will be not far from the truth regarding T.C.
And around 0.05ppm/K ( .. 0.1ppm/K) seems to be the limit what can be reproducable measured with a 24 Bit ADC.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on February 07, 2015, 03:53:53 pm

Any ideas?  Are resistors with welded connections in reality some hidden diodes?


This may not be pertinent to your measurements but Edwin Pettis had written an article where he mentioned that some resistor welds acted like diodes...

http://www.edn.com/design/analog/4427940/1/The-last-half-century--Wirewound-resistors-Part-two (http://www.edn.com/design/analog/4427940/1/The-last-half-century--Wirewound-resistors-Part-two)
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 07, 2015, 09:42:10 pm
Hello Ken,

as described on page 1 I am using a LTC2400 and not a 3458A.
Since I am measuring ratiometric I use the VREF of the LTC2400 for the resistors the MUX and the ADC itself as supply.
The input voltages when measuring T.C. are all within 0.5mV to VRef-0.5mV -> within conversion + supply range of the ADC.
So far away from leakage of ESD diodes.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 08, 2015, 02:58:34 pm
Hello Andreas,

Apologies if I'm somewhat lazy, but is it possible to make a brief summary of your results? The amount of data is kind of overwhelming  :)

A couple of related things: The S102 are epoxy molded. Do you have any thoughts on improving the humidity transfer by for example  dipping these in additional layers of epoxy? Epoxy dipping may of course also have other effects, influencing hysteresis. It may be of for DIY and small scale use. If someone could test, I could supply some S102K or S from the same batch, with and without additional dipping into epoxy.

I also started a thread relating to your post #1 on alternative methods of bonding sensitive components like these, onto PCBs.

https://www.eevblog.com/forum/testgear/methoods-for-connectig-sensitive-components/ (https://www.eevblog.com/forum/testgear/methoods-for-connectig-sensitive-components/)

Jan
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 08, 2015, 04:41:07 pm
Hello Jan,

from time to time I post some collected overviews.
The last was in post:
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg509712/#msg509712 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg509712/#msg509712)

Just dipping into epoxy will not really help. (May only change the time constant).
The only way to really avoid humidity effects is to use a hermetically sealed package.
Metal or ceramic with glass sealing for the wires.
The wires have to be out of Kovar in this case to adapt the thermal expansion to the sealing.
Unfortunately the Kovar wires have a large thermal EMF (39 uV/K) against copper.
So avoiding the humidity problem will need further effort at the thermal layout side.

To really test humidity you will need a stable (hermetically) reference resistor.
(Which I do not have). And much time.
The reaction time to humidity is usually in the range of several (3-7) days.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on February 08, 2015, 06:08:17 pm
I have to disagree about hermetic seals are the only way to avoid humidity effects.  The design of the resistor determines the sensitivity to its enviroment, in the case of film resistors, hermetic is the only way to completely avoid it because the film elements are sensitive to humidity.  In the case of my resistors, there are no significant effects, unless they are put into a high temperature, high humidity enviroment for a prolonged period and even then, the effects are very small compared to all other resistor types.  It all has to do with the design and construction of the resistor.  In a normal environment, humidity can be ruled out as a effect to worry about.

In film resistors, the resistive element is very small and the line widths/thickness are also very tiny, the element has no protective covering, it is bare and unless sealed against the enviroment, reacts to anything coming inside of its packaging.  Although wire wound resistors can use extremely tiny wire sizes, as small as 0.0004" diameter, all wire is enamel coated and pin holes are required to be few and far between.  The main culprit of humidity sensitivity in precision wire wound resistors was the lack of a true welded joint at either end of the resistor, this allowed many problems to affect the resistor's stability in the past.  That problem was solved years ago but only one manufacturer has that technology.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 08, 2015, 08:30:31 pm
metal foil resistors in mold compound package may age from oxygen and humidity.
That's something, Vishay themselves explain, when they describe the advantages of their hermetically sealed AND oil filled VHP packages..

(These truely give < 2ppm/ 5years stability.)

To protect ordinary molded types, by an additional epoxy around , you would first have to bake them, to expel humidity, as you have to do that with each IC, which was exposed to air, in order to reflow solder them, without cracks.
The mold compound is like a sponge, which stores humidity.

I think, PWWs are protected quite naturally, because good quality wires have a very tight and relatively thick lacquer around them.

The equivalent were conformally coated metal foil types, maybe, like AE = Alpha Electronis (now also Vishay) offers them.
This is obviously an additional epoxy coating against humidity, etc.
See attached document .
The stability figures are not stellar, either. 


Mr. Pettis, you claim 5ppm/year stability for your resistors.. would you mind explaining, how you measured that, or how this stability figure can be derived from the mechanical / electrical design of your resistors?

Thanks a lot!
Frank

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on February 08, 2015, 08:57:23 pm
Hello Dr. Frank,

There are three methods for testing drift, the 'sitting on a shelf' at room temperature with periodic measurements, works great but takes a long time and also has increasing uncertainty in the readings over time if you are splitting hairs.  The next method is putting the resistors in an oven at a specified elevated temperature over a period of time and taking periodic readings, this takes less time but costs more to do, has the advantage of faster results and minimal changes in uncertainies.  The third method, which is what was used with my resistors, is the thermal shock testing, it is more expensive but provides results in a few hours and the uncertainty factor is essentially the same as when the testing began.

As I mentioned elsewhere, a third party subjected 50 resistors to 50 thermal cycles (+125°C to -55°C, 30 minutes, with no more than 2 minutes between temperature, well beyond the normal 5 cycles, the resistors were found to be superior in stability to every other resistor they had previously tested (even at 5 cycles) and with a zero failure rate.  I can state that most resistors had the equivalent drift of <2 PPM/year, but I still specify 5 PPM as some units can be a little higher.  It was also found in studies conducted by institutes of standards (Australians for one), that the Evanohm alloy (of which all of mine are made from or a derivative) has by far the best long term stability of any resistance alloy IF USED PROPERLY.   The actual design of the resistor (i.e. construction) has a very significant bearing on how well the resistor performs, while it is not impossible for other resistor houses to produce 0 ±3 PPM/°C resistors (check their specs, only over a limited range in most cases), their stability is significantly worse because of their construction.  It has been and still is believed by virtually all other resistor houses that they produce a welded, stable PWW resistor, when if fact, their own specifications prove otherwise.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 08, 2015, 09:06:36 pm
Andreas, thanks for the summary!

Epoxy; is that a physical limit, i.e. diffusivity of oxygen & vapor through epoxy well defined? There are tons of different epoxies available. I imagine the epoxies used in electronics are subject to dozens of requirements, diffusivity only one of them. Resistors for the LTZ1000 circuit, would not need high temperature life, fast curing, suitable for molding, good heat transfer etc.

Unless already well tested; I'm thinking setting up a bridge of two dipped, two un-dipped resistors, measure the ratio, soak them for a few weeks, leave measure for a few weeks to see if the ratios change. If there is no clear difference, I'd leave it at that.

I don't have a climate chamber so that's not an option for me.

I did ordered hermetical resistors about a month ago, 16 weeks delivery time :-| The dip idea is mostly out of curiosity.

Just dipping into epoxy will not really help. (May only change the time constant).
Title: Re: T.C. measurements on precision resistors
Post by: Joe Geller on February 08, 2015, 09:18:08 pm
Andreas, 

Hysteresis should cause the curve to end at different resistance values depending on how that point was approached.  For example cycling the temperature from 30 c to 28 c would give one resistance and cycling the same resistor from 26 c to 28 c would give a different end value.  That is different resistance values at the same Temperature dependent on the path taken to get to that same temperature.

On the other hand, if cycling the temperature causes a circular or oval pattern, that might not be hysteresis, but rather changes in the system such as different parts arriving at the new final temperature profile along their own slightly different paths.

Even with relatively slow cycling, I observed such curves with my small thick walled Hammond aluminum box in foam which I use as a micro-environmental chamber.  Yet, if I allowed the system to stabilize at a particular temperature, the voltage references came to the same voltage, independent of path.  If it was hysteresis, there would have been different stabilized voltages, depending on the temperature path to get to the same temperature.

One way to check your system is to choose different temperature points on the now rounded curves.  Approach the point from above or below, however once at the point, stay there for an hour or more.  It might end up that you see the curve eventually settles to a common value independent of the path to get there.  If so, that is not hysteresis, where the final path dependent values would be two different values.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 08, 2015, 10:15:12 pm

One way to check your system is to choose different temperature points on the now rounded curves.  Approach the point from above or below, however once at the point, stay there for an hour or more. 

Hello Joe,

nice to hear from you.

That is exacly what I did the last 2 days.

yesterday:
Setpoint 25 deg C for 3-4 hours
then Setpoint 47 deg C for 3-4 hours
then Setpoint 25 deg C for 3-4 hours.

today:
25 deg C for 3-4 hours with cooler on
7 deg C for 3-4 hours
25 deg C for 3-4 hours with cooler on
Just will have to evaluate ... tomorrow.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: branadic on February 09, 2015, 08:00:27 am
Quote
Epoxy; is that a physical limit, i.e. diffusivity of oxygen & vapor through epoxy well defined? There are tons of different epoxies available. I imagine the epoxies used in electronics are subject to dozens of requirements, diffusivity only one of them.

Normally expoy is modified to match thermal expansion factor of the substrate, in case of IC packagings the expansion factor of the mold is matched to that of silicon. In addition the mold is filled with glas beads to minimize humidity influence.

Regards, branadic
Title: Re: T.C. measurements on precision resistors
Post by: macfly on February 09, 2015, 06:51:36 pm
Hi volt-nuts,

the discussion comes very often to the same point: hermetically sealing.
So, why not putting the hole circuit in an oil filled case, perhaps additionaly
filled with dry nitrogen ?
My old standard resistor from L & N ist filled with 'medicale grade white oil',
which is used since today e. g. by WIKA / Germany for their resistance standards
(Castrol WOM 14).
But 'white oil' is not suggested to use with pure copper over a long time period.
I thought about a medical grade silicon oil.

Has anyone practical experience for sealing electronics with oil ?

Regards,

macfly
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 09, 2015, 08:28:58 pm
Seems simple  ;D

Can you get the glass "frit" in small quantity? What temperature is needed to melt it??
Title: Re: T.C. measurements on precision resistors
Post by: macfly on February 09, 2015, 10:07:25 pm
Wow, a clear and straight concept  8)
But for an hobbyist like me, it sounds a little bit oversized ... :)

I thougth more on a tin can case and using ceramic feed through cap's
which could be useable to get the signals in / out of the box.
These cap's are also available with copper / tin legs.

The questions at the moment are:

1. How do I handle various air pressure.
2. How do I handle temperature depending elongation.
3. Is there any type of corrosion between the used materials and silicon?
4. Will the feed through cap's be leak-tight enough for the next 20 years (or so) ?

... and possibly a lot more questions.

Regards,

macfly
Title: Re: T.C. measurements on precision resistors
Post by: splin on February 09, 2015, 10:12:42 pm
... and don't forget to do a helium leak test - it would be unfortunate to go to all that trouble and then spend the next several years characterizing the unit only to discover you have a microscopic pinhole.  |O
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 09, 2015, 11:00:10 pm
PTFE is very "airtight" to oxygen, probably to steam too. The bad news is that it can't really be formed by anything but mechanical machining. Warming it up does not help, it simply deteriorates.... It might be possible to make seals with PTFE and quality O-rings. Oxygen tanks / systems are made that way, both for liquid and gaseous oxygen.

Leakage test: Maybe better to fill with Argon. Almost as simple to test as Helium, cheaper. He is the second smallest molecule of all, will leak through "anything". But still, you can only detect relatively "large" leakages. One that empties the can in a few months, half a year would be very hard to find.

I thought of having a thin metal membrane showing inner pressure. As long as there is over-pressure the metal membrane would bulge out and you'd be safe from in-leakage and . A product someone?
 
Title: Re: T.C. measurements on precision resistors
Post by: texaspyro on February 09, 2015, 11:24:53 pm
Helium leak detectors can be exquisitely sensitive.  The best ones are basically mass spectrometers and can detect leaksof just a few molecules.  Helium is a very slippery little molecule and hates to be confined... if there is a way out, it will find it.

A poor man's leak detector can be a halogen (freon) leak detector used to find leaks in refrigerent systems.  Mine can detect leaks smaller than 1 gram/year.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 09, 2015, 11:44:54 pm
Well most of us don't have a MS in the toolbox. I do have a helium detector of the thermal type but doubt it can detect the low levels required.

As He has 1/5 th the density of air, one gram is ballpark a gallon / 5 liters......
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on February 10, 2015, 12:13:21 am
Is it possible to open and reuse the body of a hermetically sealed relay? I have seen pics of old bathtub style caps disassembled but I don't know if it is worth the effort.
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on February 10, 2015, 06:25:16 am
I thougth more on a tin can case and using ceramic feed through cap's
which could be useable to get the signals in / out of the box.
These cap's are also available with copper / tin legs.

Otto Schubert boxes - thats where my homebrew voltage reference lives in. With a ugly wire hole, I must confess.
You can get feed-thru caps with actual holes in it - so you can pull your inner wiring to the outside. But this forum told me that those develop cracks and don't stay airtight [citation needed]

Quote
I thought of having a thin metal membrane showing inner pressure. As long as there is over-pressure the metal membrane would bulge out and you'd be safe from in-leakage and . A product someone?


http://www.servometer.com (http://www.servometer.com)
I am aware of those, saw them on a fair once. Word is that you find a similar something in classic liquid-filled thermostats.
Title: Re: T.C. measurements on precision resistors
Post by: macfly on February 10, 2015, 07:52:42 pm
Hi volt-nuts,

thanks for your suggestions.

Hmmm, helium, argon .... please don't forget that I wanted to use medical grade silicon oil as main-filling, not gas. The dry-nitrogen (I mentioned before) is only to swamp out the remaining oxygen.

@ babysitter: why do this kind of compartments crack ?

Regards,

macfly
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 10, 2015, 08:15:03 pm
Hello,

its time to go again "on topic"

The "warm" cycle with fixed setpoints
25 deg C
47 deg C
25 deg C
minimum 3-4 hours each.

shows around 0.8 ppm deviation.
But it seems also that the time did not be sufficient to reach final value for the first 25 deg set point.
There is 0.2 K difference to the final value.
This would give togeter with around 0.25ppm/K for the resistor already 0.05 ppm.

the cold cycle (with fan from cooler box active)
shows no hysteresis.

But all in all time constants are a horror.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 12, 2015, 10:17:49 pm
Hello,

since I will still need some days for another test fixture I thought it was a good idea to test the reverse polarity again in a different way.

Instead of reversing the VREF voltage I put the resistor from face up to face down (FD) between the measurement clips.

So against measurement of 05.02.2015 in the measurement of 10.02.2015 only the resistor is sitting face down.

Surprise!! virtually no hysteresis above the noise level.

coefficients of LMS curve 10.02.2015

A 0 =  2.27445998893372E-0002
A 1 = -2.42945611854776E-0001  -> -0.24 ppm/K slope @ 25 deg C
A 2 = -7.22887593273244E-0003
A 3 = -3.87699048469777E-0005

maximum deviation:  8.57934015336459E-0001

On 11.02.2015 I reversed the VREF without any mechanical change.
So it is the same as the measurement of 06.02.2015 only with resistor face down.

coefficients of LMS curve 11.02.2015

A 0 = -2.71122470347014E-0001
A 1 = -2.45528403800187E-0001 -> -0.25 ppm/K slope @ 25 deg C
A 2 = -7.28122515035776E-0003
A 3 =  2.14549190916762E-0005

maximum deviation: 2.09427235197069E+0000


So the direction of the current is changeing the hysteresis from small to large.
And also the mechanical orientation of the resistor (face up / face down) has a certain influence (perhaps thermally).

Ok the resistor is not symmetrically. The leads are on the back side of the ceramic substrate and welded to the front side with the resistive element at the top of the resistor.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 13, 2015, 08:22:22 am
Hi Andreas,
That's very strange..
Is this resistor  identical to the Z201, you've sent me, 1k also?
Then I'll do the reversal test on my 3458A also.

Although i can't understand that behaviour.
Frank
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on February 13, 2015, 09:26:57 am
FYI I'm getting a pretty awesome 10K metrology grade transfer standard soon from Wekomm
2ppm tolerance
Long term stability better then 1ppm / year
Temperature stability better than 0.3 ppm / °C
And they are conservative specs, and they are working on producing 0.001ppm tolerance parts.
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on February 13, 2015, 09:54:34 am
Hi Andreas,
That's very strange..
Is this resistor  identical to the Z201, you've sent me, 1k also?
Then I'll do the reversal test on my 3458A also.

Although i can't understand that behaviour.
Frank

Do you have x-ray inspection at work or can you take it to a dentist to x-ray it? This might give some (pun intended) insight if it is not available in the datasheet.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 13, 2015, 09:59:17 am
Hi Dave,
This one, obviously:
http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/ (http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/)

Well, that's quite feasible for a good reference resistor, at a limited / fixed temperature range.

But not good for the described big temperature range, as the Vishay foil resistors show hysteresis.
And 0.001ppm stability or uncertainty?
Nope, never... This German engineering GmbH would need a Klitzing Hall standard.
Even the best secondary standard, the ESI SR 104 (which  practically has no hysteresis) does not approach that level..
And third, the SI Ohm is not better 'mise en pratique', than 0.2ppm .
That will probably change in 2018..

But you need now additional metrology grade , pornographic ppm instruments for testing..maybe a nice 3458A, or so.
Then,welcome at the volt-nuts!

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 13, 2015, 10:24:43 am
Hi Andreas,
That's very strange..
Is this resistor  identical to the Z201, you've sent me, 1k also?
Then I'll do the reversal test on my 3458A also.

Although i can't understand that behaviour.
Frank

Do you have x-ray inspection at work or can you take it to a dentist to x-ray it? This might give some (pun intended) insight if it is not available in the datasheet.

Me?
Nope.

There exist many Pictures of the ceramic /metal foil chip, with attached leads.. That always looks symmetrical, so no reason for asymmetric thermal behavior.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: JohnnyBerg on February 13, 2015, 10:42:56 am
Long thread to read. Very nice though  :-+

Would it be possible to get all the results in some kind of sheet or selection guide?
It would make it easy to choose a right resistor for the right job :)

Or, make a comparison against a poor mans 0.1% resistor like the RT0805BRD071KL from Yageo?
What do you get extra when paying $10 for a resitor, instead of $0,10?
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 13, 2015, 11:31:26 am
Long thread to read. Very nice though  :-+

Would it be possible to get all the results in some kind of sheet or selection guide?
It would make it easy to choose a right resistor for the right job :)

Or, make a comparison against a poor mans 0.1% resistor like the RT0805BRD071KL from Yageo?
What do you get extra when paying $10 for a resitor, instead of $0,10?

Are you kiddin'?
<sigh>

Well, you get:
T.C. as low as < 1ppm/K (thin film  25ppm/K)
Stability as low as 10ppm/yr. Down to 2ppm/6years (t.f. about 100...1000ppm/yr.)
Trimming (stable) down to 10ppm( T.F. 0.1% but not very stable due to technology. Heat and environmental influence easily or even deterioate initial trimming on the same order of magnitude)

Frank
Title: Re: T.C. measurements on precision resistors
Post by: JohnnyBerg on February 13, 2015, 11:56:44 am
T.C. as low as < 1ppm/K (thin film  25ppm/K)

I think Yageo specifies 20ppm/K

Thanks for the comparison. Do you have any idea about the spread of spec. between DUT's?
(Specialy when they sit next to each other on the tape.)

When the resistors are thermo coupled, and used as resistors in a circuit, absolute drift and accuracy is not as important as the spread between devices.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 13, 2015, 11:59:24 am
Great work! Surely enough for a paper!?!

 
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 13, 2015, 12:07:04 pm
Did you consider any SMTs?

Susumu RG and URG are avaliable to 2ppm/C, with URG series being supposedly a higher quality, better aging. Both have some kind of secret inorganic coating. Datasheet: "Unmatched Reliability and Excellent Stability at different environmental conditions".

Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 13, 2015, 12:13:14 pm
One thought;

Thermal cycling is sometimes recommended for removing mechanical stress on sensitive components. Do you have any thought on that based on the measurements?
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 13, 2015, 12:26:03 pm
T.C. as low as < 1ppm/K (thin film  25ppm/K)

I think Yageo specifies 20ppm/K

Thanks for the comparison. Do you have any idea about the spread of spec. between DUT's?
(Specialy when they sit next to each other on the tape.)

When the resistors are thermo coupled, and used as resistors in a circuit, absolute drift and accuracy is not as important as the spread between devices.

Well,
Thin film SMD resistors, like this 0805 , usually were produced from one ceramic board.
This is completely sputtered, and annealed, and then cut vertically and horizontally, to separate these hundreds of single chips from one ceramic board.
After laser trimming , attaching the metal contacts and protecting the resistance film by tight lacquer, the resistors were put into T&R.
Therefore, all resistors from a single board may have identical /similar T.C., but trimming and sorting into T&R from maybe different batches does NOT guarantee at all that you also get similar resistors by your order.. These will probably differ greatly, instead.

This is another reason, why you need the PWW or BMF technology, 'cause they already have smallest T.C. values and variations intrinsically by the technology implemented.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: quarks on February 13, 2015, 02:00:01 pm
This one, obviously:
http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/ (http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/)

That is almost identical to what I planned for a DIY project. They even have used the same binding posts (really very nice Model 2758, see att.) as I had on my BOM.

But I wonder why they use the VHA518-7 instead of VHA518-11Z (4-terminal), which I had on my BOM, before I bought a SR104.
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on February 13, 2015, 03:09:52 pm
Maybe the internal layout is similar to the Fluke 742A. Attached is a picture of the internals of my 742A-10K. It had a loose binding post and I was able to fix it before sending it to calibration.

Fluke used what appears to be four 39.992K resistors with a mica card resistor for trim. The lighting wasn't the best but you should be able to make out the five resistors.

I think they use Rose Bopla cases too. This part seems to match the dimensions http://www.digikey.com/product-detail/en/07051100%2050/902-1220-ND/2499610 (http://www.digikey.com/product-detail/en/07051100%2050/902-1220-ND/2499610)
Title: Re: T.C. measurements on precision resistors
Post by: quarks on February 13, 2015, 03:20:26 pm
Maybe the internal layout is similar to the Fluke 742A. Attached is a picture of the internals of my 742A-10K.

Thank you very much!!!
I have been searching for this inside look for years.  :-+
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 13, 2015, 04:25:30 pm
Big Thanks for showing this photo.
Very crude construction, but effective.
These four 39.99k will be matched pairwise for T.C. cancellation, to less than 0.5ppm/K, usually.
A big pity, that they did not implement a thermometer with R(T) curve.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on February 13, 2015, 04:45:45 pm
Thanks for posting the internals of the 742.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 13, 2015, 08:59:50 pm
T :bullshit:
FYI I'm getting a pretty awesome 10K metrology grade transfer standard soon from Wekomm
2ppm tolerance
Long term stability better then 1ppm / year
Temperature stability better than 0.3 ppm / °C
And they are conservative specs, and they are working on producing 0.001ppm tolerance parts.

For the tolerance, you really meant 0.001% [10ppm], not "0.001ppm", right?

I don't think there is any way to even *measure* 0.001ppm-- well, maybe with a QHR and cryogenic comparator, but I think the leads connecting to the QHR inside the liquid helium Dewar will prevent you from getting to 0.001ppm [because the leads are not superconductors].

So, a "typo" right?

Definitely something < 1ppm. Maybe one zero too much..
Comparison between two QHRs has been done -as you describe correctly- by SQUID or so, to 1^-18 uncertainty.
Problem, like JJ array output, is the transfer from quantum to analogue world, which is limited at about 1^-9, due to noise @ 300K and thermo voltages of about 1..10nV.

Therefore, I can't await 2018, ie the new kilogram, the new Volt and Ohm, and so on..

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 13, 2015, 09:13:50 pm
Can you show us a picture of what is "face up" and what is "face down" and which side is closest to your temperature measuring device in each case?

Hello Ken,

the arrangement is shown on page 15 in post
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg599876/#msg599876 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg599876/#msg599876)

picture:
(https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=133748)

So face up means that you can read the resistor value and the tolerance from top side.
Face down in this context is when you read the brand (VISHAY), date code (B0940-) and resistor type (Z201T).

The reference temperature sensor (NTC2) is the one lying left of the resistor.
NTC1 is right and NTC_Legs is between the legs.
The NTCs are not calibrated so they have up to 0.3 deg C difference against each other.
For the T.C. measurement I will need only temperature differences and not the absolute value.

When you look at the "naked" Z-Foil resistors (VAR) for "high end audio" applications you will see
that it is impossible that they behave symmetrically.

http://www.vishaypg.com/docs/63140/var.pdf (http://www.vishaypg.com/docs/63140/var.pdf)

Is this resistor  identical to the Z201, you've sent me, 1k also?
Then I'll do the reversal test on my 3458A also.

Hello Frank,

you got the Z201#2 which has the same date code (B0940-) as the Z201#3 which I have
used for the last measurements. Both are 1K resistors.
And thanks again for testing.
If you get different results then we have a problem to find the root cause.
One could be different measurement current. Around 2.5 mA in my case and around 1 mA in your case.
So I have factor 6.25 more self heating of the resistor.

There exist many Pictures of the ceramic /metal foil chip, with attached leads.. That always looks symmetrical, so no reason for asymmetric thermal behavior.
They are symmetrical if you mount them in upright position.
But from front and back side you should have different thermal resistances to the surface.
The resistive element (metal foil) is on the front side then comes the aluminium substate then the wires on the back side.
I guess that the epoxy resin is thicker on the wire side than on the metal foil side.
The problem is to know if vishay does really do consistent printing or if they dont mind printing the values on the "wrong side".
http://www.vishaypg.com/docs/63187/zseries.pdf (http://www.vishaypg.com/docs/63187/zseries.pdf)

Or, make a comparison against a poor mans 0.1% resistor like the RT0805BRD071KL from Yageo?
What do you get extra when paying $10 for a resitor, instead of $0,10?

Hello Johnny,

I already made a comparison to a "good" (more in the 1$ range) thin film resistor (RC55Y type) with 15ppm/K spec.
See page 1 of the thread. The question is how much effort is done for long term stability.

Did you consider any SMTs?
Susumu RG and URG are avaliable to 2ppm/C, with URG series being supposedly a higher quality, better aging. Both have some kind of secret inorganic coating. Datasheet: "Unmatched Reliability and Excellent Stability at different environmental conditions".

I think I have already mentioned it in the thread: Definitively no.
The reason is simple: SMTs cannot be measured alone. (At least it makes no sense).

I will in each case get the humidity coefficient and the thermal expansion coefficient of the epoxy material or ceramic substrate where the SMTs are mounted.
It is simply useless to specify T.C. coefficients of SMTs (below 20ppm/K) without exact definition of the mounting method.
With through hole devices I can decouple the resistor from PCB (expansion) by the mounting method.

I think you can reach the 2ppm/K only on ceramic substrates with same thermal expansion than the resistor.
But never on a standard epoxy PCB which shrinks and swells with humidity.

Thermal cycling is sometimes recommended for removing mechanical stress on sensitive components. Do you have any thought on that based on the measurements?
Heating removes humidity from PCB. So the following thermal cycle will get lower hysteresis.
Thats an old trick to let voltage references with plastic package look better in the datasheet.
But it will not help you in real life applications. (except when you heat up the whole device).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 13, 2015, 09:28:55 pm
Sorry I wasn't clear: what I meant, implications of cycling a whole completed assembled PCB to relief mechanical stress on components?
Thermal cycling is sometimes recommended for removing mechanical stress on sensitive components. Do you have any thought on that based on the measurements?
Heating removes humidity from PCB. So the following thermal cycle will get lower hysteresis.
Thats an old trick to let voltage references with plastic package look better in the datasheet.
But it will not help you in real life applications. (except when you heat up the whole device).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 13, 2015, 09:42:48 pm
Sorry I wasn't clear: what I meant, implications of cycling a whole completed assembled PCB to relief mechanical stress on components?
I fear my English is not good enough to understand what you mean.
But also a pcb stores humidity and gets dry by thermal cycling for the next measurement cycle.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 13, 2015, 09:54:39 pm
Or it is my English. Not native....

One more try: It is sometimes recommended to thermally cycle a whole assembled board, to get rid of stresses on components, caused by mounting and soldering. If you do that kind of cycling, then what happens to resistors? Is it possible to make a guess, if thermal cycling of a board assembly causes more harm that it does good?


Sorry I wasn't clear: what I meant, implications of cycling a whole completed assembled PCB to relief mechanical stress on components?
I fear my English is not good enough to understand what you mean.
But also a pcb stores humidity and gets dry by thermal cycling for the next measurement cycle.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 13, 2015, 10:33:08 pm
Hello Jan,

interesting question.

Usually thermal cycling or burn in is used to sort out the boards with "early failures" (bathtub curve/Weibull)
so enhancing the reliablity at least of those who survived.
So I would not stress them much more than that what can be expected in later usage.

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 13, 2015, 11:18:14 pm
 :)

About thermal hysteresis and relief by controlled thermal cycling, you might enjoy this : :)

http://www.google.com/patents/US5369245 (http://www.google.com/patents/US5369245)
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on February 17, 2015, 05:54:49 pm
Regarding the Z201T and similar physical types, the thermal resistances are not identical from the resistance element to the 'front' or 'back of the molded package.  Since the element is mounted on a ceramic substrate, using the element as the 'front' side, the thermal resistances are slightly different.  The element goes through a slight gap to the molding (which is a pretty good thermal conductor) then to the surrounding air.  From the element to the 'back' side, it goes through the ceramic substrate, through another slight gap, to the molding then to the surrounding air, the ceramic substrate being the main difference in thermal paths between the 'front' and the 'back'.  While standing the resistor on end will provide the most equal path from the sides to the air, the internal paths are not identical.  Vishay is also correct in that the leads are the primary thermal conductors to the outside world being directly connected, as such, to the resistance element.  Laying the resistor on its 'front' or 'back' will cause a larger difference in the thermal paths to the air unless the resistor is mounted to a heatsink, which will provide a lower thermal resistance but will again unbalance the thermal paths.

'Burn-in' has been around for decades, primarily used for the military who demanded the very best possible of the times.  Later it began to extend down into industrial applications where reliability was fast becoming very important as well.  As a general rule, burn-in has not been used for commodity electronics because of the cost and the general attitude that consumers really don't care much about whether the electronics (or mechanics for matter) last for more than a few years.

In the case of using burn-in, it varies with the purpose, in the case of individual components, it is often used to 'stabilize' the component and in turn, weed out the ones which are not good enough.  In the case of assemblies, burn-in has a similar intention of finding possible premature failures, in the case of high reliability assemblies, most of the components have already been through the stabilization bake, the main purpose of doing it to assemblies is to find bad connections of any type.  This particular type of 'burn-in' does not stress the assembly any higher than the maximum working temperature of the assembly whereas individual components may be stressed above their rated limits already before assembly.

As noted earlier, the type of PCB material can significantly influence board level difficulties, soldering is one of the worst offenders these days.  The TC of most common PCB materials, such as FR-4 and similar types can be quite significant and many are sensitive to humidity, all of which are a bane to SMT components.  The fact that everyone is trying to shrink components even smaller is only contributing to the problems and making it worse.  Leaded components rarely had any problems with the normal board stresses but leaded components are on the hit list, manufacturers are doing their best to drop them from active production.  Burn-in can uncover the faults on a board assembly but it doesn't fix the root cause.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 17, 2015, 07:15:35 pm
Well, the Vishay BMF resistors are symmetrical concerning the wires, i.e left / right orientation.
There is a theoretical difference in thermal resistance between front/rear orientation, that's right.

But I really think, that's not relevant at all for such measurements and concerning this observed hysteresis, or to say more precisely, temperature lagging.

As the leads transport 90% or more of the heat, there might only be such an effect, if there would be a different coupling over the wires.
Let's say, one would be directly attached to the ceramic body and the resistive foil, and the other would have been bonded..

(That's a well known problem on power semiconductors and on LEDs).

Anyhow, as the Pomonas won't show up before midth of March, I already assembled Andreas Z201 into another aluminium box..

The small alu block takes care, that  front and backside of the resistor is virtually on the same temperature, as is the NTC sensor.

This alu block is a thermal shortcut as the outer alu box, so everything inside will be on the same temperature +/- a few hundredths of a Kelvin, if the measurement is done slowly enough.


Frank
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 17, 2015, 10:38:09 pm
[...]so everything inside will be on the same temperature +/- a few hundredths of a Kelvin, if the measurement is done slowly enough.

With your measurement septup, it's not likely to happen.

Again : The main thermal conduction path is the leads of the resistor, not the case. You should make a proper thermal contact on the leads, not on the case. Your ugly purple/grey/black/white thick wires are going sink a lot of heat from your resistor, and cause a temperature difference between the resistor element and your measured temperature (on the block) in an order of magnitude which exceed "several milidegrees".

Making a precision thermal system is similar to precision analog electronics. Since you supply the heat from a finite "impedance" source (The conductivity of aluminium between your heater and the resistor), for high precision applications you have to minimize leakage from the resistor to the outside. Make your purple/grey/black/white wire as thin as possible!

Nice thermal contacts via the leads + Small leaks via thin wires and polystyren-like insulation everywhere + A low mass = A fast and precise thermal system.

You got it right, Dude,

these ugly cables are really thin, indeed. So they won't transport that much heat.

Due to the big mass of the alu block and box, there really will be no temp differences inside than more than a few 10 mK..

As the whole alu box is also situated inside an isolation box (for cooling beer), there is a very  even temperature distribution, from the bottom to the top of the (beer) box.
Through this, the cables have to pass also, again reducing possible temperature differences from the cables themselves.

Well, the most perfect setup would be, to "cover" the leads of the resistor also to the alu box, but I had no time to do so..

Anyhow, if there really is a temperature lagging between the resistor and the alu box (i.e. the NTC), this will be visible in the measurement.. If there is  no or a very small measurement loop during fast cool down or fast warm up, then your theory of heat transfer via this cable is busted.

If I get a measurement loop even on slow warm up / cool down, then you 'd be right, and I would have to improve my setup.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 18, 2015, 11:06:14 am
these ugly cables are really thin, indeed. So they won't transport that much heat.

Sure they will transport some ugly heat! :D

Let's say they're #24 AWG (~0.5mm diameter of pure copper) even if they appear thicker than that. When your chamber is at 50°C, the ambient at 25°C, and there is 30cm of such wire connecting the DUT to something at ambient temp, let's make some simple calculations of what happens :

Thermal conductivity of copper=385W/(mK). Thermal resistance of your two pair of wires : 4 wires * 385 * Pi*0.00025² / 0.3 = 0.001 W/°C <=> 1000°C/W
Thermal resistance of the block-resistor_under_test interface : 2W/°C <=> 0.5°C/W  (Very favorable. It's the typical interface resistance of a well mounted power transistor with silicone pad or grease. Or the minimum available junction to case thermal resistance of a TO220 package)

Thermal flux through the resistor interface and the wires = Delta Temp / Rth = 25°C / (1000 + 0.5) = ~25mW

These 25mW cause through the interface a temperature loss of : Tloss = 25mW * 0.5°C/W = 12.5mK

And it's a very very favorable case.

Don't forget that when you add aluminium mass to improve temperature distribution, you decrease the thermal resistance between two points, but you also add thermal capacity. At some point, the "induced" thermal capacity increases beyong what's acceptable, regarding to the benefits of a lower thermal resistance.

From a dimensional analysis point of view, the thermal resistance is a matter of contact/tranverse area, and the thermal capacity a matter of volume. The volume increases with the cube of a dimension of your system, and the tranverse area only with the square. If the figure of merit of your system is Resistance/Capacity, it varies with x^2/x^3 = 1/x ("x" being a dimensions of your system) : Small systems are naturally favored.

Yes, your approximate calculations seem to be right, and  I also think, they are even on the correct order of magnitude, i.e. not especially favourable parameters chosen.
I used 1,27mm ribbon cable, that should have AWG 28 wires, i.e. 7 x 0.125mm.
This already gives a factor of 2 higher thermal resistance, therefore in the end 6mK temperature difference only.

You also have to consider, that there is some heat transfer between the wires and the surrounding air, and on the first 5 cm, to the temperature of the alu box (it's been taped to the box there) .

The maximum temperature differences are also much smaller, 15°C at most (10°C .. 35°C in the alu box, 20°C room temperature)

So, your calculations even support my estimations which I made for the alu box itself: I.e. less than 1/100 K temperature difference caused by the wires! That's nearly a perfect thermal isolation for that used case, and can be taken out of consideration.

This is also confirmed by the first measurement, I quickly did yesterday night and this morning:
There was no observable temperature difference / shift, due to fast  cooling or heating  ..


Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 22, 2015, 05:23:48 pm
Hello,

to find out what created the results of the posts of 08.02.2015 and 13.02.2015 with "rectifier effect" and different hysteresis face down and face up I did further measurements with a new setup which are more towards the "Emmanuel" style.

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg603949/#msg603949 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg603949/#msg603949)
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg608306/#msg608306 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg608306/#msg608306)

first Img1866 shows the "face down" setup of the previous measurements.

Img1868 the new setup "face down" which takes more care of the fact that most of the heat is transferred by the wires of the resistor.
2 thin (2 mm) AL-rails with a window for the resistor body and the main NTC.
The wires of the resistor are fixed between heat conductive silicon foils for isolation and heat transfer.
The measurement clips that I used for the last measurements are replaced by solder joints as in the first measurements.
(they would not really fit between the silicon foil).

Result:
1. the dependancy on polarity has gone.
2. the hysteresis is back nearly equally on all measurements.

So I blame the dependancy on polarity to the measurement clips.
(perhaps some bad contact due to oxidation).
The hysteresis is independant wether a fan is used with open TEKO enclosure or with closed TEKO enclosure.
So there seems to be a larger temperature constant within the resistor.

With best regards

Andreas

Measurements:

15.02.2015 Z201#3 face down normal polarity
16.02.2015 Z201#3 face down reverse polarity

17.02.2015 Z201#3 face down reverse polarity with fan
18.02.2015 Z201#3 face down normal polarity with fan

21.02.2015 Z201#3 face up normal polarity
22.02.2015 Z201#3 face up reverse polarity


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 22, 2015, 06:00:15 pm
Hello,

just to see if the self heating of the resistors creates the hysteresis
I did a measurement with reduced "half bridge" voltage of 2.5V instead of 5V.
Reducing the measurement current from 2.5mA to 1.25 mA.
So I am nearer to the comparative measurements of Frank.
His HP3458A has 1mA measurement currrent for the 1K range.

Edit:

The measurement of 19.02.2015 is with FAN,
can be compared with measurement of 18.02.2015 except 2.5V

The measurement of 20.02.2015 is without fan so corresponds to measurement of 15.02.2015.

Since the power is 1/4 with 2.5V I would expect a significant (factor 4)
change in hysteresis which is not visible.

Of course the ADC noise remains the same so I get a worse signal to noise ratio within the measurements.
The noise appears about doubled with 2.5 V bridge voltage.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 22, 2015, 07:13:59 pm
Hello Ken,

2.5 mA in 1K gives 6.25 mW during measurement.
Pulsing is not possible within my setup since this would affect signal to noise ratio.
(I have to average many measurements to achieve around 1uVpp noise).

Temperature slope is 0.12K/minute. So with 7.2 deg C / hour I am not far away from your proposed 5C/hour.
And already this takes a whole day to measure when regarding that I cannot use the first 2 hours
because otherwise I get a "open" hysteresis curve. This fact is a indication that the mold around the resistive
element has some "creeping" effect.

Edit: see attached measurement from 21.02.2015 from 20 deg starting downto 10 deg up to 45 deg then down to 15 deg
instead of using only the part of 10 deg up to 45 and then back to 10 degrees C in the previous post.

Frank already emailed me that he gets similar results with his setup for my Z201#2 and S102#1.
But not with his old (green) Z201 with 49.999K
So for now the best guess is that the changed mold (green to black) of the resistors is guilty for the hysteresis.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on February 22, 2015, 07:28:23 pm
All resistors have hysteresis, even those Thomas 1 ohm standards that have been sitting in constant temperature baths at NIST for decades has hysteresis by virtue of the fact that every time any, and I emphasize any, power is applied, it creates a history of change, the current 'value' of a resistor depends on its current state plus all of its prior states over time that has affected it.  Depending on the type of resistor and how it is made determines just how much reaction a particular resistor will have to these changes in states.  The more stable a resistor's characteristics, generally speaking, the smaller the hysteresis will be to the current stimulus.

Foil resistors can be quite 'stable' but can also have relatively high hysteresis because of their construction, the foil has very little mass and will suffer greater hysteresis to the same stimulus than a wire wound resistor might.  It is not solely the function of mass that bears on hysteresis but it can have a significant effect on the out come.  Hysteresis can also be difficult to isolate from other temperature effects, for example, the starting temperature of the resistor should be very accurately known because after the stimulus, the temperature should be returned to the original value to accurately assess hysteresis.  Any errors in temperature will manifest as an apparent hysteresis.  Another source of error is the measuring instrument, just how stable is it over the given time period in which the hysteresis is to be measured, when attempting to measure very small changes, every little detail in the chain becomes very important.  You can't measure PPM or sub-PPM measurements accurately unless you can control all of the variables sufficiently to minimize the error sources.  Are thermal EMFs known and under control?  The entire measurement chain is part of the measurement and result.

If you have analyzed your circuit needs accurately and have correctly picked the right resistors for the job, then hysteresis will likely play little if any role in the long term operation of your circuit.  In the case of these LTZ1000/A reference circuits, hysteresis, unless unusually large, will play an insignificant role in the performance and long term stability.  Under the normal operating conditions of the LTZ (and that includes fairly harsh environments) the noise floor is the limiting characteristic, most of the DC performance is buried in it, for these commercially available versions, real drift on the order of 1-2 PPM/year is as good, on average, as it is going to get.  Very long term drift may even get a bit better with time but like all Vref chips, their performance varies a bit chip to chip, someone is going to get a really terrific sub-PPM drifting chip (after some long period of time), others wouldn't be that good even after a few years.  Trying to change the chip's inherent characteristics by tweaking outside components or internal currents is pretty much a waste of time.  This chip was designed for best performance with the given information from Linear Tech.

The folks at Linear Technology knows as much as anybody about how to get the best performance out of them, if better performance is possible (commercially) then I'm sure Linear Tech would announce it, as it is, the LTZ is the best chip you can get your hands on and the best thing you can do to keep that performance is to use very stable, low noise resistors around it (and that eliminates a lot of resistors, including most of Vishay's).
Title: T.C. measurements on 5 precision resistors
Post by: Dr. Frank on February 24, 2015, 08:56:46 pm
I measured R(T) / T.C. for 5 different precision resistor technologies, all with the same setup, i.e. using the 3458A with 4W and Offset Compensation, a precision NTC, in the same thermal box, with an aluminium DUT holder, and with equal temperature profile.

The leftmost, black  ones are 2 bulk metal foil resistors from Andreas, Z201 #2, Z-foil, TC < 1ppm/K and S102JT #1, C-foil, TC ~ 2ppm/K both have 1.0000k, 0.01%, Vishay brand.

The 3rd, green resistor is also a Z201, 49k99, 0.01%, but in an older packaging from about 2004.

The 4th, blue resistor is a bulk metal foil from Alpha Electronics, type FLCY,  TC typ. 2.5ppm/K, 25k, 0.1%

The 5th is an econistor from G.R., 1k, 0.1%, TC < 5ppm/K

The 1k resistors were measured at 1mA, the 25k and 49k99 at 100µA

Stability over time of the 3458A was monitored by an external 10.00000k reference resistor, and its deviation after the hours of experiment was < 0.5ppm in all cases.

The thermal box was inserted in a small beer cooling box, so that a stable air column was formed inside, which gives a continuous temperature layering, from cold at the bottom to room temperature at the top of the box. (picture shows measurement on another box)

By this 3 fold thermal encapsulation, and the thermal inertia of aluminium block and box, NTC sensor and DUT were on the same temperature during the slow temperature changes.

Inserting a cooling pad quickly cooled the assembly down to 10°C, and an incandescent lamp, by its IR emission, quickly heated to about 40°C.


The results show a pronounced hysteresis figure of both black BMF resistors, especially as the end values do not match the  starting values.
Also, a virgin curve can be observed.

After the thermal cycle, both resistors were held at a constant temperature of about  26°C.
A creeping of the (hysteretic) resistance value, directing towards the initial value, could be observed, in both cases about 1ppm/30min.
The Z201 shows a very bulged / distorted hysteresis loop, probably due to the creeping behaviour.

At this point, I have to apologize Andreas, for casting doubts on his measurements!
His measurements setup, using a much lesser costly equipment, gave qualitatively same or very similar results, and they were nearly on the same order of noise figures.
Well done, Andreas!

Compare here: https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462299/#msg462299 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462299/#msg462299)
and here: https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303)



The three latter resistors do not show hysteresis, identifiable by the fact that the initial resistance value at ~ 26°C always closely matches the value after the temperature cycle.

Therefore, only a slight temperature lag between sensor and DUT can be observed, due to fast cooling/heating.


For the black BMFs, I can hardly assign a T.C., due to the hysteresis.
The Z201 changes about 5ppm in a window of 30°C, that's about  0.2ppm/K average TC.

The S102J is practically useless as a stable resistor, as the big hysteresis spoils the 2ppm/K specification.

The old Z201, 49k99, has a linear TC of +0.5ppm/K, w/o hysteresis.. very good.

The AE FLCY, 25k has a slightly hyperbolic characteristic; the linear TC would be around -0.9ppm/K.

The econistor has a relatively high, linear TC of about 4.6ppm/K.
I've already seen a better sample, of about +2ppm/K, recently.
Anyhow, I will have to measure all the econistors, 5EA of 1k and 12k, for my LTZ1000 , to match these pairs for best overall TC of the reference circuitry


There's one question left, for discussion:

I really wonder, why these newer, black BMF from Vishay have this hysteresis and creeping effects, in comparison to the oder Z201, and also the VHP202Z (in AT51 XTAL type can) I also have measured.
Maybe, that the smaller mold case causes this effect, or they may have simplified the inner construction, maybe omitting a mechanical buffer layer over the resistance element.
 
Title: Re: T.C. measurements on 5 precision resistors
Post by: janaf on February 25, 2015, 10:27:22 am
Good to see Dr Frank's results match Andreas's!

I'm not that surprised that Andreas is getting useful and accurate results. The setup uses differential measurements all the way and the signal changes are small, ie ADC DNL, noise, resolution and short term stability means more than INL and absolute accuracy. However hard to quantify....

It also seems the "thermal side" of his setup has also developed a lot compared to the first tests.

Yes, Well done Andreas :-+

At this point, I have to apologize Andreas, for casting doubts on his measurements!
His measurements setup, using a much lesser costly equipment, gave qualitatively same or very similar results, and they were nearly on the same order of noise figures.
Well done, Andreas!
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 25, 2015, 10:52:11 am
The lead material of the S10XX series is solder coated copper while the Vishay hermetrics are Kovar. I did not look up the lead materials of the other resistors.

The EMF of Kovar - Copper is 40uV/C while solder - copper is somewhere near 1uV/C

The 1uV/C doesn't make me worried but 40uV/C does. Any thoughts on that? Something seen on measurements?
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on February 25, 2015, 11:10:20 am
The lead material of the S10XX series is solder coated copper while the Vishay hermetrics are Kovar. I did not look up the lead materials of the other resistors.

The EMF of Kovar - Copper is 40uV/C while solder - copper is somewhere near 1uV/C

The 1uV/C doesn't make me worried but 40uV/C does. Any thoughts on that? Something seen on measurements?

The 3458A Ohm function cancels all such emf voltages, by its Offset Compensation feature.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: quarks on February 25, 2015, 11:14:42 am
The lead material of the S10XX series is solder coated copper while the Vishay hermetrics are Kovar.

When I have a look at all my VPG Z-Foil resistors, they are all tinned.
I wonder where and why VPG should use Kovar.
Can you share where you saw the Kovar information?
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 25, 2015, 11:20:41 am
Still worried abouth the 40uV/C in real life applications. A change in 1/40C between the two pins of a resistor would cause 1uV error even if this can be cancelled with current reversal in measurements.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 25, 2015, 11:22:16 am
I may be completely wrong but I was assuming the hermetic / metal cans use Kovar?

The lead material of the S10XX series is solder coated copper while the Vishay hermetrics are Kovar.

When I have a look at all my VPG Z-Foil resistors, they are all tinned.
I wonder where and why VPG should use Kovar.
Can you share where you saw the Kovar information?
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 25, 2015, 11:26:02 am
Wrong thread, I know, but it's the same problem with the LTZ1000ACH.

From the LTZ1000ACH datasheet:

Quote
The kovar input leads of the TO-5 package...
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on February 25, 2015, 12:14:00 pm
Hi Dave,
This one, obviously:
http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/ (http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/)

Yep, that's the one.

Quote
But not good for the described big temperature range, as the Vishay foil resistors show hysteresis.

These are not standard Vishay parts, they are:
Quote
custom specific made part, together with a special packaging.

Quote
And 0.001ppm stability or uncertainty?
Nope, never... This German engineering GmbH would need a Klitzing Hall standard.
Even the best secondary standard, the ESI SR 104 (which  practically has no hysteresis) does not approach that level..

They claim to be working with  PTB (the german national standard institute) to achieve that tolerance on demand.

My resistor has been tested and is about 0.5ppm / °C
and is about 7.7ppm off nominal 10K.
They expect that to drift by up to 0.1ppm by the time I get it.

Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on February 25, 2015, 12:20:59 pm
For the tolerance, you really meant 0.001% [10ppm], not "0.001ppm", right?
I don't think there is any way to even *measure* 0.001ppm-- well, maybe with a QHR and cryogenic comparator, but I think the leads connecting to the QHR inside the liquid helium Dewar will prevent you from getting to 0.001ppm [because the leads are not superconductors].
So, a "typo" right?

No, that is what they claim they are working on in conjunction with PTB
http://www.ptb.de/index_en.html (http://www.ptb.de/index_en.html)
I have no idea if it is feasible or not.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 26, 2015, 12:35:38 am
I looked up a few Vishay metal can hermetics like VHP100 series and the DON't use Kovar! Relieved!
Quote
Lead Material #22 AWG (0.025 Dia) Solder Coated Copper
Title: Re: T.C. measurements on precision resistors
Post by: barnacle2k on February 26, 2015, 01:42:34 am
I looked up a few Vishay metal can hermetics like VHP100 series and the DON't use Kovar! Relieved!
Quote
Lead Material #22 AWG (0.025 Dia) Solder Coated Copper


I don't think that info is correct.
Because copper to glass seals don't work and break due to dissimilar thermal expansion coefficients.

Edit: Ok thanks to diligent for the more detailed info on that.

Anyways: the results for the Z201 Resistors are ALARMING and i think at this point it we should contact Vishay about this.

I am now thinking about ordering those Audiofoolery naked Zfoils for my Project, since the whole circuit will be hermetically sealed and under transformer-oil anyways.
(Much cheaper then the hermetic zfoils)
Title: Re: T.C. measurements on precision resistors
Post by: juani_c on February 26, 2015, 01:03:51 pm
A quick question:
I've seen T.C.R expressed as "±5x10-6 /K", is that the same as "ppm/ºC"?
Title: Re: T.C. measurements on precision resistors
Post by: bdivi on February 26, 2015, 01:05:51 pm
Yes, ppm is abbreviated Parts Per Million which is exactly 1.10e-6
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on February 26, 2015, 05:33:24 pm
Either regular solder or silver solder may be used for hermetic sealing, depends on the application, I sealed many a mil grade hermetic resistor at Ultronix with regular solder, in some cases, particularly the large, oil filled units, sealing could be a bit tricky.  Not all hermetic parts contain oil but they are not nearly as common as the oil filled units.  I have not heard much about the use of no-lead solder for hermetics, most of the time the hermetic parts are exempted anyway.

In real life, it is not that difficult to isolate the LTZ and circuitry from air currents, it is being done many times over in critical applications without any sign of excess drift.  Encapsulating the unit is not difficult, foam works excellently and is not difficult to obtain or use, most anything with very low thermal conductivity.  If the Kovar leads were a real problem, the folks at Fluke, HP, NIST, et al, would have been screaming at Linear Tech about it.  This is not the first time Kovar has had to be used in ICs.  Yes, it does require a bit more effort than if Kovar hadn't been used but obviously Linear Tech deemed Kovar leads necessary.
Title: Re: T.C. measurements on precision resistors
Post by: texaspyro on February 26, 2015, 06:36:45 pm
A seller on Ebay is selling currently sheets of aerogel insulation for affordable prices.  Search Ebay for "SPACELOFT".
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 26, 2015, 09:22:01 pm

Anyways: the results for the Z201 Resistors are ALARMING and i think at this point it we should contact Vishay about this.

I am now thinking about ordering those Audiofoolery naked Zfoils for my Project, since the whole circuit will be hermetically sealed and under transformer-oil anyways.
(Much cheaper then the hermetic zfoils)

Hello,

I do not find this alarming. This is only a feature of the resistor which is not specced within the datasheet like humidity sensitivity. (datasheet = advertisement).  And you still have the possibility to use hermetically sealed resistors.

Usually you should read between the lines: which parameters that are of concern for your application are _not_ covered within the "datasheet".

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 26, 2015, 10:27:09 pm
Andres, not sure I have seen: the S102 are available in C, K and J where the K alloy should be slightly more temperature stable than C, while J is far behind. You measured the J and it was walking around a lot. But then there are measurements of S102x, x not specified. I think the early S102 later  became S102C?   

So, have you measured C and K alloys, can you see any difference?
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 26, 2015, 10:45:06 pm
Andres, not sure I have seen: the S102 are available in C, K and J where the K alloy should be slightly more temperature stable than C, while J is far behind. You measured the J and it was walking around a lot. But then there are measurements of S102x, x not specified. I think the early S102 later  became S102C?   

So, have you measured C and K alloys, can you see any difference?

Hello,

From S102 the C and J are the same alloy. Only the lead pitch differs (3.81 mm / 5.08 mm).

I do not know where you looked at but some values differ for the low ohmic and high ohmic types.

Up to now I did not get a K-type alloy (which is not so commonly stocked at the distributors).

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: janaf on February 26, 2015, 11:09:48 pm
I can send a couple of S102K if you like.
560R, got plenty.
2K, 4K, 10K I could spare 2 each. All NOS.
Sorry, no 1K.
Which values are best for you?
PM your address and I'll put them in the mail.
Title: Re: T.C. measurements on precision resistors
Post by: barnacle2k on February 27, 2015, 07:40:14 pm
I do not find this alarming. This is only a feature of the resistor which is not specced within the datasheet like humidity sensitivity. (datasheet = advertisement).  And you still have the possibility to use hermetically sealed resistors.

Usually you should read between the lines: which parameters that are of concern for your application are _not_ covered within the "datasheet".

[rant]

Am i really the only one upset that Vishay creates one of the lowest tempco resistor on the planet and then packages it into a package that completely negates the advances in the resistor technology?

If i pay more then 30 bucks for a resistor i expect superb quality and not horrible hysteresis behavior that makes that resistor completely useless for precision applications.
These results just ruled out anything but their most expensive resistors for my project.
The hermetic's are at least double the price.
(Maybe i'm just mad about blowing the budget again)

[/rant]
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 07, 2015, 07:03:59 pm
Hello,

to come back to Franks measurements of my Z201#2 resistor with a direct comparison of his and my results.

- First attachment is the measurement of Frank.
  Z201_1k.jpg

- to make the comparison easier I have transformed it to my more or less standardized format.
  20150221_TC_Z201_2_LMS_Frank.PNG

- since he uses around 12000 seconds for his 32 degree temperature profile (around 0.36K/minute)
  this corresponds more or less to my "fast" measurement of 0.3K/minute.
  20140729_TC_Z201_2_LMS_temp_corr_fast.PNG

- since I have seen that at least with my setup (with the wires outside of the metal case)
   the results are not so good reproducable than on Franks setup I have reduced the ramp speed to 0.12K/minute.
   Another difference is that on my ramps there is some rest time at the temperature end points so that the
   creeping effect of the mold has more time at the end points.
   And of course the measurements slightly differ in end temperatures
   since with my setup it is easier to reach higher temperatures in summer
   and lower temperatures in winter which has also a slight effect on the hysteresis.
   20140801_TC_Z201_2_LMS_temp_corr.PNG
   20150123_TC_Z201_2_LMS_temp_corr.PNG

All in all there is a good coincidence of the measurements if regarding the boundary conditions.
Of course Franks measurements have less noise with his better setup.
But the resulting LMS curves look very similar under same conditions.
My target is to select from some resistors that I have the best fitting resistors
for my next LTZ1000A reference.
So for the voltage divider (1K/12.5K) this is a similar behaviour with T.C.
And for the other resistors(120R, 70K) I will select for minimum T.C.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 08, 2015, 11:53:31 am
Hello,

and similar with the S102#1 resistor a comparison with his diagram to my measurements.
I did not standardize and calculate the LMS 3rd order curve since it is simply meaningless.

- Franks measurement with around 0.36 K/minute
  S102JT.jpt

- one of my measurements of summer with 0.3K/minute 3 times
  starting from 25 degrees to negative values and a final setpoint jump from 21 deg to 30 deg.
  20140714_TC_S102JT_1K_1_temp_corr.PNG

- a slow measurement from this year which ressembles more the measurement of Frank.
  20150119_TC_S102JT_1K_1_temp_corr.PNG

so the form of the hysteresis seems to be dependant on the "history"

With best regards

Andreas


Title: T.C. measurements on 1k econistors
Post by: Dr. Frank on March 08, 2015, 04:34:04 pm
Hello,

here are my measurements on a batch of 6 econistors, 1k, 0,1%.

All have the same date code 1435, and seem to come from the same manufacturing lot.
So I expect similar T.C.s over the whole batch.. and this shows...

As there is practically no hysteresis, and the T.C. behaviour is quite linear over T, I only applied a linear fit for calculation of T.C., in this case.

The T.C. is higher than the typical value, but within the 5ppm/K max. limit, except one sample.

All econistors, also the 12k, 120 Ohm ones, are all very well inside the R25 tolerance, i.e. they are better than 0.01%!


Title: T.C. measurements on 12k econistors
Post by: Dr. Frank on March 08, 2015, 04:36:27 pm
Here are the measurements for 5EA 12k , 0.1%resistors.

They have much lower T.C., but show pronounced  hysteresis and non linear T.C. over T.

I applied linear fit and box method.
Both values are well below the typical T.C. value of 3ppm/K.

Depending on the shape of the hysteresis and the dependence of T.C. vs. T, the box method may give either lower or higher values than the linear fit, but it will always be on the same order as the linear fit.

This observation is important for the estimation of the T.C. behaviour of the next 120 Ohm resistors.

I would like to use pairs of 1k/12K resistors, matched to a T.C. difference of about 3.0 .. 3.4 ppm/K, for the LTZ1000 reference.
That would give a contribution to the LTZ temperature coefficient of  about -0.04ppm/K (calculatory 1/78 of T.C. of resistor divider)
Title: Re: T.C. measurements on 120 Ohm econistors
Post by: Dr. Frank on March 08, 2015, 04:44:46 pm
And here are the measurement on two (of five) 120 Ohm, 0.1% resistors.
The 4W measurement was done 10mm apart from the resistor tube, therefore the copper T.C. of the leads would not greatly add to the resistor.

The resistance curve is relatively linear over T, and hysteresis is also not very pronounced.
Therefore, these parts are for sure totally outside specification, even if one would measure in the spec limits of -55...85°C, or so.

The supplier is currently investigating on the root cause, and will hopefully supply me resistors with sufficiently low T.C.

This T.C. would give an additional -0.08 .. -0.14ppm/K, (1/100),  maybe much less of that in practise, according to the results of JANAF.
https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg615470/#msg615470 (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg615470/#msg615470)

That calculatory result is much too high, and also in the wrong direction, to cancel the 1k/12k divider T.C.


PS: the econistors are made in U.S. by General Resistance, which belong to Prime Technology. Here's the G.R. datasheet:
http://www.primetechnology.com/cp/images/PartsDocument/163323.pdf (http://www.primetechnology.com/cp/images/PartsDocument/163323.pdf)

According to that datasheet, leads of an econistor have 53µOhm/mm, that makes about 5mOhm in total, if I would measure on the ends of the leads (my samples have 50mm long leads) .
Copper has a T.C. of about 4000ppm/K, and that would contribute in worst case 0.17ppm/K for a 120 Ohm resistor.
Therefore, this too high a T.C. can not be caused by the copper leads.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 08, 2015, 06:15:42 pm
First, a definition of hysteresis as applied to resistors: A failure of the resistor to return to the initial value of resistance after an applied stimulus.  As I have noted elsewhere, all resistors have hysteresis, only the degree to which they have it is different.  Technically, the initial value of resistance is that obtained after the manufacturing processes, in many cases the ‘final’ value is calibrated after the resistor has been made in the case of precisions.  This is the process of adjusting the ‘final’ value to within the specified tolerance band.

Depending on the resistor specifications, further ‘enhancement’ may be applied to increase its long term stability, the particulars of how this is done tends to vary with the type of resistor but in general thermal conditioning is used.  The success of this depends on how the resistor is designed and built, a loose rule of thumb is the lower the long term drift specification (particularly under power), the better the stability of the resistor.  Stability is a result of the resistor’s design and construction, stability is an inherent characteristic of the design, no amount of external stimulus can compensate for a poor design.

Hysteresis is the accumulation of all previous permanent changes, from the initial to the current.  Drift is a completely separate phenomenon which is generally defined as a change in resistance without stimulus, the ‘sitting on a shelf’ specification.  The long time ‘drift’ due to power stability specification is a misnomer; this is actually hysteresis in action as it is due to an applied stimulation.

When measuring hysteresis, it is important that the initial conditions be accurately specified, ambient temperature and initial value, not to mention the accuracy and repeatability of the measurement system.  If the observer wants to measure the effect of a change in temperature between two points, how fast or slow the external temperature changes is irrelevant, only that the resistor is given enough time to stabilize at the second temperature internally is important.  The same thing applies when returning the resistor to the initial conditions.  Additionally, the greater the stimulus change, the greater the hysteresis, this is not to say that the hysteresis will be a large value but hysteresis tends to be larger with larger stimulus change.

The application of thermal shock cycles will produce a more pronounced hysteresis than just a single change in temperature, even if that temperature change is 125°C, thermal shock can separate the ‘boys from the men’ so to speak, small hysteresis changes are indicative of highly stable resistor designs.

By industry test standards, the relatively gentle temperature shifts of 30°C being used in these measurements will not show much hysteresis except in the probable case of a bad resistor.  If you are going to use these resistors in an application in which the operating conditions are relatively stable, with only a few degrees change, then hysteresis is not going to be of much consequence, the long term drift of the resistor is going to be of much greater importance here.

Primary standard resistors are kept at a constant temperature for a purpose, stability, those Thomas one ohm resistors made of Manganin would otherwise show a significant hysteresis if this was not the case, applied power is kept to a minimum as well, under these conditions any really good resistor will show very little change over time.  In other words, gentlemen, you are basically fussing over a characteristic which in your application will be of little significance compared to other terms.

Long term drift of some of the resistors you have been talking about is much more significant, up to possibly 17 times higher (or more) than any of the hysteresis values you have measured here.  Once the LTZ1000/A or LM399/A have reached their lowest drift coefficients in time, the long term stability of the resistors then become more important as they can and will introduce an additional drift component, albeit not too large but significant, compared to the Vref.  The other 900lb. gorilla in the room is noise, this is the ultimate limitation, and noisy resistors will definitely add to the total noise over and above the Vref’s.

Dr. Frank, as I recall, the econistors specify 0±3PPM/°C TCR 0°C to +85°C, your batch is out of specification, the 0±5PPM/°C TCR applies above +85°C or below 0°C.  Given that they claim a lengthy thermal conditioning cycle (7 days), something or someone got sloppy on production.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 08, 2015, 07:28:52 pm
Hello Mr. Pettis,

thanks for your elaborate analysis, which I mostly agree, apart from two aspects:

If the observer wants to measure the effect of a change in temperature between two points, how fast or slow the external temperature changes is irrelevant, only that the resistor is given enough time to stabilize at the second temperature internally is important.  The same thing applies when returning the resistor to the initial conditions.  Additionally, the greater the stimulus change, the greater the hysteresis, this is not to say that the hysteresis will be a large value but hysteresis tends to be larger with larger stimulus change.

That's a kind of contradiction in itself.. as that's the crucial point , if the  measurement is done in quasi equilibrium, or not..

The hysteresis measurement very well depends on the speed of the temperature change!
Andreas measurements with different temperature change rates illustrate that.

The physical effect, which causes hysteresis, mostly can be described like something like a friction force.
If this force as is "weak", you will get fast recovery or fast creep effects.
If this force is strong, you will get a very stiff hysteresis, and maybe no practical observable creeping (glass state).

Equivalently this may also be described with a relaxation time constant for these hysteresis effects, (strong force <=> long relaxation time constant and vice versa)
I observed something like 1ppm/30min in one of the measurements on Andreas resistors.

Therefore, if the temperature change is much slower compared  to this time constant, the sample can relax to the equilibrium state at the different temperature points, and you may get a very narrow hysteresis loop, compared to the case, when the change is much faster than the time constant.

For the measurement of T.C. curves, that may also give totally different results, if you determine T.C. linearily or by box method.


Dr. Frank, as I recall, the econistors specify 0±3PPM/°C TCR 0°C to +85°C, your batch is out of specification, the 0±5PPM/°C TCR applies above +85°C or below 0°C.  Given that they claim a lengthy thermal conditioning cycle (7 days), something or someone got sloppy on production.


Well, the econistors are not very well specified (similar to Vishay)

That 3ppm/K is a TYPICAL value only, therefore most of the resistors are definitely within spec.

The MAXIMUM 5ppm/K over -55.. + 125°C strongly indicate that the 14 and 7.7ppm/K are outside, but here again, I have the same specification struggle as with Vishay, that is the averaging calculation by a box or a butterfly definition..

For metrological purposes (18..35°C), I need instead the physical definition, i.e. T.C. = dR/dT, or exactly the R(T) curves we are measuring here.

The box and butterfly definition mostly determine the T.C. by measuring at 3 or 5 temperatures only, and then average the resistance change by the temperature range.

This may give much lower T.C. values, if the R(T) curve is not linear over temperature, but has minima and maxima.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 08, 2015, 07:29:05 pm
Hello Frank,

thanks for sharing.
That confirms me that for a good result I will have to
test and pair/select the resistor T.C. s for my references.
And probably the 120R resistor is the most critical.

I always asked me why the LTZ#2 where I paid more
attention on the stability of the cirquit seems to have
a slightly worse 1000hrs standard deviation than #1.

Probably I should measure the resistors some days.
(if I have better references).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 08, 2015, 07:47:43 pm
Hello,

just to see if further reduction of temperature ramp speed could have a significant influence on hysteresis I did a measurement starting with a warm cycle on 24.02. from 20 deg C with 0.06K/minute and a cold cycle on 25.02. with 0.04K/minute for Z201#3.

Other setup is the same as on measurement from 22.02. with 0.12K/minute which is again attached as comparison.

Against the measurement of 22.02. the hysteresis does not change as a factor 2 like the ramp speed for the warm cycle.
For the cold cycle the reduction of hysteresis is larger since there is more time to creep back to the 20 deg point over night and the lower ramp speed.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 09, 2015, 12:35:46 am
Dr. Frank,

Let me restate what I said in a slightly different manner, same conditions.  Using the same 30°C temperature change (roughly) that is generally being used here, are you trying to say that if the resistor is subjected to exactly the same temperature excursions, up to and back down again, that supposedly the rate of change is going to make a difference?  Sorry, I disagree, after seeing thousands of examples of resistor hysteresis over the years, the rate of change is irrelevant (read on below before jumping to conclusions) .  A change of 30°C will produce exactly the same amount of hysteresis no matter how slow or fast the change was obtained IF the resistor obtained equilibrium internally at both temperature extremes and other conditions are met.  The key here is the internal equilibrium of the resistor with the change in temperature, given that the resistors being discussed here has a rather low thermal conductivity, it will take the hysteresis loop of the wire against the mandrel a significant  amount of time to equalise to the ambient temperature.  If insufficient time is not given for the internal stabilization of the resistor then the hysteresis effect will not be consistent from one run to another and that is possibly what may be seen here (again, see below).  How much time, don't know, unknown variables involved.

A further note about the alloy(s) used in PWW resistors, they have an extremely high value of tensile strength, this alloy will not and cannot be stressed in any way that changes it, 150°C is nothing to it, what causes real hysteresis (in shelled resistors) is the hoop stress on the layer of wire against the mandrel, if this stress is not removed during the manufacturing processes and this is important, the bobbin materials must be chosen correctly, the hoop stress is not entirely removed from the resistor during manufacture and further hysteresis can result from temperature changes.  This results from the bobbin material never completely 'relaxing' against the wire turns and this remaining stress can be there for the life of the resistor.  There have been many claims by manufacturers (I know, I used to work for one of them) that they chose their wire and bobbin materials to match each other, this is a steaming pile of hog wash (the British call it rubbish), the materials do not match each other correctly, the problem being that they match in the wrong way leading to different stress and the manufacturers don't seem to understand this because they keep making the same mistakes over and over again.  Molded resistors, which are very popular because they are cheaper than shelled resistors have their own set of problems with internal stress, this partially being caused by the molding process itself which imparts an unpredictable stress onto the entire resistor's windings and this stress changes with time, it can also appear as a significant component of 'drift' in both shelf and powered long term stability.  There is also stress imposed by the bobbin material generally used in this type of resistor as well, more hoop stress and the list goes on.

Another factor that can enter into this apparent hysteresis effect is the resistor's design and construction methods, many precision wire wound resistors will indeed exhibit what appears to be only hysteresis but in fact, other mechanical forces are working internally to cause an additional instability factor which appears as hysteresis during a temperature cycle.  This same effect can also appear as part of a drift term in parts sitting on a shelf long term.  Unless the precise method of design and construction of the resistor is known, it is not possible to identify exactly the cause of this effect but it is definitely inherent in many resistors.  I have seen and identified sources of this effect in many different PWW over the years.  This effect could easily be what is showing up in your tests as variations in hysteresis depending on the ramping speed of temperature as they react differently to the stimulus.

When you are looking at very small term effects in a resistor, it becomes a highly complex combination of electrical and mechanical terms which manifest themselves at the terminals of the resistor, it is like the 'black box' puzzle so many professors like to throw at their students.  There is a component or circuit inside the box and without opening it, you try to figure out what is inside of it.  Unfortunately, that really doesn't work with PWW resistors because so much of it does depend on what is inside of it and how it was made, trying to separate all of the various terms is near impossible from the outside.

At best, a properly designed and constructed PWW will exhibit little hysteresis and will not have any of the effects described above in the second paragraph.  That is not easy to achieve or else all PWW resistors would be pretty much the same in characteristics.  Remember, these parts were intended to be working, not sitting on a shelf or being kid glove treated like a resistance standard, there will be some effect, however small, from the fact that operating conditions are not constant with time.  They may indeed be too small to measure over just one or even several change cycles but tiny changes there will be even in the best, only difference is that those changes are much more likely to be consistent instead of varying response with the same stimulus.

I am not disputing the fact that you gentlemen are measuring differences in hysteresis as such, what I am trying to do is explain why you may be seeing these abberations and are possibly attributing them to the wrong effect.  Feel a bit confused, don't, there are many resistor engineers out there that don't understand it either and/or refuse to understand it.

It would take a book to describe and detail all of the things that go into PWW design and manufacture, one reason you won't likely see one is that to explain some of this stuff, proprietary information would have to be explained and as you know, that isn't likely to happen.  It is most amusing how every PWW manufacturer thinks that their resistor design is proprietary when nearly all of them are a conglomeration of ideas 'borrowed' from competitors over the decades.  General Resistor's design is nothing new nor does it solve many of the problems exhibited by this construction and manufacturing.  I've seen and dissected a lot of resistors over the years and they are all very similar to everybody else's resistors out there.  There are variations of course, but basically they are still all essentially the same result.
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on March 09, 2015, 06:55:06 am
Hi, it seems that you guys are using different definitions of temperature hysteresis leading to misunderstanding, let me summarize:

Edwins definition seems to be considering just the net effect of putting a heat cycle - beginning and ending at the same temperature only. As seen from the resistor, this will earn you a difference in the resistance before the temperature cycle to the resistance after the temp cycle.

The hysteresis error Andreas and Frank are looking for is the maximum difference in resistance at any temperature when approaching the temperature setpoint first with increasing and then with decreasing temperature.
This explanation, supported by the measurements (or vice versa) will give you the idea why the temperature ramping must be slow enough to give the resistor time to get to a equilibrium - caused by right what Edwin describes, the temperature "cycle" must be done before every single measurement. And also giving the data to specify what happens to that certain resistor if you give it the "Edwin style" treatment.

Let me call that "A sensor guys" definition of temperature hysteresis, as I work in sensing and is exactly what I look for at work. The resistor here is treated like a temperature sensor.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 09, 2015, 08:44:23 pm
Hello together,

which effect it really is does not really matter for me.

My world is much simpler:

- all what appears as closed curve (so it is not ageing) during
  temperature test which has a "eye" (opening) with different temperature direction I call "hysteresis"
  -> this is something that I cannot correct with a temperature sensor without knowing the history.

- all what leads to a non closed curve during temperature test -> it is ageing or humidity drift

- all temperature dependant what can repeated -> T.C.

Vishay states in the Z201 datasheet that within 1 second the final value of temperature drift is met within 10 ppm.
So 20 seconds should fit to meet final temperature resistance value within sub-ppm.
The hysteresis or creeping effects that last for some hours (or over night) are something that I do not want to have in my references since I cannot calculate it out with a simple temperature sensor.

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 09, 2015, 09:09:07 pm
Hello,

after several months now I have found one "typical" (= golden) Z201#6 resistor. (Date code B1305-)
At least when only regarding the 10-45 deg C temperature range.

The LMS approximation (green) in the diagram indicates 1.76 ppm maximum difference between min/max over a 35.5 deg C temperature span. Giving 0.05 ppm/K (average) with the box method.
Hysteresis is around +/- 0.3ppm on the warm cycle when not regarding the noise of the measurement.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 09, 2015, 09:56:45 pm
Nice!

Andreas, I have to ask about the terms in you plots. Is this correct:
- With drift you mean the resistance variation over the temperature cycle?
- The hysteresis plot, you use your LMS line as mid-level and plot variations from that mid-level but in a larger scale?
Did I get that right?
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 09, 2015, 10:10:39 pm
Hello Jan,

Yes. Drift = temperature drift from a virtual "zero" near 25 deg on the left scale.
(since I have no absolute measurement).

Hysteresis (on the right scale) is the difference between LMS value and measured value.
With these low differences of Z201#6 the diagram gets somewhat confusing.

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on March 10, 2015, 05:28:52 am
... and after all this work, Andreas is in the business of selling golden, selected Resistors. AMG - Andreas' Manufactur für Gute. Vishay is going to buy him, new name will be VISHAY-AMG.  :-DD
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 10, 2015, 06:06:37 am
after several months now I have found one "typical" (= golden) Z201#6 resistor. (Date code B1305-)

So, how many of these expensive foil resistors do I have to buy to find one that has "typical" performance of 0.05ppm/K ?

Easy, that's all about Normal Gauss Distribution, I think?

Frank
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 17, 2015, 04:56:51 am
Look what just turned up:
(https://pbs.twimg.com/media/CARbfQjVIAAjFFo.jpg:large)
(https://pbs.twimg.com/media/CARcBKZVEAAER3v.jpg:large)

My 34470A is currently saying it is 10.000200
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on March 17, 2015, 05:08:49 am
Nice!

You should get two additional drills in the enclosure and put a characterised Thermistor inside! Thats how Dr. Frank does it to get to know the temperature of the resistor element.

Fun fact: If I google for Wekomm engineering GmbH, the residues of their incomplete website show they are active in both areas, Metrology and model trains... wonder what that means? :)
Title: Re: T.C. measurements on precision resistors
Post by: 3roomlab on March 17, 2015, 05:38:35 am
Look what just turned up:

My 34470A is currently saying it is 10.000200

just curious, how much (ballpark) does it costs?
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 17, 2015, 06:30:02 am
Hello,

more interesting would be if there are any accuracy specs for the values in the datasheet.
(including ageing)
The absolute value seems to be that what they have read from the 3458A multimeter.

For the T.C. it would be interesting over which temperature range did they get the coefficients.

A teardown with photos would be nice. (if the resistor is hermetically sealed).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 17, 2015, 06:42:01 am
Look what just turned up:
..

My 34470A is currently saying it is 10.000200

Hi Dave,

looks really solid, but it lacks a thermometer for that uncertainty grade..

Although 28ppm deviation is within specification of the 33470A...

Did you correct for ambient temperature, and did you use 4W and offset compensation?

Would also like to see the inner construction.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 17, 2015, 06:44:25 am
Hello,

more interesting would be if there are any accuracy specs for the values in the datasheet.
(including ageing)
The absolute value seems to be that what they have read from the 3458A multimeter.

...
With best regards

Andreas

The 3458A delivers 7 decimal places only.
Maybe they use another 8 1/2 digit DMM, or more probably, a precision resistance bridge with another reference resistor.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 17, 2015, 07:10:45 am

The 3458A delivers 7 decimal places only.


Even if I use the HPIB connection?
The 28 Bit converter should give a total of 8 digits.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 17, 2015, 07:26:48 am

The 3458A delivers 7 decimal places only.


Even if I use the HPIB connection?
The 28 Bit converter should give a total of 8 digits.

With best regards

Andreas

For the 3458A (and also all other 8.5 DMMs) the 8th digit is meaningless.
Transfer accuracy, i.e. comparison to another external reference resistor really is 0.2ppm at best, and absolute uncertainty of the internal references is about 2ppm/24h.

Therefore, if they write 8 digits on their box, they either must have used something else, an ESI SR104, or maybe measured at PTB (what I doubt w/o a thermometer inside), or they are not very serious.

Btw.: Their homepage is not working, I can't access their metrology site and can't retrieve the spec.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on March 17, 2015, 08:37:01 am
Measurements International has the following brochure...

http://www.mintl.com/media/pdfs/accubridge.pdf (http://www.mintl.com/media/pdfs/accubridge.pdf)

Does this mean all the extra digits (10's of ppb) are meaningless or do they have a useful application?

I have not seen any videos of these in operation so I am guessing they are not as easy to operate as their sales people advertise.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 17, 2015, 09:15:49 am
Measurements International has the following brochure...

http://www.mintl.com/media/pdfs/accubridge.pdf (http://www.mintl.com/media/pdfs/accubridge.pdf)

Does this mean all the extra digits (10's of ppb) are meaningless or do they have a useful application?

I have not seen any videos of these in operation so I am guessing they are not as easy to operate as their sales people advertise.

That's a resistance bridge, not a DMM!

Bridge type instruments are much more precise and sensitive.
1e-10 resolution is possible, but 1e-9 is more serious, due to 10nV noise limit vs. 10V max. DUT voltage.
See also IET Labs 242D Bridge specification.

For uncertainty, at room temperature the best known standard is the ESI SR104, and this is limited to about 0.1ppm uncertainty, maybe 0.01ppm by prediction, in a well controlled lab and with very frequent comparison to a Quantum Hall standard.

Therefore, 0.1ppm uncertainty or 7 digits are serious for a non - primary lab.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 17, 2015, 11:13:57 am
just curious, how much (ballpark) does it costs?

About US$5500  :o
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 17, 2015, 11:25:39 am
Therefore, if they write 8 digits on their box, they either must have used something else, an ESI SR104, or maybe measured at PTB (what I doubt w/o a thermometer inside), or they are not very serious.

I can assure you they are very serious.
The unit was calibrated at Germany's national standards lab, and from a brief look at the cal documents, tested for drift at 0.5degC intervals over a large range.
They are working very closely with Vishay to get their very best technology and the resistors are custom designed for them. They are not just buying a stock series resistor sticking it in a box and whacking it on a 3458A.
It took many weeks to characterise and calibrate this resistor for me.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 17, 2015, 12:03:20 pm

I can assure you they are very serious.
The unit was calibrated at Germany's national standards lab, and from a brief look at the cal documents, tested for drift at 0.5degC intervals over a large range.
They are working very closely with Vishay to get their very best technology and the resistors are custom designed for them. They are not just buying a stock series resistor sticking it in a box and whacking it on a 3458A.
It took many weeks to characterise and calibrate this resistor for me.

Wow, then congratulations on that fine standard!

All right, as assumed, that's our PTB - Physikalisch Technische Bundesanstalt, maybe they used their Quantum Hall effect facility directly for that. In this case, 5500 bucks would be a real bargain.

And you really paid that price? Are you really goin' volt-nuts??  :-+

I'd like to see that calibration document, please!!
And a picture of that blue cal sticker on the top, that might be the PTB - blue.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: TiN on March 17, 2015, 01:04:46 pm
Whose 10k is more 10kish?
This PDF (http://www.vishaypg.com/docs/63625/63625.pdf) on VPG say's it's VHA518 (http://www.vishaypg.com/docs/63120/hzseries.pdf), very similar to my 1M0000 HZ resistor :)
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 17, 2015, 01:16:33 pm
And you really paid that price? Are you really goin' volt-nuts??  :-+

I ain't nuts!
They contacted me out of the blue and offered one, it's a new product they are working on.

Quote
I'd like to see that calibration document, please!!
And a picture of that blue cal sticker on the top, that might be the PTB - blue.

Yep, I'll scan and post more, plus I plan a video using it to calibrate my Fluke 5450A
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 17, 2015, 03:48:42 pm
For comparison, gentleman, my ESI SR104 is in excess of 15 years old and exhibits the typical sub-0.1PPM drift per year as is common with these standards.  My SR104's nominal value is at +2PPM (10,000.02 ohms), Alpha is -0.06PPM/°C and Beta is -0.008PPM/°C.  The built in temperature sensor resistor is 10,000.08 ohms with a TCR of +1000PPM/°C and the SR104 also has a thermometer well.

This 'new' resistance standard appears to be starting off well but it is going to take years to find out just how good it is for long term stability, unfortunately for standards grade resistors you can't use the usual tools to predict long term stability except by time alone.  As can be seen by comparison to my SR104, this standard has some more work to be done on it, a very good start though.  The SR104 still holds the top honors in this group and I rather doubt it is going to lose that position any time soon.

The ESI 242D bridge is still exceptional even after all these years, with direct comparison to a SR104, the uncertainty is 0.1PPM or a bit less, the bridge can resolve 0.01PPM or a bit better (I can resolve another digit) by direct comparison.  I can resolve two more digits beyond that by using the standard multiplier but the absolute accuracy degrades a bit because of a slight additional uncertainty added.  Only a DCC bridge can obtain a higher accuracy at 10K or the National Primary Standards Labs.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 17, 2015, 04:15:42 pm
These are not standard Vishay parts, they are:
Quote
custom specific made part, together with a special packaging.


http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/ (http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/)

Quote
Wekomm engineering paired the VHA518-7 resistor with carefully selected components to form a transfer standard product.
VHA518-7 (= 7 resistors in series)



VHA518 long term drift measurements against the Quantum Hall by the Dutch Metrology Institute:

http://www.vishaypg.com/docs/63620/63620.pdf (http://www.vishaypg.com/docs/63620/63620.pdf)

VHA518-11 (= 11 resistors in series)
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 17, 2015, 05:01:38 pm
Hmmm, a case study of essentially one  or two resistors, that is more anecdote than data per se, they do not say just how many of these 'units' were tested, it appears to be just one each and neither one was a 10K unit.  I am also rather curious about Dr.ir. Gert Rietveld's use of the phrase "appeared to be less than 0.5PPM", that makes it sound like there is some question about the measurement.  While I have no qualms about the Dutch VSL, I have always found Vishay's proclamations to leave something to be desired.  The Alpha and Beta quantities are significantly worse than the SR104 and yet they seem to be claiming possibly better linearity of the linearity line, this is conflicting.

The SR104 has a long history behind it and a long track record of proven performance over decades, this supersedes the measurements of a few resistors even over a relatively short period of 5.5 years.  Frankly, I'm not sure the NIST would accept this as a standard in the same performance of the SR104, the SR104 is classed as a primary standard  and therefor must meet very high performance standards.

Just so there isn't any question, I am fully cognizant of the fact that these Vishay (Wekomm) units are made up of quite a few individual resistors, but as a whole, they are still referred to as a single resistor as a unit.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 17, 2015, 05:58:59 pm
they do not say just how many of these 'units' were tested, it appears to be just one each and neither one was a 10K unit.

"Vishay typical" ?

While I have no qualms about the Dutch VSL, I have always found Vishay's proclamations to leave something to be desired.

The document I linked is Vishay marketing material. Most likely VSL will publish / has published scientific article of their experiments.

Bulk metal foil resistors have been used for a long time in primary metrology for AC/DC transfer. But not for anything long term...
Title: Re: T.C. measurements on precision resistors
Post by: jaxbird on March 17, 2015, 06:16:59 pm
Look what just turned up:
(https://pbs.twimg.com/media/CARbfQjVIAAjFFo.jpg:large)
(https://pbs.twimg.com/media/CARcBKZVEAAER3v.jpg:large)

My 34470A is currently saying it is 10.000200

This is a good read :) Might not legally apply, but still...

https://www.ftc.gov/tips-advice/business-center/guidance/ftcs-revised-endorsement-guides-what-people-are-asking (https://www.ftc.gov/tips-advice/business-center/guidance/ftcs-revised-endorsement-guides-what-people-are-asking)

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 17, 2015, 06:43:34 pm
they do not say just how many of these 'units' were tested, it appears to be just one each and neither one was a 10K unit.

"Vishay typical" ? 

  Yes, typically Vishay posts incomplete or 'fuzzy' specifications, 'fuzzy' test results or tests that don't even accomplish what they are supposed to do.  Vishay has done this for decades and the practice still continues.  I know, I've been here for nearly all of it.

While I have no qualms about the Dutch VSL, I have always found Vishay's proclamations to leave something to be desired.

The document I linked is Vishay marketing material. Most likely VSL will publish / has published scientific article of their experiments.

Bulk metal foil resistors have been used for a long time in primary metrology for AC/DC transfer. But not for anything long term...
   

  Precisely, Vishay Marketing BS, it makes the grass greener and much of Vishay's so-called scientific papers have flaws in them and misstatements.  Bulk metal has been around for years but PWW have been around a lot longer and currently surpass many of the claimed Vishay performance data.  Unlike you folks, I've been in the resistor industry for over 4 decades, I have the experience and knowledge which no one else in this forum has an apparent claim to.  Yes, for the record again, Vishay resistors do have some really good attributes but rarely do they perform to the overstated claims of Vishay.  Read back through some of the threads on this forum and some of the professional forums and you will find that this is backed up many times over.

For those of us in the know, some of those purported Vishay scientific papers are just plain laughable and inept.
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 17, 2015, 11:41:21 pm
For those of us in the know, some of those purported Vishay scientific papers are just plain laughable and inept.

Have people published detailed research papers in rebuttal?
I have not researched any of this of course, but putting my skeptical hat on, I find it hard to believe that one of the leaders in the field of precision resistors for many decades produces "laughable and inept" scientific papers.
Mistakes? I'm sure that happens, but "laughable and inept"?
Proof please.
And BTW, proof is not "go read the professional forums" and "Unlike you folks, I've been in the resistor industry for over 4 decades, I have the experience and knowledge which no one else in this forum has an apparent claim to."
/skeptical hat off
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 18, 2015, 12:24:16 am
Yes, laughable and inept, I can produce some e-mails from Bob Pease, for example who was quite critical of some of Vishay's nonsense.  Alot of the stuff Vishay has put out about PWW resistors has been and still is laughably wrong, they are not experts on PWW resistors, never have been but that has never stopped them from making wrong (possibly intentional) claims about PWWs.

If you really do 'research' it, you'll find that it is inevitably Vishay who has been calling themselves the industry leader for the most part, some of their competitors may not be quite ready to call them that.  I know of many engineers and companies who have tried Vishay resistors in the past (recent as well) and have returned to PWW resistors as the Vishays did not meet specs.  I can give an example of one such case, there are many more.  Vishay is consistent in their fudging specifications over the decades and in some cases have actually ceased production quietly after problems were reported by customers.  As it appears you seem to think I am the only one of this opinion, I am not by far but of course there are people out there who think Vishay is the greatest thing since sliced bread too.

If you have been paying attention to what I write about Vishay, I do not solely bash them on the head, they do make some very good parts of course, but there is also a lot of hoodwinking going on over the years and it is quite surprising how many people are clueless.  It is solely up to you whether or not you want to believe me or not, I have no control over that but there are people who also agree with my position on Vishay and also agree that I classify as an expert in resistors.

The proof, if you want to take the time to find it, is 'out there' and professional forums are generally a qualified source of valid data (not always of course).  If you want skeptical, I had to convince Bob Pease that what I was saying was valid and I did.  Bob was no easy pushover either.

When I get a little time, I'll be happy to post some, and yes, my 40+ years does qualify me as an expert whether you accept it or not, I do accept your opinion, you are welcome to it but on the flip side, what qualifies you to say that I am not what I say I am or my opinions are valid or not?  It does work both ways.

BTW, I made standard resistors for Tegam (before they sold the line off) and my 10K was made from one resistor and it was within 2PPM of nominal.

One more thing, I may sound like I'm swinging a sledge hammer here (I'm told I do that at times) but that is not the intention. 
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 18, 2015, 12:34:29 am
It is solely up to you whether or not you want to believe me or not

No, it's not up to me, it's up to the evidence.
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 18, 2015, 12:45:38 am
When I get a little time, I'll be happy to post some, and yes, my 40+ years does qualify me as an expert whether you accept it or not, I do accept your opinion, you are welcome to it but on the flip side, what qualifies you to say that I am not what I say I am or my opinions are valid or not?

I was not doubting your qualifications. In fact I did some research and you do seem legit.
But once again, I go with the evidence, not because someone just declares so on a forum because they are an expert.
You've made a bold claim that Vishay's scientific papers are "laughable and inept" so you are the one that gets to prove that, or else don't be surprised that some people might be skeptical of that claim.
That's how it works.
This is not about doubt of your experience in the field, nor is it meant in any way personal.
It has nothing to do with whether or not your opinion are "valid" or not. Your opinion is just that, an opinion. Sure an experts opinion might be worth more at face value than some anonymous forum user, but ultimately it always comes to evidence to say who's opinion is right. This isn't philosophy we are talking about, this is engineering where there is a right and wrong answer based on  the evidence.
Even if Bob Pease came on here and declared Vishay's scientific papers "laughable and inept", I'd still ask for the same proof. I'm a hard arse like that  ;D
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 18, 2015, 12:54:07 am
The cal sheet for my Wekomm standard
http://www.eevblog.com/files/WekommResistanceStandardCalSheet.pdf (http://www.eevblog.com/files/WekommResistanceStandardCalSheet.pdf)

Looks like they use this bridge:
http://www.mintl.com/media/pdfs/accubridge.pdf (http://www.mintl.com/media/pdfs/accubridge.pdf)
and a Genrad 1444A standard
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 18, 2015, 03:14:48 am
Hi Dave,

Thank you for posting the calibration document, interesting.  A bit of a clarification here, I did not say that all of Vishay's papers were laughable or inept, just some of them and yes, they are somewhat self-serving in nature, that doesn't necessarily mean they are all junk.  If one has a good understanding of the subject matter, reading their papers or watching their videos supports my conjecture without a doubt.  Unfortunately, most people watching or reading this stuff does not have the necessary knowledge to separate the wheat from the chaffe. 

The Measurements International is a DCC ( direct comparison current) bridge, hence it has a bit of an advantage over my ESI 242D at ?10K, the 242D has the advantage above 10K.

DiligentMinds is correct in that Vishay's papers are a mix of scientific and opinion which is questionable at times, I do not know of any of the Vishay 'papers' being peer reviewed as such, for that matter I constitute a peer and as such review them now and then.  There are some well written 'white papers' (a more correct term than scientific) that Vishay has put out, it is just difficult for most people to know the difference between the flowers and the manure, that is kind of where I come in.  I, in no way, thought your comments were 'personal', there was really nothing personal in your manner nor are my comments meant to be.

<chuckling> In the case of Bob Pease, if he were here, no doubt he would tell you in no certain terms that he didn't give a ____ but Bob usually didn't pull any punches no matter what the object of his comments thought.

You must agree that qualifications and opinions go hand in hand, one who is qualified on the subject matter is also qualified to make opinions and those are usually well informed.  Of course, the corollary of this is that some people who think they are qualified also think highly of their misinformed opinions and don't know any better.

I do thank you for your voicing your concerns, I never expect everyone to agree.
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on March 18, 2015, 03:34:12 am
The GR1444A was introduced in 1970($600.00), and was not listed in the 1978 catalog. It appears to be an analog to the ESI SR104 resistor. The earliest mentionof the SR104 is in a 1971 ESI242D manual(I didn't try very hard). It is easier to find GenRad infformation than ESI. I wish IET Labs hosted some of ESI's documentation.
 http://www.ietlabs.com/pdf/GR_Experimenters/1970/GenRad_Experimenter_March-June_1970.pdf (http://www.ietlabs.com/pdf/GR_Experimenters/1970/GenRad_Experimenter_March-June_1970.pdf)
Reading further I see H.P. Hall was used in the creation of the article :D .
Title: Re: T.C. measurements on precision resistors
Post by: 3roomlab on March 18, 2015, 04:44:34 am
These are not standard Vishay parts, they are:
Quote
custom specific made part, together with a special packaging.


http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/ (http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/)

Quote
Wekomm engineering paired the VHA518-7 resistor with carefully selected components to form a transfer standard product.
VHA518-7 (= 7 resistors in series)



VHA518 long term drift measurements against the Quantum Hall by the Dutch Metrology Institute:

http://www.vishaypg.com/docs/63620/63620.pdf (http://www.vishaypg.com/docs/63620/63620.pdf)

VHA518-11 (= 11 resistors in series)

more noob questions  :P

when its so many resistors in series, does it mean they use the "+" and "-" ppm drifts to cancel out each other, after knowing the characters? and that is the primary thermo/electrical "pairing" criteria? and probably sorting thru thousands of resistors, arrange a series that give a linear tempco?

what is this "dutch quantum hall" being mentioned?

edit : this 1 could be a tough question. why did they choose 23oC as the zero deviation centre temp? maybe dave ... ask KCB this question?
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on March 18, 2015, 06:22:03 am
Have a look at the data what KCB (which is not the PTB!) is accredited for. I use my advantage of being a native german speaker... or at least almost, as I am coming from a part of germany where they speak something I wouldn't necessarily call german :)

(translation mode on)Resistance: 0,1*10E-6 * R best reportable uncertainity.(translation mode off)

But why do they give a tempco in their test report? :wtf:

Neither do they list a device which was used to realize the temperature slope or what it is referenced to, nor does the DUT itself provide a temperature measurement like the SR104 which could be specified together with the main resistor.
They can use whatever they want to heat up their unknown reference sensor, what does it tell us about resistor temperature, especially in still air?

(They are not even accredited for temperature calibration.)

I think that is a fact they should note.

Title: Re: T.C. measurements on precision resistors
Post by: MK on March 18, 2015, 06:25:12 am
One troubling thing about the vishay VHA518-11 is the 6.5 K version is not showing root hours settling down, it was drifting faster after 2000 days then it was at the beginning! so that means it does need constant recals to trust its behavior in my eyes.
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on March 18, 2015, 06:27:37 am
That is a good question. I have some American equiptments datasheets dating from the 60's that specifies the cal temp of 23degrees C, while my Solartron is specced at 20deg.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 18, 2015, 09:32:46 am
this 1 could be a tough question. why did they choose 23oC as the zero deviation centre temp?

That is a good question. I have some American equiptments datasheets dating from the 60's that specifies the cal temp of 23degrees C, while my Solartron is specced at 20deg.

The early British metrology equipment, like standard resistors, had 17 degrees Celcius calibration temperature. Not sure when the change to 20 C took place, could be in the 1950s. I quess they have 23 C nowadays like the rest of the world.

Back then cooling was not available, heating only. The calibration temperature needed to be higher than the room temperature. Quite cold in those Victorian buildlings.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 18, 2015, 10:07:41 am
Well, the GenRad 1444-A was the precursor to the ESI/Tegam/IET-Labs SR-104 10K resistor.

The GR1444A was introduced in 1970($600.00), and was not listed in the 1978 catalog. It appears to be an analog to the ESI SR104 resistor. The earliest mention of the SR104 is in a 1971 ESI242D manual(I didn't try very hard).

The SR104 tested against the computable capacitor in NPL Australia was manufactured in 1965. The earliest commercial SR104 I have seen dates back to 1967.

The GR 1444-A was introduced later as a competitor to the SR104 but turned out to be less stable. The GR unit wasn't silicone oil filled like the SR104 but nitrogen instead and constructed from only two mica card resistors.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 18, 2015, 10:22:06 am

Have people published detailed research papers in rebuttal?
I have not researched any of this of course, but putting my skeptical hat on, I find it hard to believe that one of the leaders in the field of precision resistors for many decades produces "laughable and inept" scientific papers.
Mistakes? I'm sure that happens, but "laughable and inept"?
Proof please.
And BTW, proof is not "go read the professional forums" and "Unlike you folks, I've been in the resistor industry for over 4 decades, I have the experience and knowledge which no one else in this forum has an apparent claim to."
/skeptical hat off


Hi Dave,

I'm also a singulary voice only, as I agree in parts what Edwin Pettis is saying.

But I'm also very hard-arsed, concerning reputable specification for components, as myself, I was creating automotive component specifications for a long time.. and have some experience on Vishay, including many of the companies they took over, all were our  suppliers..
Beyschlag once was one of the best manufacturers of Thin Film resistors, with the most reputable specs.. until Vishay swallowed them..


So I can really assess the quality of the current Vishay specs, and find the ones about BMF technology exaggerated quite often.

It is very obvious for every engineer, if you only look carefully on the data sheets, that the typical data are much too optimistic, partly advertised in big letters, compared to the upper limits they specify.

For example, they always claim a typical T.C. of 0.05ppm/K, on top of the spec, even in the headline, but on the left lower part of the page, they specify a T.C. of +/- 0.2ppm/K +/-2ppm/K maximum.

Only that latter specification parameter is reasonable and serious, so they really do not screw the customer, in the end.


My personal experimental experience with my 5 VHP202Z resistors was exactly, what you doubt, that such a reputable manufacturer tries to lead the user astray by these advertising methods.

These components in reality (according to my own measurements, which I also posted in the EEVBLOG forum) all showed a T.C. between -0.3 to -1.0 ppm/K, no sign of a positive parabola shaped R(T), with a typical 25°C minimum point, as the Z-foil technology would imply, according to several scientific papers from Vishay, where even the honourable Dr. Felix Zandman had signed.

After my complaint about this bad performance of the parts, the Vishay representative had to admit that one can not rely on the typical data and on the intended technology characteristics so far, as advertised...and that there is no guarantee from Vishay about these typical characteristics...
So the new standard from Weekom should be observed very critical also..


In the end, the BMF technology is still very good, and beats PWW components in some aspects, but the production variance is much too big.. an indicator, that Vishay is not really able to control this technology in last consequence.

My impression also is, that with the early death of Dr. Zandman, the respectability of Vishay decreased...as the bean counters took over instead, maybe.

I disagree to Mr. Pettis, that all the papers from Vishay are toast..
Some are very interesting, and several claims really reflect reality, for example the long term stability of these oil filled, hermetically sealed types..again from my own measurements on these 5 resistors.

These have drifted less than 1ppm apart in the last 4 years.. so 2ppm/6years as specified TYPICALLY seems to be reasonable.

Well, that's a singulary finding also, but every bits and pieces sum up to a reliable picture.. in negative and in positive aspects.

I append one of the key documents from Vishay about their BMF technology, regarding the characteristics of the different foil types, C, K and Z. (Z like Zandman)
These characteristic shapes also show up in the BMF specification, but Vishay measures the T.C. not by the physics method, T.C. = dR / dT, but by 3 temperature points only, i.e. the butterfly method.
Also some sort of misleading..

Frank

Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 18, 2015, 10:31:34 am
After my complaint about this bad performance of the parts, the Vishay representative had to admit that one can not rely on the typical data and on the intended technology characteristics so far, as advertised...and that there is no guarantee from Vishay about these typical characteristics...

If they are "typical" characteristics, then, well, there is no guarantee! Either a spec is typical or it is guaranteed.
If it's typical and doesn't meet spec, bad day for you.
If it's guaranteed and doesn't meet spec, bad day for Vishay!

Quote
So the new standard from Weekom should be observed very critical also..

FWIW I've been assured that the resistor in the Wekomm standard is a custom part and hand selected. So in theory, no matter how bad Vishay's standard production spread is, it's possible to pick the good ones.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 18, 2015, 10:54:59 am

If they are "typical" characteristics, then, well, there is no guarantee! Either a spec is typical or it is guaranteed.
If it's typical and doesn't meet spec, bad day for you.
If it's guaranteed and doesn't meet spec, bad day for Vishay!

FWIW I've been assured that the resistor in the Wekomm standard is a custom part and hand selected. So in theory, no matter how bad Vishay's standard production spread is, it's possible to pick the good ones.

Dave, you missed my point.. I was fully aware of the maximum limits..

BUT if you specify typical values, these should be met for a reasonably big quantity, if you buy several.. if you understand typical parameters correctly, in the sense of a statistical distribution.. and the typical value representing the median.

And also, a clear characteristics of the Z-foil technology is the parabola shaped R(T) behaviour (see appended document) .. in other words this R(T) shape is strictly required, if the compensation technique between metal foil and ceramic substrate is correctly applied.

So, if you don't see this parabola, even not over a wider temperature range (and the Vishay representative told me, they could also not guarantee this shape), then the very logical reverse conclusion is, that this component has no Z-foil characteristics, and that this Z-foil compensation method did not work for the DUTs.
And as a last conclusion, that also explains the 10..20 times higher T.C. than typical...


And for the Weekom standard:
Correct, you may pick the resistor with the lowest T.C., that won't change over time, probably.
In fact, 0.45ppm/K is not stellar.. comically on the same order than my single chip VHP202Z types.

But you cannot pick for best long term stability, as accelerated testing is not possible, and only time will tell, as Mr. Pettis correctly stated.
Frank 
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 18, 2015, 11:36:09 am
RS9010 tempco from the calibration certificate compared with a good SR104
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 18, 2015, 12:12:58 pm
OK, here I have another very interesting CERN document .. in several aspects.

It describes the construction of an ultra precise / stable 10mA current source for the calibration of the SC coils of the accelerator.

One author is John Pickering, godfather of the LTZ1000 volt reference Datron / Fluke 7000, cofounder of Datron (1271 / 1281 DMMs => now Fluke 8508A, 4910 reference (?)). He also invented the hysteresis cancellation on the LTZ1000, and the statistical TaN divider.

In this paper, the current source relies also on a 10k Z foil resistor, which is needed for ultra low T.C.
The current source shows an overall  T.C. of typ 0.07.. max 0.14 ppm/K for the output current, that also means that the 10k Z-foil also has to have even better characteristics...

Therefore, there really exist Z-foil resistors with that ultra low T.C., so Vishay may be able to control the technology sometimes..

Or how many resistors did they need to sort out for getting some really good ones?


Therefore follows another argument: If the yield of these "good ones" is high, then the company really has control over the technology.
If the yield is low, or this special feature shows up randomly only, then the company fails to control this technology..

For latter case, in the end, you get some quite reasonably good resistors, but which are not remarkably better than PWW technology.

You may decide by your own.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 18, 2015, 12:27:02 pm
Dr Frank, it may also be that the "good ones" are cherry picked for important customers while us mortals end up without those golden ones ???
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on March 18, 2015, 03:24:11 pm
Dr Frank, it may also be that the "good ones" are cherry picked for important customers while us mortals end up without those golden ones ???

That means, that there would be the possibility that Vishay regularly offered T.C. selected parts, and they would have the possibility to make a 100% screening of production parts.

I asked the Vishay representative about that, exactly, and he responded 2x "NO".

The only possibility would be to buy a 100 fold quantity and select on your own.

Or they offer other parts with guaranteed lower T.C.
The trick with that special type is, that Z-foil is NOT used at all, but one C and one K foil element in series!
C has positive parabola, K a negative one.. obviously more predictable than Z foil..

I think, it was VHD100.., or so. VHP101.. maximum 10ppm change over 30°C window, gives 0.3ppm/K average.


So I really wondered, what they offered to Mr. Pickering, they did not want to offer to me..
His 1281 was full of Vishay BMF, so he had a better standing, obviously

Frank
Title: Re: T.C. measurements on precision resistors
Post by: acbern on March 18, 2015, 03:50:22 pm
Besides the fact that the Wekomm part in my opinion is way overpriced (even when considering its calibration), one should put things in perspective. The SR104 is a true primary standard (historically at least, today it is rather Quantum Hall of course). A resistor standard based on VPG hermetic foil resistors is both pretty stable (I can confirm Dr Franks stability data based on cal data) and has a low TC (0.2ppm/K is not bad, I have seen SR104s with similar and higher TCs). Compare this to the not really cheap Fluke standard resitors, and you will see that this is a very good alternative to the SR104s and Flukes out there, given its price. Nobody considering to buy a SR104 would seriously consider the Wekomm anyways, it is adressing another market in my opinion, more a competiton to Fluke.
How Wekomm came to the conclusion to charge 5k for it is beyond me though; even the most expensive VPG hermetic 4 terminal 10k resistor I have been offered was a few hundred bucks only, plus say 500 for a good cal if they cannot do it themselves.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 18, 2015, 04:51:33 pm
Besides the fact that the Wekomm part in my opinion is way overpriced (even when considering its calibration), one should put things in perspective. The SR104 is a true primary standard (historically at least, today it is rather Quantum Hall of course). A resistor standard based on VPG hermetic foil resistors is both pretty stable (I can confirm Dr Franks stability data based on cal data) and has a low TC (0.2ppm/K is not bad, I have seen SR104s with similar and higher TCs). Compare this to the not really cheap Fluke standard resitors, and you will see that this is a very good alternative to the SR104s and Flukes out there, given its price. Nobody considering to buy a SR104 would seriously consider the Wekomm anyways, it is adressing another market in my opinion, more a competiton to Fluke.

Yes, it is not fair to compare a Volkswagen with a Ferrari. But it would be, if they had the same price tag. And with the Wekomm resistor that is actually the case. It costs the same as a new SR104 (or 4-5 used). First I suspected a decimal error though...

For comparison the VHA518-7 resistor which is the heart of the unit was approximately $50 when I last asked. And based on the graph I draw from the calibration data, there seems to be no real improvement in tempco to the off-the-shelf VHA518.

Even if there was, why pay $5500 for a tempco selected resistor when you can buy a basic VHA518 for hundred times less and ovenize it. With an aluminium block and a relatively simple thermostat the tempco will be excellent . And you also get rid of the possible irriversible or long time constant errors caused by the temperature variations. The power comsumption of the oven will be minimal because the tiny resistor is easy to insulate very well.

The drift is another issue. 1 ppm/year vs. SR104 <0.1 ppm/year typical. The two resistors measured by the VSL showed drift rates of 0.4 and 1.9 ppm/year. Most importantly it is not (yet) predictable. Accelerating like some other Vishay resistors?

I know this a tough business and there are not too many new products introduced outside the big players. So we should treat the new company nicely and understand that their first product may not be a direct hit. But I still feel uncomfortable with their "marketing first" approach which reminds me a little of the high-end audio business.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 18, 2015, 08:12:08 pm
To Dr. Frank, reply #367, to clarify again, I did not say all of Vishay's papers were laughable and inept, only some of them are and those particular ones were reviewed by some other people (Bob Pease was one of them) and agreed that they were what they were.

It was my understanding that Wekomm was a young independent Dutch company like so many others, bought out by Vishay.  As I've said many times in the past, Vishay does not like competition, they will either buy them up or try to belittle them if they believe they are any kind of a threat to them, that is a legacy of Felix.  Vishay has destroyed many reputable resistor companies over the years with no qualms about it.  I agree that Vishay does make some very good resistors but the customer must be fully aware of Vishay's 'tricks' in advertising and data sheet shenanigans or else you run the risk of not getting what you thought you were.

The wonky TCR curve noted in the Wekomm calibration sheet is indicative of multiple resistors being 'matched' together for a given low TCR, while this can work exceedingly well for a narrow temperature range, it does not work well over wider ranges unless the TCR curves are linear and not one single Vishay part has completely linear curves over temperature.  The curves are still wavey even if they are of low TCR values and this may not be an issue in a given circuit if the TCR only needs to stay within a given range but most designers would much rather have a linear TCR that is predictable over range.

As to Dr. Franks speculation on Vishay's cherry picking resistors, if they are doing it and that is highly likely in some cases, I rather doubt they would publicly acknowledge it as they would consider it a blight on their reputation that they could not control their manufacturing processes close enough and in some instances, Vishay has 'quietly' acknowledged this fact off the record.  I very seriously doubt you will ever get that comment on record from Vishay.

Given that the Wekomm part is made up of so many resistors, the time and labor involved to cherry pick all these resistors for one unit is probably is sufficient enough in their eyes to warrant a $5,500 price tag, however, until this standard can consistently demonstrate characteristics long term to warrant that kind of money, I think they are over pricing themselves out of the market for the time being.  While I have not been terribly impressed with the Fluke standards construction (they cherry pick too), they at least do have something of a track record for reasonable long term stability but I also think they are a bit over priced as well for what is in there vs performance.
Title: Re: T.C. measurements on precision resistors
Post by: MK on March 18, 2015, 09:31:56 pm
One thing that concerns me about that Vishay foil "std" resistor, no thermal well for the thermometer, how can you confirm it is at a known temperature without having a very very steady temp in the cal lab...
Title: Re: T.C. measurements on precision resistors
Post by: EEVblog on March 18, 2015, 10:38:45 pm
I think the Wekomm resistor is meant as a "secondary transfer standard", to compete with the likes of the Fluke 742A series [which they are discontinuing].

Of course it's a transfer standard. Not sure why anyone would think it's a primary standard?
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 18, 2015, 11:05:17 pm

The good old ESI SR1 nowadays manufactured by the IETlabs. Manganin (Zeranin) on mica card. Stability specification 2 ppm/year, much less in practice. Tempco 1 ppm/C.

The 10 kohm version costs $560 according to the 2012 price list.

http://www.ietlabs.com/decaderes/resistance-standard/esi-sr1-calibration-resistor.html (http://www.ietlabs.com/decaderes/resistance-standard/esi-sr1-calibration-resistor.html)
Title: Re: T.C. measurements on precision resistors
Post by: texaspyro on March 18, 2015, 11:12:14 pm
I have a box with an SR-104,  10V reference,  and GPSDO in it.   The thermal control temperature is set at the upper point where the SR-104 TC curve crosses 10,000 ohms.  Self-heating from the GPSDO / Vref / PWM'd fan mixing in external air does the temperature control.  The components are baffled/isolated from air currents from the fan.  RMS variation of the internal temperature is in the millidegree range.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 18, 2015, 11:34:18 pm
Question : Manganin alloy naturally offers "parabola" shaped resistance/temperature curve, with a perfect zero slope near 25°C. Produced since May, 1893 (Wikipedia said). Some of you pointed out that at this time it's impossible to find a resistor with such characteristics, even from the best manufacturers like Vishay. Since the material is available for more than a century, I don't understand why this performance isn't achievable today. Is it about manganin manufacturing related issues (Soldering)? Did I miss something?

Manganin and its relatives are still widely used (even in bulk metal foil resistors). It is easy to solder, you just need to be fast.

But there are even better alloys available (stability, tempco, resistivity, corrosion). Edwin is the expert.
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on March 18, 2015, 11:46:21 pm
Snip*
If you want to double your accuracy, you have to more than double your price
SRL - $1974.00-$4895.00, for the almost 5K unit I would guess it is for the low/high ohms units.
Adjustment to norm/year drift/ 18-25C, with 23.0 nominal
1 k?
±2 ppm /±1 ppm / 3 ppm tot
 
1.9 k?
±2 ppm / ±1 ppm / 2 ppm tot
 
2 k?
±2 ppm / ±1 ppm / 2 ppm tot
 
4 k?
±2 ppm / ±1 ppm / 2 ppm tot
 
10 k?
 
±2 ppm / ±1 ppm / 1.5 ppm tot
 
19 k?
 
±2 ppm / ±1 ppm / 2 ppm tot
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 19, 2015, 01:29:10 am
Hello EmmaneulFaure,

Manganin does not have a perfect zero slope TCR at the cardinal temperature, at the very least not consistently, the hyperbolic TCR curve of Manganin is inherent in the alloy, it is 'considered' relatively 'flat' for about ±5°C from the cardinal reference before the TCR starts rapidly increasing.  Zeranin, a relatively new derivative, increases the relatively flat portion of the TCR curve to about ±10°C from the cardinal before taking the same serious change in TCR rate.  I believe there is one or two other derivatives which make small tweaks to the 'flat' portion of the curve and extends the upper range to about 65°C, this not to say the TCR remains as low as at the cardinal point, this extension was made by changing the mix of metals in the alloy slightly, the overall characteristics are still basically the same.

Low ohm resistors and shunts are still made with the Manganin family although it is not generally used for standards any longer being replaced by the Evanohm family alloys, they not only have a very linear TCR but their stability is better and are tough to boot.  Manganin cannot be operated at elevated temperatures, not much above 65°C because they anneal at very low temperatures and that changes them permanently.

Manganin's lack of use is primarily because they have been replaced by superior alloys and better technology in using those alloys.  Manganin was easy to use because it could be soldered (carefully), Evanohm cannot be soldered, it must be welded.

No matter what the alloy used for a low TCR / high stability resistor, the processing of the resistor demands very careful controls and depending on the technology used, those processes are considerably different.  My processes are very different from Vishay's as there are different variables involved.  Another consideration is that current metallurgy cannot control the batch mix of metals tight enough to provide consistent characteristics batch to batch, the fact that we can produce very low TCRs with very long low stabilities these days can be attributed to some 'cherry picking' of the alloy characteristics as every batch of alloy is different.  While the resistor industry can push the alloy providers for ever tighter mix controls there is a limit as to how tight they can get it for a reasonable price.  My most expensive wire runs over $18,000 a pound which depends on the wire size, I have no control over that aspect.  I have a very limited number of alloys which can meet my wire specifications, outside of those, there are none.

Just remember, whether it is a PWW or a metal film/foil resistor, your TCR limits is going to cost more and more as you demand zero TCR because those very low TCRs are in short supply, we really can't create them at will unfortunately.
Title: Re: T.C. measurements on precision resistors
Post by: MK on March 19, 2015, 07:56:41 am
does that mean that for the LTZ1000 temp circuit it would be cheaper to get a matched pair od 1-3ppm resistors than to insist on 0 ppm for both?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 19, 2015, 06:03:07 pm
Yes, once you start specifying TCR under 0±3PPM/°C (as an example), asking for TCRs 0±1PPM/°C will immediately increase the price over and above the tolerance cost (tighter tolerance, higher price), then depending on who you are asking, matching (tracking) TCR will involve another price multiplier and if the values are far enough apart, you may encounter yet another price point.  Vishay does supply multiple resistors in a matched configuration, some are even on the same substrate, I'm not sure what their pricing for these 'custom' networks are of late, but as I recall, the price point is fairly high and the delivery is long.  These networks do have very good specs within their limits.

I do not have the capability to put multiple resistors on a single bobbin mainly due to physical problems, I have given a lot of thought on how to do it, no practical solution as yet.  The PWW solution to multiple resistor networks has always been to encapsulate them in an appropriate box, while this works, there are thermal limitations to it.  It also runs up the cost for the labor and materials to do this.  I have been asked by customers for a 'dual' resistor bobbin design, I am looking into it.

I have supplied resistor networks to customers with the recommendation that they wrap a piece of copper tape snugly around the resistors, side by side and after assembly, encapsulate the entire board in foam to prevent air drafts.  While this arrangement has a bit longer thermal trail, once the board reaches thermal equilibrium, everything tracks tightly and stays in track.  I use certain manufacturing techniques in making these networks, they do cost more than a standard line part but their performance has been reported as exceptional and they have enhanced processing for long term stability.

Sorry I can't divulge the customer's names, they are under NDA so I can't say much about the details either except that tracking has been well under 1 PPM°C.  I can build the networks for anyone else as they are my designs of course.
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on March 19, 2015, 07:52:43 pm
Edwin,

Would the use of a thermal epoxy like this http://www.arcticsilver.com/arctic_silver_thermal_adhesive.htm (http://www.arcticsilver.com/arctic_silver_thermal_adhesive.htm) be a good way to thermally couple resistors and have you heard of any issues arising from its use?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 19, 2015, 08:15:55 pm
Hi,

I don't foresee any problems using this stuff although it might be more expensive than copper foil tape but it should provide a good thermal conduit between the resistors.  I certainly wouldn't us it on bare foil resistor though. <grinning>  I would still strongly recommend the foam encapsulation of the board, it doesn't have to be too thick and it really works, keeps everybody inside at a pretty even temperature.  It definitely would not bother my resistors to use this adhesive.
Title: Re: T.C. measurements on precision resistors
Post by: texaspyro on March 19, 2015, 08:28:48 pm
Even the best thermal expoxies are rather poor conductors of heat.  I make my own out of either silicon carbide or diamond.

Also, when using copper foil to link the two resistors together you should use a thermal transfer medium (i.e. thermal paste) between the copper and the resistors.  Small air gaps between the copper and the resistors have a great effect on the thermal coupling.  And, as with all thermal pastes,  use the bare minimum amount needed to fill the gaps...  thermal pastes also have horrible thermal conductivities.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 19, 2015, 08:54:25 pm
While it is true that unfilled epoxy has lousy thermal conduction properties, there are many 'filled' epoxies that do have much improved thermal conduction.  Glass fibers or balls, metallic oxides (for non-conduction) such as alumina, ceramic and in the case of the filled epoxy asked about here, it is filled with silver.  All improve thermal conduction.  Depending on the formulation, the thermal conduction properties can be increased considerably, the drawback is extra cost.

The object here, in the case of thermal equilibrium is not necessarily the lowest thermal conduction possible but keeping the resistors in the network at the same temperature (or close to it, nothing is perfect) and at the same temperature as the assembly is a plus.  Therefor one does not need the best possible thermal bridge between the resistors, after a bit of time has passed, they will reach equilibrium and any apparent drifting will cease.  Copper tape does achieve this and does it quite well at a reasonable cost, thermal paste may or may not be an advantage here, the adhesive on the tape tends to fill in most of the gaps and any micro gaps tend not to be significant.

Perfection sounds great in theory but in reality, it doesn't work that way, there is a point of no return for effort, trying to get that last degree can be very costly and in the end, not significant over all.
Title: Re: T.C. measurements on precision resistors
Post by: paulie on March 19, 2015, 11:38:33 pm
I have supplied resistor networks to customers with the recommendation that they wrap a piece of copper tape snugly around the resistors

I believe the concept, known universally as "passive heat-pipe technology", might have first been put into practice by vref pioneer and chief engineer of Calibratory Corp and designer of the highly acclaimed D105. At least held in high esteem here on Eevblog forum in the thread devoted to that product.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 19, 2015, 11:57:03 pm
Nope, that copper tape concept has been around for quite a few years, nothing new.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 20, 2015, 09:50:31 am
If you are referring to a, by now, conventional Heat Pipe, it's a pipe partially filled with liquid and vacuum, to start with. The pipe is hermetically sealed in both ends. The liquid has low boiling point and boils of in the hot end and condenses in the cold end. It will transfer much more heat than solid copper! But it works best / only with the pipe vertically, hot end down. It may only work with significant temperature differences, I don't really know.

I believe the concept, known universally as "passive heat-pipe technology"
Title: Re: T.C. measurements on precision resistors
Post by: paulie on March 20, 2015, 12:51:55 pm
Sorry, that was another poor attempt at sarcastic humor relating to the somewhat horrific feeding frenzy in this thread:

https://www.eevblog.com/forum/testgear/calibratory-d-105-dc-precision-voltage-reference-standard/240/ (https://www.eevblog.com/forum/testgear/calibratory-d-105-dc-precision-voltage-reference-standard/240/)

you came here with a superiority complex....and then made wild, bogus claims about things like "heatpipe" technology....which your device clearly does NOT employ.

We can start with claims about your miraculous "heatpipe" technology.....how about claims of somehow bending the laws of physics, to improve Ti's technology?  Face it, the ref you are selling simply proves the quality of Ti's REF102C package...DESPITE your best efforts to corrupt it's implementation.....

The fellow was a little off his rocker with the vampires and God stuff but the voltage reference itself performed well beyond expectations in multiple reviews there and elsewhere too yet was the object of IMO mostly unjustified technical criticism. Personal attacks from otherwise professional and knowledgeable individuals were despicable.

However I should mention that the teardown (literally) there of Awesomes product was the first time I saw anything like that tape trick and the earliest photos searching the internet were from his implementation. I'm sure he wasn't really the first and true it's not what's commonly called "heatpipe" but still...  Let's drop the subject before this excellent thread gets contaminated any more. I apologize for the diversion.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 20, 2015, 01:27:20 pm
OK, get it. I followed that thread in the beginning too but dropped out, so yes, please, lets drop it here too.
Title: Re: T.C. measurements on precision resistors
Post by: IanB on March 20, 2015, 04:24:28 pm
It is about 4000 times more efficient at moving heat than a similar sized copper bar might be-- and is much lighter as well.

That number of 4000 is very large and rouses the skeptic in me. For instance the linked Wikipedia article has a statement,

Quote
The effective thermal conductivity varies with heat pipe length, and can approach 100 kW/(m?K) for long heat pipes, in comparison with approximately 0.4 kW/(m?K) for copper.

--and even that ratio of 100/0.4 = 250 seems itself remarkable.

I suspect this is an area where the implementation details make all the difference and generalized statements are of limited applicability.
Title: Re: T.C. measurements on precision resistors
Post by: MK on March 20, 2015, 07:14:45 pm
One company I used to work at used heat pipes, and at a low delta T they do not perform as well as one might hope.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 20, 2015, 08:03:06 pm
Hello,

it´s time to come back on the topic.
For those who have forgotten: its T.C. measurements on precision resistors.

After a long journey I picked up some resistors from local customs office today.

5 days from Grand Junction to San Francisco
2 days from San Francisco to East Coast
2 days to Germany
and finally after nearly one week intensive customs treatment I could pick them up.

In the mean time I have rearranged my T.C. setup with the "golden Z201" as reference resistor.
So after having made a suitable fixture for the Ultrohm Plus resistors I will start with T.C. measurements.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 20, 2015, 08:24:29 pm

This is going to be interesting. Especially the hysteresis (or lack of it).

After a long journey I picked up some resistors from local customs office today.
5 days from Grand Junction to San Francisco
2 days from San Francisco to East Coast
2 days to Germany
and finally after nearly one week intensive customs treatment I could pick them up.

Based on the plastic colour they traveled from the 1970s.
Title: Re: T.C. measurements on precision resistors
Post by: 3roomlab on March 20, 2015, 11:09:39 pm
@andreas, how much (ball park) did those cost you?
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 21, 2015, 04:38:21 am
Hello 3roomlab,

pricing for the 0.1% 3 ppm/K standard tolerance which I ordered is on the LTZ1000 thread:

https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg602719/#msg602719 (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg602719/#msg602719)

So its comparable to that what you will pay for UPW50 or 8G16 resistors from RS / Farnell in small quantities.
But with the service that you can get every individual resistor value.

Shipment was around +11% for the 33 resistors with USPS.
And import tax (=VAT) adds +19% on the total in germany.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 22, 2015, 11:04:14 am
Hello,

after measuring the golden Z201#6 I had the hope that the other Z201#7 that I ordered at the same time would be behaving similar.

But first surprise: the datecode was B1315- (and not B1305- as on the "golden" resistor)

Second surprise: T.C. + hysteresis is much larger.

The results of Z201#7 Datecode B1315- on 21.03.2015
T.C. with box method: 0.818591506 ppm/K

3rd order LMS interpolation referenced to 25 deg

A 0 = -1.43422451376099E-0001
A 1 =  9.64007179982438E-0001
A 2 = -1.26483309879412E-0002
A 3 = -3.34940837358552E-0004

So T.C. at 25 deg is 0.96 ppm/K

max deviation from LMS 2.57933723463504E+0000 ppm (hysteresis + noise)

In the mean time I am in doubt what the data sheet terms
"typ. TCR" and "max spread" really  mean.
"typ." seems to be the average value over a large number of batches and samples.
And with "max spread" I am not shure wether the 1 sigma or the 3 sigma limits are meant.
Up to now I had expected that max spread means something with 3 sigma. But I have already
2 samples which are above +/- 0.2 +0.6 ppm/K on Z201 at least for the slope at 25 deg C.
But the data basis is still not sufficient for any statistics.

I have also attached the overview and included Franks results for the 1K00 resistors.
The hysteris cannot be compared directly since the noise
on Franks HP3458A is about a factor 5 lower than my setup
and is determined "graphically" at 25 degrees.

The test fixture for Edwins Ultrohm Plus 805 style resistors (I will refer to them as UP805) is nearly ready.
So I will most probably start with first measurement on 1K resistors tomorrow.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 23, 2015, 07:48:45 pm
Would appreciate feedback on this idea:
https://www.eevblog.com/forum/testgear/temperature-test-box-for-component-characterization/msg635672/#msg635672 (https://www.eevblog.com/forum/testgear/temperature-test-box-for-component-characterization/msg635672/#msg635672)
Title: Re: T.C. measurements on precision resistors
Post by: 3roomlab on March 23, 2015, 10:28:17 pm
Hello 3roomlab,

pricing for the 0.1% 3 ppm/K standard tolerance which I ordered is on the LTZ1000 thread:

https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg602719/#msg602719 (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg602719/#msg602719)

So its comparable to that what you will pay for UPW50 or 8G16 resistors from RS / Farnell in small quantities.
But with the service that you can get every individual resistor value.

Shipment was around +11% for the 33 resistors with USPS.
And import tax (=VAT) adds +19% on the total in germany.

With best regards

Andreas

thanks for that. they look rather affordable  >:D
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 24, 2015, 02:03:13 pm
Andreas, I don't want to hijack your thread, but I'm doing my first measurements right now, showing results "real time" in thread
https://www.eevblog.com/forum/testgear/temperature-test-box-for-component-characterization/msg636219/#msg636219 (https://www.eevblog.com/forum/testgear/temperature-test-box-for-component-characterization/msg636219/#msg636219)
Title: Re: T.C. measurements on precision resistors
Post by: branadic on March 24, 2015, 08:56:23 pm
Edwin, please let me ask, why are your resistors look like they do? Is this (ugly) yellow color some sort of corporate design? My guess is some black jacket would have been more effective in advertising.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 24, 2015, 11:08:28 pm
Well, despite the fact that yellow is my favorite color, I didn't pick the color, my colleague picked it and it is the original color.  For one thing, it does stand out.  In the many instruments our resistors went into, you couldn't miss them.  We weren't the only one, one other resistor house here in the States actually had different colors for each of their different series including red, blue and green.  I must admit, that green was kind of sick looking, matter of fact, my bobbin supplier actually insisted that I take some bobbins that he had made up in this 'green' color, I've still got some of them in stock.  Print will show up quite nicely on the yellow, black particularly well, white will also work.  Some of the other resistor houses do use a black shell but they really don't 'stand out' and trying to wind wire on a black bobbin....doesn't work very well at all.  For the time being, they will have to stay this color, particularly as long as the inventory holds out but even then, I do not think I will go for black.  Ultronix used a relatively dark blue but then the supplier stopped making it, they ended up having to use a sickly green color as there was only one source for the stuff.  Ultronix did make a line of shelled resistors which were black, I still have some of them.

Besides, yellow has been used many times, Sprague Atom electrolytics for example and there are still some capacitor manufacturers that are still using yellow so there is old precedent for the color.

I must say, you are the first one to say they are ugly <grinning>
Title: Re: T.C. measurements on precision resistors
Post by: babysitter on March 25, 2015, 12:51:55 am
Hey Branadic, go get your thermal camera to Andreas and figure out the time how long it takes for a given power to heat up the shell/ cool after a given (even tiny) power pulse/how hot it gets/etc. ! :)
Title: Re: T.C. measurements on precision resistors
Post by: TimFox on March 25, 2015, 05:15:28 pm
A former coworker had worked for a company who always used a very green color for their panels.  My buddy dummied up a unit with a more "normal" looking combination of metal and black and showed it to the company president.  He merely replied, "Green is the color of money", and kept the panel color.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 25, 2015, 10:18:39 pm
Andreas, do you normalize all measurements to 25C? I should use the same as you  8)
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 25, 2015, 10:44:53 pm
Hello,

first results from Ultrohm Plus resistor (UP805) 1K #1  0.1% +/- 3ppm/K
shipment date 1510 (not printed on device).

First the pictures of the test fixture.
keeping the wires (solder junctions) as good as can to the same temperature.
Similar to the latest Z201 test fixture.
Remembering that most of the heat is transferred by the wires.

Measurement of 23.03.2015

T.C. Box :  -1.147786492 ppm/K
T.C. LMS @ 25 deg: -1.14174590659788 ppm/K
max deviation from LMS: 2.62806722886730 ppm (Hysteresis + noise)

Measurement of 24.03.2015 (reverse polarity of reference voltage)

T.C. Box: -1.118012147 ppm/K
T.C. LMS @ 25 deg: -1.15136479955225 ppm/K
max deviation from LMS: 2.22772720963622 ppm (hysteresis + noise)

Measurement of 25.03.2015 (normal polarity again)

T.C. Box: -1.153801472 ppm/K
T.C. LMS @ 25 deg: -1.14061584826123 ppm/K
max deviation from LMS: 2.35918437692836 ppm (hysteresis + noise)

On the measurement of 23.03. it was not clear wether there is some ageing drift or if it is only some creeping effect (hysteresis) at lower temperatures.
but since the measurement of 25.03. has only 0.5 ppm difference at 25 deg LMS offset value (which is within hystersis) I think that it is not ageing.

The hysteresis of this sample is comparable to my UPW50 resistors.
T.C.  (1.15ppm/K) is well below the spec of 3ppm/K.
And better than most of my UPW50 samples.
(and well below the 8G16 resistors of Frank).

@Edwin: There remain following questions:
Is the temperature treatment to 150 deg a standard procedure or only on request?
In the datasheet the wattage of the 805 style resistor is not given.
Which material is the connection wire? (copper?)
Is it possible that the epoxy still hardens? Is it hardened thermally or by a hardener?

with best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 25, 2015, 11:02:23 pm
Andreas, do you normalize all measurements to 25C? I should use the same as you  8)

I do this only for the LMS calculation.
(and the difference to LMS).
For me the T.C. around 25 deg is the most important.

In metrology you would use 23 deg. (+/- 1 .. 5 deg).
Since the temperature in my "lab" ranges from 18-32 deg (winter/summer) I use the 25 deg.
All components (resistors, references) are usually also normalized to 25 deg.

And this is mainly due to the fact that the mathematical (rounding) errors
during LMS calculation are lower when I normalize the values near zero.
Although this is usually not relevant for 3rd order approximations with double/extended floats.
But if you do a 5.th or 6.th order approximation you will see differences.

You can use what is more convienient for you.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 25, 2015, 11:37:47 pm
first results from Ultrohm Plus resistor (UP805) 1K #1  0.1% +/- 3ppm/K
shipment date 1510 (not printed on device).
So your has about -1.1ppm/C while mine has something like +0.7ppm/C at this temperature range  :-DMM
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 26, 2015, 07:49:34 am
@Andreas:

You should probably move the sense wire closer to the body of the resistor-- Edwin's spec is 3/8-inch I think [but check the data sheet].  Copper has about 4000ppm/K TCR, and because this is a lower value resistor [1K?], this might affect your data a little bit [and certainly in the sub-ppm range].

I'm not believing the hysteresis, I think that there is a time lag between what temperature the resistor is at and the reading from the temperature sensor.  I could be wrong, but if you sweep the temperature slower and pulse the drive current on the resistor [and sensor if it takes current] only while taking a reading, then I think they will track better.

That said, I think Edwin's resistors are showing very well so far...  Time for me to order some...

Hello Ken,

as Frank already calculated for his 120 Ohms resistors the wire lenght does not really contribute to the T.C.
Edwins wires are more stiff than others. Actually the wire diameter is 0.8 mm (AWG20) so having 33 uOhms / mm.
so if I would shorten the wire lenght by 30 mm on each side I would get a lower resistance of 2 milli Ohms.
With respect to the 1 K resistor the +4000ppm/K contribute only 1/500000 to the total T.C.
So for the 1K resistor please subtract these 0.008ppm/K from the result (giving rounded -1.16 ppm/K instead of -1.15 ppm/K)

Since I cannot reduce further the ramp speed I will do a accellerated test today with 0.3 K/minute instead of 0.12K/minute.
(just to see if the temperature lag increases).

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on March 26, 2015, 10:46:19 am
The cal sheet for my Wekomm standard
http://www.eevblog.com/files/WekommResistanceStandardCalSheet.pdf (http://www.eevblog.com/files/WekommResistanceStandardCalSheet.pdf)

Kalibriercentrum Bayern. Not a primary lab, but very high capability. Volt/ohm nut friendly???

http://www.kalibriercentrum.de/pdf/DAkkS_Urkunde_Bayern.pdf (http://www.kalibriercentrum.de/pdf/DAkkS_Urkunde_Bayern.pdf)

(http://www.kalibriercentrum.de/pix/CIMG1916.jpg)

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 26, 2015, 05:47:31 pm
Andreas (reply 416),

The standard process thermal conditioning which exceeds 150°C is done on all resistors, this reduces aging and drift to below the given specs.  There are additional processes which I can apply to further reduce such effects, some customers do require such processing but most do not as the standard stability exceeds most other PWWs to begin with.

That power rating of the 805 bobbin is 0.33W @ 125°C ambient.

The leads are oxygen free copper, tinned.

The epoxy uses a hardener, the manufacturer recommends room temperature curing but also specifies a 60°C bake if time is short.  The epoxy sealant on the end uses an epoxy based color ink which does recommend a short bake @ 121°C for two hours or 150°C for 15 minutes, this colored epoxy is the same stuff used for printing on the resistors.  I'm curious, why do you ask?
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 26, 2015, 08:31:29 pm
The leads are oxygen free copper, tinned.

The epoxy uses a hardener, the manufacturer recommends room temperature curing but also specifies a 60°C bake if time is short.  The epoxy sealant on the end uses an epoxy based color ink which does recommend a short bake @ 121°C for two hours or 150°C for 15 minutes, this colored epoxy is the same stuff used for printing on the resistors.  I'm curious, why do you ask?

Hello Edwin,

oxygen free copper: you could sell them to audiophiles for a much higher price.  >:D

I only asked because on other resistors up to now I have seen potrusions from baking the epoxy.
On your resistors I see shrinking epoxy like on some capacitors.
I am just curious to find out how the components are built.

On the first day of measurement I thougth that there could be some ageing drift due to hardening of the epoxy.
But since I measure only 0.5 ppm during 3 days (cycles) which could also be on the reference resistor or
due to hysteresis I do not think that there is a significant ageing drift.
I think I will let the resistor in the thermal chamber and do one comparison measurement after all the other 1K resistors.

And the power rating should be on the invoice the next time in germany (just in case the customs asks).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: TimFox on March 26, 2015, 08:38:12 pm
If nothing else, OFHC copper can handle many more mechanical cycles of bending than can ETP copper.
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 27, 2015, 08:10:37 am
I have seen the comments on hysteresis or not. I do not believe they are time lags. For example the Z #7, the difference in temperature domain is at maximum around five degrees between up / down. It is imo very unlikely that you have such a big temperature difference with the type of setup and slow ramp that you have. Could you simply stop the cycle at mid point, both up and down, let it stabilize for an hour to see if the effect is still there or not? 
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 27, 2015, 09:08:55 pm
I have seen the comments on hysteresis or not. I do not believe they are time lags. For example the Z #7, the difference in temperature domain is at maximum around five degrees between up / down. It is imo very unlikely that you have such a big temperature difference with the type of setup and slow ramp that you have. Could you simply stop the cycle at mid point, both up and down, let it stabilize for an hour to see if the effect is still there or not?

I already did this experiment with a UPW50 resistor: (see page 1)
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462300/#msg462300 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462300/#msg462300)
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=98751 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=98751)

and also with Z201#3
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg606730/#msg606730 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg606730/#msg606730)
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=135525 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=135525)

even with 120-200 minutes (2-3 hours) settling time there is a remaining offset.
It needs one whole night to reach the previous value.
For the Z201 in the data sheet a thermal settling time of 1 second is specified.
So after 1 minute the resistance should be settled within 0.001 ppm or less.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: acbern on March 28, 2015, 08:30:41 am
The cal sheet for my Wekomm standard
http://www.eevblog.com/files/WekommResistanceStandardCalSheet.pdf (http://www.eevblog.com/files/WekommResistanceStandardCalSheet.pdf)


Kalibriercentrum Bayern. Not a primary lab, but very high capability. Volt/ohm nut friendly???

Well, my experience, pretty expensive, and thus not volt/ohmnut friendly in my view. I did a search some time ago in Germany for calibration of 10k and 10V standards. Got many quotes and found that ESZ is pretty cost efficient for both (ISO certificates), and when asking for an ISO certificate they can certify accuracy limits below 0.5ppm for both, so I use them for my standards. And: I can drive by and drop off my standards and pick them up, thats good too (and from time to time have a little chat with the lab guys). For thermal converters I use Testo, because they have an automated procedure, so very good accuracy and at the same time cost efficient because few labor hours involved. RF power I do at Rohde/Schwarz, acceptable prices and they can adjust the EEPROM in their power heads.
These are all the calibrations I need.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 28, 2015, 05:42:19 pm
Hi Andreas (reply 423)

All resistors have many interacting mechanical forces on them, they are complex in nature and like most things, decreasing the effects of one force may amplify another.  While the mechanical forces in PWW and film/foil resistors share many common effects, there are differences because of the different types of construction as well.

PWW have hoop stress from winding the wire onto a bobbin, film/foil do not but film/foil have a similar type of stress from being attached to a ceramic substrate.  Encapsulation stress is present in both types for different reasons, both have partial remedies.  Film/foil are sensitive to moisture, partly due to absorption through the encapsulation and around the leads, mine have no significant affects.  Film/foil are particularly sensitive because the resistive element is bare and the circuit paths are very small and close together allowing water molecules to bridge them easily, baking is a temporary fix.  Foil/film resistors have an applied voltage coefficient, PWWs do not, again this is a artifact of film/foil construction.  Both types of resistors use very similar alloys, the basic aging rates are normally nearly identical.  Drift rates are complicated, mainly the result of residual mechanical stresses which are very difficult to completely remove.  This involves additional 'enhanced' (as Vishay likes to call it) processing to further reduce those stresses, i.e. costs more.  Oil filled hermetic sealing tends to be more beneficial to film/foil as it removes more of the external sensitivities that film/foil has to their environment such as moisture and barometric pressure which PWW resistors have insignificant sensitivity to.  One other important characteristic, film/foil are noisier than PWW resistors, particularly mine.

"I only asked because on other resistors up to now I have seen protrusions from baking the epoxy.
On your resistors I see shrinking epoxy like on some capacitors." - Andreas

I am not sure what is causing the protrusions you are seeing on the other resistors, it may be caused by a filler they are using in the epoxy.  The epoxy end 'seal' on my resistors is purely mechanical in nature, to hold the resistor inside the shell and support the lead assembly, the epoxy may not form a complete seal around the metal lead, this is of no concern as it does not affect the resistor's electrical characteristics.

Aging tends to be of a nonlinear nature, as exhibited in the better resistors, aging (which is a form of drift) tends to decrease with increasing time reaching a fairly level, stable rate after some period of time.  Just how much the initial rate is and the eventual stable rate becomes depends on the resistor technology and quality of construction.  As I've pointed out before, the more 'perfect' a resistor you want, the more expensive it is going to be and that cost tends to rise exponentially.  You are also going to rapidly run into the always present law of diminishing returns, that last little bit of improvement becomes so expensive, it isn't worth the trouble.

Speaking as an design engineer, respectfully:

Fluke, HP (Agilent, Keysight), ect. all recognise this, that is why their voltage references perform to a certain level for that cost because it is a happy balance.  It is much cheaper to have those references checked against a primary standard periodically than to try and put a primary standard inside their instruments.  Would anyone be willing to pay say five or ten times more for a 3458A with a primary standard inside?  I really doubt it, the performance of the LTZ1000/A with good resistors around it provides the necessary performance at a reasonable price.  The Fluke voltage standard ups the anti by using multiple LTZs to improve the performance but those aren't cheap and no one is going to make a single LTZ perform as well as that Fluke standard no matter how good the resistors are.

Those LTZ resistor sensitivity numbers given in the data sheets/apps are only approximate as stated clearly there, every LTZ is a bit different.  If you want to know about this stuff, just ask Linear Tech, the people who designed the chip.  If there were any relatively 'easy' tweaks that could be done to improve the performance, don't you think Linear Tech or Fluke would have already discovered them after all this time?

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 28, 2015, 07:07:16 pm
If there were any relatively 'easy' tweaks that could be done to improve the performance, don't you think Linear Tech or Fluke would have already discovered them after all this time?

Hello Edwin,

perhaps they have already discovered some tweaks. But they have to earn money with their products. Some tweaks may be so time consuming that they will not get paid for that.
For me its a challenge to find out how the components work in ppm ranges. And to see how far can I go with self made equipment to create stable references.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 28, 2015, 07:15:22 pm
Hello,

as promised some further measurements

on 26.03. I did a "fast" measurement with 0.3K/minute to see if its temperature lag of the NTC-sensors which creates the hysteresis.
If its temperature lag the hysteresis should increase significantly which is not the case.

on 27.03. a normal slow measurement 0.12K/minute again.
Interesting is that the hysteresis/creeping effect at low temperartures
 is decreasing significantly after several temperature cycles.
(compared to measurement of 22.03.)
So my guess is that there is some humidity effect.

Today I am running the first cycle of UP805#2 1K resistor.

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 28, 2015, 08:10:06 pm
If there were any relatively 'easy' tweaks that could be done to improve the performance, don't you think Linear Tech or Fluke would have already discovered them after all this time?

Hello Edwin,

perhaps they have already discovered some tweaks. But they have to earn money with their products. Some tweaks may be so time consuming that they will not get paid for that.
For me its a challenge to find out how the components work in ppm ranges. And to see how far can I go with self made equipment to create stable references.

With best regards

Andreas

If there were any relatively 'easy' tweaks that could be done to improve the performance, don't you think Linear Tech or Fluke would have already discovered them after all this time?

Hello Edwin,

perhaps they have already discovered some tweaks. But they have to earn money with their products. Some tweaks may be so time consuming that they will not get paid for that.
For me its a challenge to find out how the components work in ppm ranges. And to see how far can I go with self made equipment to create stable references.

With best regards

Andreas

Edwin, Andreas,

LT would have an interest if they did not already "own" this market. No competition in sight. Fluke have no interest spreading their knowledge but obviously have the knowledge to build standards approaching 0.1ppm/year stability.

As Andreas wrote in his first post in this thread, his ambition was to see if it's possible to select & match resistors so they cancel errors. I think that is an approach that the commercial actors might not be able to do, just about nobody would be prepared to pay the cost. I have never seen a survey like the one Andreas is doing. I certainly learned a lot.

The measurements I have done on the LTZ1000ACH, I have not seen elsewhere. They seem to show systematic and significant differences compared to the datasheet (repeated on several different devices by me and by others). Who knows where this ends.................
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 28, 2015, 10:22:41 pm
Jan,

Just what systematic and significant differences are you talking about?  If you are referring to the data table in the data sheet/app that shows the effects resistors have on the output of Vref, those are approximate and nothing more, they are going to differ from chip to chip and there are no specific limits put on them.  There are differences in all of the parameters of the LTZ, some do have specified limits, others do not.  There are intrinsic differences, chip to chip, which cannot be compensated for by external components, long term.  You might be able to do a tweak which will reduce drift temporarily but it will not be permanent.

Basically what Fluke is doing is well known, they are aging and measuring their LTZ chips over a long period, then the chips characteristics, within possible limits, are selected and put together in multiple LTZ summing circuits, just like the technique that was used with the LM399/As years ago.  This time consuming method can and does produce a unit device which can produce stabilities in the neighborhood of the 0.1PPM you are quoting, that may actually be on the better side of average frankly.  The long term stability of an LTZ varies, chip to chip and cannot be predicted accurately, it will settle down to within a given smaller drift range over time which, with multiple averaged chips, could produce the 0.1PPM but with rare exceptions, no single LTZ chip can produce such stability, certainly it could be less than 1 PPM/year in time and may even drop below 0.5PPM / year with more time but otherwise the plain answer is no.  Even if you put perfect zero TCR resistors around an LTZ you are not going to get any better drift rate than the intrinsic rate of the chip.

You are right in that specific aging techniques have been found over the years to age the LTZ chips faster than just letting them sit powered up for many months, from what I hear it is relatively involved and also takes quite some time as well, months.  You cannot subject the chip to too much abuse without causing unwanted changes, for that matter, you cannot subject most components to abuse without causing unwanted changes.

It would seem to me that a Vref stable to around 1 PPM/year that cost less than $100 has more than enough stability to meet most demands.  I would be putting more time into figuring out how to get an stable  ~10V output from this chip that is close to comparable to the LTZ's stability.  There are ways of achieving this, besides, contrary to common view, a 10V standard does not have to have an absolute 10.000000V output, only a stable output, it is easy to calibrate the scale to that offset and it isn't any less accurate, maybe not as easy as an absolute 10V but no cal lab usually has an absolute 10V standard, your DVM is calibrated to that offset voltage in the lab, simple calculator math.

Title: Re: T.C. measurements on precision resistors
Post by: janaf on March 29, 2015, 12:15:32 pm
Edwin,

Lots of things to reply to. The short version would be; please see my previous posts, the last one from the day before yesterday. But here is and example:

The datasheet states an attenuation of 100 of "R1" the current setting resistor. I have measured, on three different devises from two batches, two different board designs, an attenuation of -700. This has also been done by one Chinese poster and verified by TiN on this forum. We all measure an attenuation of near (-)700 (+/-5%) on five different ICs , versus the datasheet +100. I call that a significant and systematic difference.

Another undocumented aspect is how the time drift over temperature looks. We pretty much know that lowering temperature lowers drift, but by how much, what's the spread? I honestly don't even know the sign or if there is one or not. The measurements have also pretty much shown that these values are "known" errors, not uncertainties in the sense that the attenuation is near -700, not +/-100 as you might get the impression in the datasheet. The same is true for the other resistors in the LTZ100ACH circuit, but the differences are less dramatic.

I do not rule out the possibility that the datasheet data is valid for the thermally un-insulated LTZ1000CH.

If, like Andreas, you are trying to match TCR of resistors to the circuit based on the info in the datasheet, you'd definitely be mislead.

Fluke: I'd compare to resistors, it's "well known" how to make them but the devil is in the details, and surely include in-house secrets and lots of details and experience that take you from "good enough" to "the best". To my knowledge, the Fluke standards use a single LTFLU reference each, while it's common for large institutions to have four 732B units in one rack. The now obsoleted 7001 used a single LTZ1000 per module but was also available in multi-unit racks.

10V; yes surely looks a bigger challenge than "only" the raw LTZ1000 circuit output voltage. For some the 7.xx volt stability may be enough, for others approximately 10V is enough while others may require 10.000 000 0
volt.

In the end, what we do depends on our objectives. On a tight budget? Want to learn something? Volt-nut? Need a specialized reference? Are you happy with the 4ppm/year of the 3458A? Then fine. You can buy a stand-alone board on Ebay. If not, simply lower the temperature, maybe up-grade resistors and decrease drift to 1ppm-ish/year as many have reported.
 
I'm in for the learning. I'd like to get down to the nitty gritty details of this. It would have been a lot cheaper for me to buy a used 732B if I "only" wanted a good voltage reference.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 30, 2015, 09:28:17 pm
Hello,

Results of 2nd 1K resistor UP805#2:  shipment date 1510
28.03.2015 first measurement unfortunately had problems with communication connection of one ADC so started too late for a complete cycle.
29.03.2015 reverse polarity measurement. Due to switching to daylight savings also one hour shorter than usual.
30.03.2015 normal polarity like on 28.03.

Evaluation of 30.03.2015

Box: 37.70573087 ppm / -32.51923093 K = -1.15949024 ppm/K

LMS evaluation:
A 0 = -3.89958540734108E-0001
A 1 = -1.20262068020544E+0000
A 2 = -2.06316059338346E-0002
A 3 =  5.13160024309751E-0004

so T.C. at 25 deg is -1.20 ppm/K
max. deviation from LMS: 2.87036703474386 ppm (Hysteresis + noise)

So values are very similar to UP805 #1 with slightly higher T.C. and hysteresis / creeping effect.

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg637341/#msg637341 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg637341/#msg637341)

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on March 31, 2015, 06:33:42 pm
@DiligentMinds,

Looks can be deceiving, the devil is in the details of the resistor construction and manufacturing processes.  I've been busy processing a complicated set of resistor orders and they have to come first of course.

First hand data on what you are asking for is quite limited, secondary information is limited to the data which was provided by other testing labs at potential customers.  The data sheet I posted earlier on the Ultra Precision Reference thread gives considerable indication as to long term accelerated aging, I refer to the 50 cycles of thermal shock (1500 component hours) MIL-STD-202 treatment showing unprecedented small changes in resistance, as is the norm, changes decreased with time.  The amount of change did vary slightly depending on the bobbin style and the winding operator's (one of the significant mechanical variables) abilities.  Unfortunately, the lab only provided the cumulative data figures and no individual resistor data.  As pointed out in the data sheet, one resistor bobbin style had unusual shifts which were later determined to be an error in the selection of wire size, the bobbin was slightly overfilled with wire.

The first hand data comes from a set of matched resistors that Bob Pease had requested 5+ years ago, as is my usual practice, extra resistors were wound in case any were lost during production.  The resistors were 200R0, 807 bobbin (0.250" D x 0.750" L), two resistors turned out within less than 2 PPM of each other  (tolerance), the other two were within <10 PPM.  I sent the closer matched pair to Bob and kept the other set, over the years my two resistors have stayed within the measurement uncertainty / accuracy of my 242D which is always kept calibrated to capability, not specs.  This performance was the result of the enhanced processing carried out in addition to the standard processing of my resistors, I cannot guarantee this level as I do not have accumulated physical detailed data beyond what I've mentioned here.  The long term drift under enhancement is less than half the standard long term drift factors which I've mentioned here as well.  The first hand and second hand data tends to indicate that long term drift appears to be under 2 PPM/year or less. for most bobbin styles.  Without further hard data I would not be willing to absolutely say that enhanced resistors will have essentially no drift as my resistor set has demonstrated, it is too small a data set to base a specification on at this time.

All of the reference resistors I make are enhanced, roughly, a 10K, ±0.005%, 0±3 PPM/°C, 807 bobbin is approximately $12.  Other specifications are additional such as a tighter tolerance, lower TCR, certificate of calibration, etc.

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 02, 2015, 09:01:43 pm
Hello,

Results of 3rd 1K resistor UP805#3:  shipment date 1510
31.03.2015 first measurement
01.04.2015 reverse polarity measurement.  (no april hoax)
02.04.2015 normal polarity like on 31.03.

Evaluation of 02.04.2015

Box: 28.92999192 ppm / 32.37332611 K = -0.893636688 ppm/K

LMS evaluation:
A 0 = -1.23983866206313E-0001
A 1 = -9.09276513282291E-0001
A 2 = -2.23106751095097E-0002
A 3 =  4.32564638954812E-0004

so T.C. at 25 deg is -0.91 ppm/K
max. deviation from LMS: 2.42265360986628 ppm (Hysteresis + noise)

so this resistor is below 1ppm/K

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 06, 2015, 09:40:48 am
Hello,

Results of 4th 1K resistor UP805#4:  shipment date 1510
03.04.2015 first measurement
04.04.2015 reverse polarity measurement.
05.04.2015 normal polarity like on 31.03.

Evaluation of 05.04.2015

Box: 35.77847918 ppm / 32.16232419 K = -1.112434504 ppm/K

LMS evaluation:
A 0 = -3.05581250800723E-0001
A 1 = -1.12251269574014E+0000
A 2 = -2.22106743141866E-0002
A 3 =  4.18486182138598E-0004

so T.C. at 25 deg is -1.12 ppm/K
max. deviation from LMS: 2.04536528569842 ppm (Hysteresis + noise)

so this resistor T.C. is again very near to #1 + #2

While the other resistors where below 100ppm against my Z201#6 in resistance value.
This one has around +150 ppm offset.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 09, 2015, 09:46:17 pm
Hello,

Results of 5th 1K resistor UP805#5:  shipment date 1510
06.04.2015 first measurement
07.04.2015 reverse polarity measurement.
08.04.2015 normal polarity like on 06.04.

Evaluation of 08.04.2015

Box: 35.28859792  ppm / 32.40994882 K = -1.088819921 ppm/K

LMS evaluation:
A 0 = -2.23342937070244E+0000
A 1 = -1.13611717010502E+0000
A 2 = -1.86452762076308E-0002
A 3 =  4.20604123685255E-0004

so T.C. at 25 deg is -1.14 ppm/K
max. deviation from LMS: 3.36389728597688 ppm (Hysteresis + noise)

so this resistor T.C. is again very near to #1 + #2 + #4
But hysteresis is rather high and the ageing drift during the 3 days was around -2.2 ppm
Offset against Z201#6 is around +370 ppm
Generally the "creeping effect" during cold cycle decreases over the 3 days.

And again a overview over the 1K resistors.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 13, 2015, 09:28:40 pm
Hello,

now a Z201 1K resistor. (Z201#4)

This one had a special treatment before measurement:
A 28kHrs "light load life test" with 100mW loading at room temperature.
Intermittend load 2 hrs cycle with 1.5 hrs on and 0.5 hrs off.

Results:
10.04.2015 first measurement
11.04.2015 reverse polarity measurement
12.04.2015 normal polarity like on 10.04.

Evaluation of 12.04.2015:

Box: 15.66604454 ppm / 33.59927393 deg C = 0.466261401 ppm/K

LMS evaluation:
A 0 =  2.99261984205150E+0000
A 1 =  5.53842688490914E-0001
A 2 =  7.90175918799547E-0004
A 3 = -3.96404076296749E-0004

so T.C. at 25 deg is 0.55 ppm/K
max. deviation from LMS:  0.856984434770559 ppm (Hysteresis + noise)

The hysteresis itself is very low at this sample.
But in the cold phase the previous curve is never met on the 2nd time.
So there is a large ageing drift of 3.6 ppm within 3 days

So probably the pre-ageing procedure (load life) had introduced
 a larger hysteresis which will need several temperature cycles to wear out?

will be interesting how Z201#5 will behave (with same load life test).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: lars on April 14, 2015, 05:46:06 pm

The hysteresis itself is very low at this sample.
But in the cold phase the previous curve is never met on the 2nd time.
So there is a large ageing drift of 3.6 ppm within 3 days

So probably the pre-ageing procedure (load life) had introduced
 a larger hysteresis which will need several temperature cycles to wear out?

Could the Z201 resistor be humidity sensitive so that the pre-aging process have dried the resistor? In that case the cold cycle will fasten the recovery I guess. I have never tested Z201 resistors but my S102 resistors have seasonal variations that I guess comes from humidity changes.

Should be interesting to know if temperature cycling in any way can predict the seasonal variations due to humidity? For voltage references in plastic packages the "hysteresis" is smaller than the seasonal variations what I have seen.

Lars
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 14, 2015, 06:42:09 pm
Hello Lars,

interesting questions.

Perhaps one day too late now: I have soldered in already Z201#5 and the first temperature cycle is nearly done.

If I had thought of that I could have made some room temperature testing over one week with#5
Usually humidity has a time constant of 4-7 days so most of the humidity effects should be gone before cycling.
On the other side: 100mW at room temperature will give a temperature rising of about 15-20 deg C.
(derating is 0.3W from 70 deg C to 125 deg C).
I am not shure if there is a significant drying by this temperature step.

Further: Z201#5 had 4 complete days more than #4 after temperature cycling to acclimate.
(I have switched off the load life test on evening of 09.04. when soldering in Z201#4).
So if it is really humidity there might be some difference between the 2 samples.

@Ken: I still have not found any catalog distributor who sells Z1 foil resistors.
Ordering from Vishay would need around 14 weeks from my side.
So thats not a real option for a quick test.

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 18, 2015, 03:10:32 pm
Hello,

and the last Z201 1K resistor. (Z201#5)

This one had also the special treatment before measurement:
A 28kHrs "light load life test" with 100mW loading at room temperature.
Intermittend load 2 hrs cycle with 1.5 hrs on and 0.5 hrs off.
But 4 days recovery before first measutement.

Results:
14.04.2015 first measurement
15.04.2015 2nd measurement (no polarity reversal to better see the ageing)
16.04.2015 3rd measurement

Evaluation of 16.04.2015:

Box: 24.45159402 ppm / 33.6496634  deg C = 0.726651965 ppm/K

LMS evaluation:
A 0 =  1.18021451556126E+0000
A 1 =  7.87390996304856E-0001
A 2 = -5.23359467608180E-0003
A 3 = -1.50254441552998E-0004

so T.C. at 25 deg is 0.79 ppm/K
max. deviation from LMS: 1.38927451445198 ppm (Hysteresis + noise)

ageing drift of 1.8 ppm within 3 days

So this could be a indication that Lars is right with his humidity assumption.
I think best would be to repeat the ageing cycles during the measurement of the 120 Ohms resistors.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 18, 2015, 05:49:03 pm
Hello,

attached the overview of the (easily available) 1K resistors.

The quint-essence:
there is no really ideal (COTS) resistor when diving down below the 1 ppm range.

With the Z201 you can have luck if selecting between enough samples:
but the stray of T.C. is relative high even from the same lot. (B0940-)
When comparing to "typical" specs of the data sheet the T.C. is often far away.
(someone should do the maths how often you can get a "golden" resistor with 0.05 ppm/K within that stray).

With Edwins PWW resistors (UP805) the absolute T.C. is in average somewhat higher.
The stray of T.C. is very close at least from the same batch.
So these resistors might be better for voltage dividers built out of the same resistor value.
The 3 ppm/K spec of the datasheet is very conservative as all measured resistors were near 1 ppm/K.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: blackdog on April 18, 2015, 06:29:13 pm
Hi Andreas,

I just want to say THANKS! for sharing al your hard work.

Kind regarts,
Blackdog
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on April 18, 2015, 11:51:07 pm
Andreas,
Your findings show another important detail:  For most practical applications the Pettis UltraOhm resistors offer a much better price / performance ratio than any of the expensive Vishay resistors we've tested.

We used the Pettis resistors for quite a while now on various Vref systems, typically with LM399 / LTZ1000 and have never had a problem.  I don't work for the fellow, but we do use his resistors in precision designs, and always get rock-solid results.  And at much less cost than the Vishay's we've used over time.  Less noise also - that "precision" foil is not as quiet as PWW resistors in low freq circuits.

This might be a moot point since I think you said you are only going after "relative" measurements, but even so:

I have never, ever witnessed the UltraOhm (or Vishay hermetics) changing at ~2ppm over a few days, and we've never seen any humidity issue with his product.  This maybe in a previous post (forgive me) but what is your measurement technique, and how are you confident that you can actually measure down below into the 1~2ppm area - and how are you characterizing your equipment noise floor before every test?  I notice you have an astonishing number of digits in your measurement math values also.  Normally we will only work the math out to only the real physical significant digits of measure.  I am not questioning your final results, I was just wondering how you characterize the true noise floor of your technique. For instance:  When you calculated the value drift of 2 or 3 ppm over a couple days when measuring values, do you think you could have been observing your own equipment drift?  And how do you prove or disprove that?  Do you cross-check with a "real" resistance standard other than a single Vishay to measure against?

For best results in the lab, we don't rely on a single DVM to measure resistance, we always cross-check with several bridges and several resistor standards that are at least an order of magnitude better than what were are trying to measure - say SR-104 etc.  Always within NIST calibration interval, so that we have a reasonable expectation of measurement uncertainty.

Most Metcal labs will have several different meters, of different makes and models when we start measuring down past 5~10ppm.  Generally you don't rely on one meter or one resistor to make a test, and in general, your measurement system hardware & technique wants to be at least 10 times better than the results you're after.  That does not take away any of your work though Andreas.

Again:  Thanks for your final results!!  It matches the trends we've been noticing for years... Mostly that the expensive Vishays, in general, are not worth the price that you pay to get mostly no actual better performance in the end.  Then only "end" performance we've seen is in the wallet. :)
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on April 19, 2015, 04:10:40 pm
DiligentMinds,

You have completely missed the point of MisterDiodes constructive critique, using the term 'relative' does not release anyone from doing due diligence to make sure that the measurements are at the very least stable and repeatable in order to produce a certain level of measurement.  In this case, PPM and sub-PPM measurements are being attempted but with little to no verification of the measurement system's ability to have sufficient stability and repeatability even in the short term.  MisterDiodes was trying to point this out and he did stray into the area of accuracy to make the point.  Certain 'flaws' in the measurements being posted was noticed and was pointed out with absolutely no intent to demean anyone's attempt at measurements, however, spinning one's wheels is a waste of time and does not produce anything of real use.  As to your comment about VPG foils are not "garbage", I do not see where MisterDiodes said anything of the sort, he was pointing out that VPG's resistors often fail to meet 'typical' specs and are often at the limits of the specs, they are within the stated maximum limits but are often not within typical or even close to typical.  SMT packages have their own batch of problems in addition to the usual leaded parts problems.  Yes, hermetic sealing does protect the resistors to some degree from their inherent sensitivities but it does not entirely eliminate them, particularly if they are being used in the real world or being used with kid glove care.

I do agree that foils/films do have advantages over PWW in some AC applications but they also have some drawbacks because of their inherent design, the proper specification of a resistor depends entirely on the application of it and what parameters are chiefly important to the design.  The AC characteristics of PWW resistors are an entirely different subject entailing a lot of misconception of their AC characteristics but that is another complex subject that should be discussed independently of DC and relatively low frequency applications.

The subject of shelf stability and long term use stability has been talked about some before, I have mentioned the stability of a pair of matched resistors that I can verify long term drift on personally.  My resistors have been proven by independent metrology labs (General Electric - AEECD, Foxboro) to have exceptional long term stability and reliability, testing which even VPG's hermetics would not stand up to with the same results.  As noted by Andreas, my specifications tend to be conservative, as are the specifications for long term drift, while I guarantee my resistors to meet published specs, that is not to say that they do not exceed those specs.

When I get some time, I plan on commenting on the measurements Andreas made on my resistors to draw attention to the details of why those measurements are being questioned over in the Ultra Precision Vref thread.  We are NOT criticizing the efforts of anybody, we are criticizing the quality of measurement in relation to what is being attempted.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 19, 2015, 09:46:07 pm
I just want to say THANKS! for sharing al your hard work.

Thanks for this work Andreas.

Thank you blackdog + Ken for honoring the work.


This might be a moot point since I think you said you are only going after "relative" measurements, but even so:

I have never, ever witnessed the UltraOhm (or Vishay hermetics) changing at ~2ppm over a few days, and we've never seen any humidity issue with his product.  This maybe in a previous post (forgive me) but what is your measurement technique, and how are you confident that you can actually measure down below into the 1~2ppm area - and how are you characterizing your equipment noise floor before every test?  I notice you have an astonishing number of digits in your measurement math values also.  Normally we will only work the math out to only the real physical significant digits of measure.  I am not questioning your final results, I was just wondering how you characterize the true noise floor of your technique. For instance:  When you calculated the value drift of 2 or 3 ppm over a couple days when measuring values, do you think you could have been observing your own equipment drift?  And how do you prove or disprove that?  Do you cross-check with a "real" resistance standard other than a single Vishay to measure against?


Hello MisterDiodes,

yes you are right: I am doing only relative measurements.
And only to a "reference resistor" which has changed 2 times (from UPW50#2 to Z201#1 and then the "golden" Z201#6) during the measurement row.
So if I observe a drift over 3 days I cannot say that either the DUT or the reference resistor has driftet.
The probability is larger that the DUT which is thermally cycled is changed during the test and not the reference resistor which is heated to 27.5 degress.

By the way: did you thermally cycle the DUT during your "not seeing any drift of 2ppm" or was it at constant temperature with constant humidity?

You will find further description of the setup beginning on page 1.
Heart of the measurement is a 24 Bit ADC (LTC2400).
It is self calibrating during each single measurement so Offset + Gain + their temperature drifts of the ADC are canceled out.
I am using further compensation techniques as: ratiometric measurement.
The resistors are supplied from the same voltage reference as the ADC -> VREF cancels out.
I am measuring in 4 wire technique by pseudo differential measurement -> Wiring resistance + Offset of buffer amplifier cancels out.
Since I am using only a small range of the ADC -> INL is negligible.

What remains is
 DNL (24 Bits no missing codes) = 0.06 ppm of 5V or 0.12 ppm for the 2.5V over the resistor
Temperature error: around 1K over 33-34K span giving 3% relative error of the measured over all ppm value.
Noise: which is typical 0.5ppm to maximum 1ppm over the integration period of 1 minute for 1:1 resistor values.

All in all: I have a repeatability of around 0.03 ppm for the T.C. at 25 deg. (most due to reaching of different extreme temperatures)
And I consider all "offset" drifts of the curve at 25 deg of more than around 1 ppm as significant. (above noise level)

Sorry no daily calibration against a standard.
Only 2 measurements from Frank of 2 of my resistors with his HP3458A show me that I am in the right ball park.
And I am doing at least 3 repetitions of the measurements at the moment to get further information about repeatability.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on April 20, 2015, 12:03:25 am
Andreas,

As to your assertion about a DUT thermally cycled vs a 'reference' resistor that was not, your 'thermal cycling' is quite gentle by thermal standards, a 33°C-35°C temperature change is nothing (at least to my resistors), not even over three days, it is the next best thing to sitting on a shelf.  Why you may ask, my resistors are subjected to at least three different temperatures during a normal manufacturing process of standard line resistors.  All three temperatures (at different times in the process) are well in excess of your gentle cycling, therefore there will not be any 'thermal shock' to my parts as they have already withstood far greater 'shocks' during manufacturing.  My resistors are designed such that once they have gone through manufacturing, there is very little change left in them.  In the case of high stability, I apply enhanced techniques which, for all intents and purposes, removes any further changes.  The proof is in the physics of the design of my resistors, the materials were selected which allowed the removal of any 'stress' (no matter the type) from the wire/bobbin assembly, once the resistor has gone through processing it cannot be further stressed within the military temperature range, period, there are no other possibilities.  Whether it is cold or hot, it has been removed, anything you have measured beyond the initial measurement (you did record an initial measurement before proceeding of course) is external to the resistor.  This has been verified time and again by direct resistance bridge measurements which cannot be argued against unless you have a DCCC bridge or a standards lab measurement system.

I do applaud your work as well but your measurements, when in conflict with a primary bridge such as an ESI 242D and SR-104 cannot be assumed as accurate, the source of error is in the measurement system not the resistor.  I understand that you cannot afford a 242D/SR-104 and are doing the best you can otherwise but you must recognise the limitations of such a system.  There are many more sources of error in a system such as you are using than a bridge has and those are well known.  That is precisely why NIST does resistance measurement directly by bridge method, indirect means is inherently less accurate.  Did you read my earlier posting?  General Electric tested my resistors and found them superior, perhaps you are wondering what the AEECD is, that is GE's jet engine division, do you not expect them to be able to accurately read resistors better than any of you or perhaps even me?  Foxboro is a major military/aerospace contractor, same thing there.  My resistors had zero failures and very small resistance changes, no one else has ever achieved that and blew through the military specs easily.  You can't get any better reliability from anyone else, many of Vishay's resistors would likely fail after being put through the wringer as mine were.

There is much more to specifying a resistor than just zero TCR and some drift specs, if you want nearer zero TCR and near zero drift, I can certainly provide those but are you really willing to pay for them?  I provided standard line resistors as requested, if you want enhanced, I can certainly provide such.   I hear grumbling about the cost of Vishay's hermetics and they still have some weaknesses.  Hermetic sealing doesn't protect them from the voltage coefficient which is around 0.1PPM/volt (mine don't have that at all), they still have stress from being mounted to an unforgiving ceramic chip (yeah, that's been ironed out to a wavey curve), get them too hot while soldering, oops more stress and don't kid yourselves, some of that is a permanent change, at least they removed the barometric pressure problem and humidity but then, that doesn't bother mine.  Mine are not perfect either but at least they don't have near the list of problems that film/foil resistors do and have some significant advantages to boot.

Your TCR measurement appear to be reasonably accurate (for a relative measurement), on the other hand, comments about hysteresis and 'cold creep' (whatever that is) are irrelevant in my case, they do not exist to any degree that you could possibly measure reliably otherwise my 242D would have detected it (oh, by the way, I have up to eleven decimals of resolution that are quite repeatable over the short term of a few days, note I did not say accuracy, that is entirely a different beast), plus I know my measurement uncertainty is normally within <0.2PPM as verified against the SR-104.  Yes, when necessary, I do indeed verify my system's accuracy which is certified to NIST, I am qualified to do calibration (since 1973) and can certify standards within my limits.

Please do not think I am bashing anyone, what MisterDiodes and I are trying to do is point out possible error sources that you may not be aware of or effects of.  Even relative measurements demands due diligence if they are going to be done correctly.  If the system is operating as best as possible then your accuracy may possibly improve as well.  Please take these comments as constructive critiques, they are not meant to be personal in any way nor to discourage from pursuing measurements.  We are merely pointing out inconsistencies in the measurements, nothing more.
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on April 20, 2015, 03:31:39 pm
Andreas,

Looking at your setup, this a good attempt at an "economical" test, but I suspect it is not in the range of a few ppm for numerical accuracy.  Yes, we have tested the Pettis resistors over a range of temperatures and humidity.  Never have we seen a huge change of ~2ppm over a few days.

Not to take away from your hard work, but the difference between your economical test technique and a calibration lab technique shows many more error sources you might not have considered.  Typically all of these together might push you out of well out of the capability of measure into the sub 2 or 5ppm area, even for relative measurements. Again, this does not take away from any of what you did, but consider:

What I see in the photo: 1. Soldered connections - if you are not very, very careful to keep those soldered joints at exactly the same temperature, you will measure thermal drifts, not resistance drifts.  When we are doing precision resistance tests of a component, normally the pressure contact points in the test jig are measured and apply a known, very repeatable clamping force on the test leads in an isothermal fashion and typically in a dry nitrogen atmosphere (or at measured humidity) during the test run.  2. You have random length long test wire laying out on what looks like a non-ground plane, and what probably doesn't include 4-wire (+, -, Guard and Ground)  guard circuitry in your measurement device.  Usually you want a short & tidy test lead layout with crimped, verified spade lugs or heavy clamp connections, and make good use of guarding techniques, especially since you are primarily measuring voltage here, not resistance. 3 You really want to not rely on an LTC2400 for all your measurements, as it only as good as the supplied voltage reference driving it.  You want to cross check with other meters and really, for resistance measurements we want to use bridge measurement techniques against a known standard.  Try this: attach a known accurate reference standard to that voltage divider and watch what happens to your LTC2400 readings when you take a measurement coming from "zero" ohms vs coming at it from "Infinite Ohms (open)".  You will see that the LTC2400 self calibration (and if you apply math averaging) is not quite always perfect, and you will see some hysteresis right there.  4.  For characterizing a resistor build batch, typically we measure several hundred resistors to see the overall trends, not just a few.

I could go on all day, but to get to the point.  You did a very economical test on a shoestring budget, and you got what you paid for:  In the end, even on your economy test setup you found out that spending a lot of money for the magical Vishay, at say 8 to 10 times more money does NOT get you 8 to 10 times the performance of a good precision wirewound resistor like the UltraOhm (at lower frequencies).  I think those are the main results from your test even though I'm not completely convinced about some of the numerical results. 

But thanks for the results in any case!  I would be willing to share more, but since my employers pay for the results - and they do not wish to make that public knowledge - I'll have to stop here.
 
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 26, 2015, 08:44:16 pm
Hello,

it's always a good excercise to question a measurement setup. Especially when going hard to the ppm limits.
So I had a lot of thinking wether there could be further improvement.

On the other side I am not shure If you really have understood what I am actually doing.

In fact I have a sort of bridge measurement.
On the one side of the bridge I have the DUT and the reference resistor. (first half bridge)
On the other side (2nd half bridge) I have the ADC (using the same reference voltage) comparing the ratio of both resistors.

as it only as good as the supplied voltage reference driving it. 
Since I am comparing ratios only instead of voltages the voltage reference cancels out.
It has only some influence on self heating of the resistors and the signal to noise ratio.
(see also the measurements of 19.02. and 20.02. with 2.5V reference instead of 5V reference for the resistors).

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg615657/#msg615657 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg615657/#msg615657)

What I see in the photo: 1. Soldered connections - if you are not very, very careful to keep those soldered joints at exactly the same temperature, you will measure thermal drifts, not resistance drifts. 

Good point: but also for this I made a measurement with zero reference voltage to see if there are differences in offset on the different pins. (all below 0.2 ppm when regarding difference voltage since using the buffer amplifier with Z201#3).
https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg603931/#msg603931 (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg603931/#msg603931)

Of course that brings me to the idea: On radial resistors like Z201 the solder junctions on PCB are much closer than with axial (wire wound) resistors. So in real world applications like a voltage reference the thermal design is more critical with axial resistors.

So I do not know if the enhancement to the setup suggested by Emanuel is really that what I have later in reality.

When we are doing precision resistance tests of a component, normally the pressure contact points in the test jig are measured and apply a known, very repeatable clamping force on the test leads in an isothermal fashion and typically in a dry nitrogen atmosphere (or at measured humidity) during the test run.
Do you have a photo of the setup? Would be interesting. Perhaps I can adapt some ideas for my measurements.

Typically all of these together might push you out of well out of the capability of measure into the sub 2 or 5ppm area, even for relative measurements.
For absolute measurements you are right: I simply cannot prove it in lack of a suitable standard resistor.
Any drift can either come from DUT or from the reference resistor.
For relative measurements the statistics of repeated measurements indicates a well below 1 ppm deviation for T.C.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on April 28, 2015, 01:18:54 am
Andreas,

I think that MisterDiodes and I have a very good understanding of what you are attempting to do and how you are attempting to do it, it is a good attempt but it is still flawed when trying to make precision measurements, whether or not you are trying to do absolute or relative is not really the point.  The point is that even trying to do relative measurements requires due diligence to eliminate all sources of error in the measurements and even relative measurements requires a 'reference' that is known to be sufficiently stable in its own characteristics, therein lies the conundrum, you cannot use a 'reference' part characterized by the same uncalibrated/relative system you are using to make relative measurements because you do not know what the characteristics of that system is well enough.

For example, I have a 242B bridge, unlike the 242D which is trimmable to ~1PPM, the 'B' is not trimmable and is specified to 0±20PPM accuracy which is quite good.  Now let's say I need to measure a resistor to 10PPM, obviously I cannot use the 242B since it is only 'known' to be somewhere within ±20PPM (we are ignoring the fact that I have standards), the fact that the 242B may actually be accurate enough at a given resistance is unknown.   What solution do you think will solve the problem?  You could possibly purchase a regular resistor with specifications good enough to check the 242B close to the required value or you might know someone with such a resistor you could borrow.  My point is that you simply cannot make even relative measurements unless your reference is at least known to be within sufficient parameters to compare to, it doesn't have to be a calibrated standard but its properties must be known.  Your ADC, good as it is, does not qualify as a reference because you are not measuring resistance with it, you are measuring the ratio between two resistor voltages, neither of which you know the parameters of well enough to claim one is a reference, the accuracy of the ADC becomes irrelevant per se as you are not measuring its accuracy.

Using a Z201 as a reference, as you apparently are, you can only assume that this resistor is within the maximum stated limits unless it is measured by an actual accurate resistance bridge and tested accurately for its TCR over the temperature range in question, you cannot assume any of its parameters are better than worst spec.  Therefore your limits of reading, relative or otherwise, is the worst specifications of the resistor you are using as a reference.  Since you folks do not have access to any equipment with which to 'calibrate' with, you cannot assume anything better than worst specs.  Your relative readings are no better than the worst specs of your reference and that assumes that you know what all of your error sources are in addition to the 'reference'.  Given the limiting specifications of a Z201, your uncertainty of measurement must be equal to the Z201's worst specs, you cannot presume anything else without that resistor's actual specifications being known.  In short, your relative measurements are nowhere near as good as you think they are despite some repeatability, repeatability is not accuracy.  In your case, all you know is that your Z201 has appeared to remain 'stable' as far as you know but that doesn't change the requirements of measurement.

As I stated earlier, your TCR measurements of my 1K resistors appear to be reasonable, as far as accurate, they look like they are reasonably close but since I did not do TCR measurements on those resistors, I cannot say that your measurements are accurate, only relatively accurate.  That said, reread my earlier post, I found that your 'readings' indicating aging or 'cold creep' were not valid for the temperature range you were using.  That should have been sufficient to indicate there is a problem of some sort in your system as I nor other demanding customers have found such aging or 'cold creep' in very accurate measurements.  Please recall that I am using a fully calibrated resistance measuring system which well exceeds any of your setups in accuracy and repeatability.  If such characteristics were present I could easily measure them.

For your Z201, here are the major limiting factors (uncertainties), tolerance (0±50 PPM), TCR +0.8, -0.6 PPM/°C, PCR (depends on power level, <±5 PPM), VCR 0.1 PPM/volt applied.  You may be able to get a little better reading on the Z201 resistance if a DVM is available and sufficiently more accurate and calibrated.  Unless the actual TCR of the Z201 can be verified, you have a minimum uncertainty of +0.8 to -0.6 PPM/°C, that means a reading of +1 PPM could be anywhere between +1.8PPM/°C to +0.4 PPM/°C, you cannot do any better than that.  PCR can be fairly accurately calculated knowing the power dissipation but the uncertainty is still ± whatever that is.  VCR is an additional 0.1PPM/volt.  We also know that the Vishay TCR curve is not linear so unless it is held steady in temperature, that could also vary by an unknown amount.

The problem is that even relative measurements still have to depend on known quantities of the reference being used otherwise the measurements will be of limited use.  You cannot use 'typical' stated values obviously because you don't know that you have a 'typical' part in hand.  When it comes down to the details, your 'relative' measurements still depend on having a reference with known parameters, since you are using a Z201 as your 'reference', you absolutely cannot assume that part has specifications any better than the figures I stated above nor is your system capable of verifying those parameters because your system depends on that Z201 as the 'reference'!  You are hereby caught in a catch 22 in which the only way to get 'relative' measurements is by having a known good reference, otherwise your 'relative' is tied to the uncertainties.

At 1000 ohms, a TCR of 0.05 PPM is 1,000.00005 or .05 milliohms per degree, that is 9 digits of resolution, well within the range of my 242D, however, my 242D has an uncertainty of 0.2 PPM so that a 'relative' or accurate reading is to the 8th digit, despite the fact that my bridge has repeatability well beyond 9 digits, that does not in the least make those digits of any relevance even for a 'relative' measurement because anything beyond the 8th digit is within the uncertainty of the measurement.  I am confident that my 242D can accurately read TCR to a tenth of a PPM with an uncertainty of 0.2 PPM of the reading, I am not confident that any of the 'relevant' measurement systems being used here have an uncertainty of less than 1.4 PPM at best with the provision that all other error sources have been accounted for.  That is the problem with indirect measurements, they are never as good as direct measurements.

I must disagree also, you do not have a "bridge of sorts", you have a voltage divider being measured as a ratio, that does not constitute a bridge by definition.


QUOTE: "For relative measurements the statistics of repeated measurements indicates a well below 1 ppm deviation for T.C." 

See my comments above, you cannot use math to reduce uncertainties, it doesn't work that way.  You appear to be making the mistake that 'relative' somehow removes uncertainty, it doesn't.  I suggest you go through and make a note of all of the uncertainties in your system, from beginning to end, I think you will be surprised that they add up to a significant sum.  At the very least, your uncertainty for TCR is 1.4 PPM at best without considering any other sources.  Unless you can verify your measurements, you must observe worst case uncertainties in the system, you cannot assume anything is better than that.

Your measurements are 'relative', no question about that, the real question is, how relative and to what are they?  From all of the comments here, it appears that these measurements are being interpreted as 'accurate' rather than 'relative', the so-called 'golden' Z201 probably is the most stable of the Z201s but you are still chained to the uncertainties, you do not know just where that stability lies because of the facts I've presented above.  Relatively, that Z201 probably does have a lower TCR than my resistors, I'm not disputing that, what is in question is just where my resistors really come in at (they might be a little higher in TCR or maybe a little lower in TCR in reality), the assumption is that the golden Z201 has a near zero TCR but in fact, it can be anywhere within that uncertainty range, you don't know where.  Even with relative measurements you cannot make such assumptions.

In the case of many resistors tested here, there are obvious flaws in some of the resistors, some are even questionably meeting spec.  The real problem begins to show up when the better resistors are closer to the 'reference' resistor, possible errors and uncertainties begin to show up and must be taken into consideration, if not, relative becomes of little use.  What you really need is a characterized resistor of known quantities, that would go a long way to improving 'relative'.

I will leave any other comments due MisterDiodes to him if he would like to respond in kind.  As you pointed out:

"it's always a good excercise to question a measurement setup. Especially when going hard to the ppm limits."

MisterDiodes and I are only trying to point out discrepancies and what needs to be taken into consideration for better measurements.

If at some time in the future the opportunity presents itself at an convenient time, I will be happy to post some actual readings on some resistors, I generally do not have the free time to setup and run such tests unless a customer requests them and also asks for recorded readings (which has been quite rare).

Best regards
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on April 28, 2015, 05:31:31 am
Andreas,
I would love to post a photo of a real test jig that is good to some ppm, but owners won't let me - and I have to honor their wishes.  The testing apparatus I was working  on last week cost its owners something in the neighborhood of $250k USD just to do some precision measurements on simple passive devices, and that's not even close to the upper end of cost.  That is -just- the mechanical assembly that holds the DUT.

What you have isn't even close (don't take that the wrong way, you have made more effort than most any hobbyist), but let me offer some more very general and basic  insight - and even this is just barely the tip of a very very huge iceberg:  You never solder in a resistor under test, it is always clamped in place with a known calibrated force as I noted before.  It's not only the interaction of metallic alloys, but the size & surface finish of the solder joints themselves can cause un-even heat gain or loss at each end of the DUT.  Clamps / crimped connections only on the testing jig in general, and everyone in the lab agrees to use a single set of calibrated torque nut drivers (tested every morning) on the connection joints, on 100% clean copper 145 ("tellurium") or gold-plated joints (with a known calibrated gold layer thickness).  Test leads are part of the test jig calibration along with connection point nut torque, clamp pressure, etc.  Polarity reversals often during test will expose diode-action and thermal problems.  Make good use of active guard circuits, etc.

The clamps are arranged to be isothermal (as I said before in a known atmosphere), or maybe under oil or other fluids, and will offer essentially zero mechanical stress on the part under test.  Typically this may mean one clamp is fixed, the other clamp rides on an mag-lev slide or roller bearings or ??.  The whole system rides on a concrete pillar isolated from the building floor (embedded down into bedrock on its own) so as to not pick up vibration (noise) from the lab, and even then the test jig typically sits on an anti-vibration table.  Or the system is suspended from overhead load isolators.

In a real measurement lab you would not have an electrically noisy laptop or PC anywhere near the device under test, and especially the USB ports are forbidden because of the huge amount of noise present at all times on the USB cable itself; and -any- ground loops no matter how small are affecting your test - So typically for the last couple decades we use only fiber cable or straight old-school opto-isolated serial data lines to communicate with the test equipment, which is usually running on battery power or old-school 50/60Hz transformers for lowest noise - usually no switching power supplies allowed. Make sure everything around the test jig is at pure DC only during the test - and even most of the time we do not allow chopper amps or noisy SAR ADC's.

Incandescent - style lamps only, no fluorescent illumination.  If LED illumination is used it is not PWM, just straight DC.  Or use IR-filtered sunlight only if the test jig area is light-tight.  You really want no noise sources within several meters of the device under test, and even then good shielding / guarding practices are a must.

Once you think you get close on the test jig design, you load it with all sorts of whatever devices you are measuring, and characterize the performance of the test jig itself, and measure its uncertainty first.  That can take weeks or months on its own.  Only then do you proceed to measuring devices on it.

Etc. Etc. Etc.  Again, I could still go on all day. 

Now:  The biggest problem I see is the notion that the Voltage Ref to your ADC doesn't affect measurement.  Actually it does (and with measurable effect) because you are not only reading the voltage drift of your Vref into the measurement, but its noise power also (averaging noise does not make it go away in all cases). Remember, you are measuring the -voltage- at your resistor ratio test setup, not a true balanced "null". 

That's why we use a resistance BRIDGE with a resistance standard (of known calibration and certainty) to measure resistors in detail,  where you are well and truly looking for a NULL voltage across the balanced bridge.  Truly when you have "zero" voltage across the bridge, as measured with your ADC, "zero is zero" - always.  The voltage reference of your ADC, and its associated noise are now out of the system completely.  Better yet: Now you don't even need an ADC as there many "analog" means to detect "zero voltage" without any sort of noisy digital ADC and its noisy Vref.

Let me put it to you this way:  If you came to work at the labs I'm in, typically they would give you maybe 10 or 25 resistors to measure, along with a gold-plated tellurium wire about 25mm long or similar (to represent close to zero ohms).  The resistances would be known lab standards, of known value to whatever ppm, but not written on the resistors themselves..  You would be asked to measure the resistors on the bridge and test jig, write down the results, and hand them to the lab manager. The manager would tell you the results are wrong, then he or she will ask you what you think you did wrong,  and you go back and do it again.  And maybe the manager will give you a hint or two.   And you try again.  And again.  And maybe all your results are getting close to 5ppm of being correct after quite a bit of practice.

After a few years practice you might be getting good results to within true 1 or 2ppm. <Laughing>  Only kidding, (sort of).  Once you start chasing down into the sub 5ppm area, on some day you realize you are spending more and more time (and learning more and more nuance) just trying to validate every unknown in your test system.  The best result is to have others replicate your findings over and over again, and to question the results over and over again and measure as many different ways as practical.

Again:  Not trying to take away any of your work, but you have only just begun to delve into true metrology.  Take heart though: what you have learned is the expensive Vishay's don't come close to offering the same performance to value ratio as a good precision wirewound resistor (at lower frequencies) and that is very good (and very valuable) information to know and to share.

The numerical results you reported I respectfully do not believe are 100% valid, but that is only from my point of view and experience. I think if you acquire a good resistance standard & bridge equipment, and learn how to use it with more precision technique, I think you will find out even more how good precision wirewounds can be.  It is a fascinating field of investigation.
Title: Re: T.C. measurements on precision resistors
Post by: texaspyro on April 28, 2015, 05:45:09 am
Darn,  you mean my Simpson 260 and Chinese alligator clips won't do?   :(
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on April 28, 2015, 05:56:56 am
TexasP:
Ha! Sometimes I trust my Simpson more than anything...<Laughing> Nothing does "True RMS" like a good analog meter, if its within the right frequency bounds.



Title: Re: T.C. measurements on precision resistors
Post by: robrenz on April 28, 2015, 12:27:47 pm
+1  :-+
5µV @ 60Hz
(https://www.eevblog.com/forum/testgear/hp-3410a-ac-microvoltmeter-restoration/?action=dlattach;attach=94372;image)
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on April 30, 2015, 08:22:04 pm
Hi guys, thanks for the laugh.

I found some rather interesting information from the NIST about the ohm, concerning the quantum ohm standard, it can only be read with an uncertainty of 0.2PPM at best, no matter what national lab in the world it is, all agree on this uncertainty.  Furthermore, this uncertainty is not included in calibration certificates as it is understood by all labs that it is there, all uncertainties provided on the certificate of calibration are in addition to this 0.2PPM.  The document is here: 

http://www.nist.gov/customcf/get_pdf.cfm?pub_id=31431 (http://www.nist.gov/customcf/get_pdf.cfm?pub_id=31431)

It also provides other information about the process of calibrating primary standards, uncertainties and the bridges/methods used.

Another interesting document is the calibration pricing sheet here:  http://www.nist.gov/calibrations/resistance.cfm (http://www.nist.gov/calibrations/resistance.cfm)

You can get your SR-104 certified to the best accuracy in the world for only $3.385,00 plus shipping.

Another very interesting tidbit of information from the NIST about solid-state Vrefs:  if it is an LTZ1000/A or LM399, the design should be using PWW resistors if you want the results to mean anything when you get the device back.  They will tell you that right up front and to get that Vref certified, $2206 covers a couple measurements per day over 12 to 15 days, then they send you the average result.  Cost to ship it back is $519 to you and they would prefer that the device is battery powered for a minimum of 30 days but they will provide up to 50 watts of mains power if it has to be plugged in.

The calibration pricing for Vrefs of various types can be found here:  http://www.nist.gov/calibrations/voltage.cfm (http://www.nist.gov/calibrations/voltage.cfm)

As usual there is more information on voltage standards and particularly the solid-state references.  Note the comment on DVMs and other similar instruments on this page.
Title: Re: T.C. measurements on precision resistors
Post by: texaspyro on April 30, 2015, 09:22:10 pm
A couple of years ago, I asked IET what they charge to cal an SR104... it was around $850.   When I did not respond to them after a couple of weeks,  they sent an email to the effect of "Hey if our quote was not acceptable, let's talk... maybe we can work out a deal..."
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on April 30, 2015, 09:38:02 pm
Hi texaspyro,

Yes calibration certification does cost, mainly because there are strict procedures in place that must be followed and the instrumentation is required to be top notch to certify standards to very low uncertainties.  The NIST has tens of millions of dollars of equipment invested in to make their certifications so I really do understand why they charge so much.  On the other hand, the next tier of primary labs who do pay those NIST fees gets to spread those costs out over many standards so they don't have to charge so much but their uncertainties are almost as good as the NIST and usually it is only those primary labs that need such levels of calibration.  It certainly doesn't hurt to shop around, some of these cal labs are willing to come down in price some just to get the business.  My calibration uncertainties for resistors is very close to a primary lab's and since I don't have to pay thousands, I don't have to charge so much when someone asks for a certification (note that is not the same as 'just a reading', that is quite accurate but isn't quite as low in uncertainty as a certified reading).
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on April 30, 2015, 10:50:39 pm
I found some rather interesting information from the NIST about the ohm, concerning the quantum ohm standard, it can only be read with an uncertainty of 0.2PPM at best, no matter what national lab in the world it is, all agree on this uncertainty. Furthermore, this uncertainty is not included in calibration certificates as it is understood by all labs that it is there, all uncertainties provided on the certificate of calibration are in addition to this 0.2PPM. 

The quantum ohm can be realised with uncertainty of parts per billion, but it is not the same as the SI ohm. The ~0.2ppm is between the quantum ohm and the SI ohm.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on April 30, 2015, 11:08:52 pm
The point being is that every primary resistance standard in every world national laboratory has an uncertainty of 0.2 PPM, no one is getting a standard certified to PPB, it can't be done, the quantum ohm exists as a calculated value based on known physical references/quantities.  The statement about the 0.2 PPM uncertainty is directly from the data sheet of the NIST, you can't get any better than that, I am not making that statement if that is what you are thinking.  I know all about the quantum standard, it is a very oddball value and it cannot be directly compared to a 'normal' standard, there is a link to information about the quantum standard on the NIST resistance measurements sheet.

In absolute terms, my SR-104 has an absolute uncertainty of 0.4 PPM compared to the NIST's standards including the 0.2 PPM uncertainty.  If the NIST did the calibration then the absolute uncertainty would be 0.2PPM but I hardly think paying over four times higher calibration fee is warranted for that extra 0.2 PPM.
Title: Re: T.C. measurements on precision resistors
Post by: Marco on April 30, 2015, 11:32:29 pm
The quantum ohm can be realised with uncertainty of parts per billion, but it is not the same as the SI ohm. The ~0.2ppm is between the quantum ohm and the SI ohm.

Where does this uncertainty come from? You should be able to go straight from theoretical description of the SI Ohm to the theoretical description of the quantum Ohm to an infinite precision.

PS. oh nevermind, equivalence according to physics isn't good enough. The 0.2ppm is actually the uncertainty in the SI standard measurement setups. Do NIST and their cohorts simply not offer calibration to the quantum non-standard?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on May 01, 2015, 12:43:09 am
Hi Marco,

From what I understand from NIST, it is the difficulty in comparing the very oddball value of the Quantum to an SI standard, this paper from NIST gives information on it plus the link I gave before:  http://www.nist.gov/calibrations/upload/tn1458.pdf (http://www.nist.gov/calibrations/upload/tn1458.pdf)

The problem is transferring the quantum value to decade values, it cannot be done directly, that is where the uncertainties start creeping in.  A cal lab can compare decade values such as 1 ohm to 10 ohms to 100 ohms with a very small uncertainty but when you deviate from those decade values, which are done in a direct comparison manner, more uncertainty creeps in as there are more steps in transferring values, that is where transfer standards come in, they are used to transfer a given decade value into multiples of that decade value and by definition, that adds uncertainty.  I can measure a 10,000.000 resistor to 0.1 PPM with the uncertainty of my SR-104 (0.2PPM) but I can't measure a 8543.8588 ohm resistor quite as good because it involved the use of transfer standards to calibrate the off decade resistors, the accuracy is still better than 1 PPM but not as good as at 10K.

The NIST papers I linked to earlier also gives the uncertainties for various resistor decades which varies  from decade to decade, that is just the fact of calibration, the NIST is THE legal, best reference for the USA and other countries, you can't get any better measurements anywhere else on the planet, they may be just as accurate but not any more accurate.

All that and the fact that we cannot construct physical resistors with any better characteristics than what we've got right now, it is no minor effort to get another decade better in accuracy and uncertainty.

NIST, et al, do indeed reference to the quantum ohm, that resistance is known very well to a high degree in PPB, it is the jump to SI where the uncertainty comes from.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on May 01, 2015, 08:20:25 am
The quantum ohm system can be understood as a "standard resistor" which is parts per billion stable. But even better (if properly built and used) also the absolute value of the quantum hall is the same within parts per billion in United States and for example in Sweden.

The problem is that we still don't know the relation between the practical quantum ohm and the theoretical SI ohm better than approximately +/- 0.2 ppm. As far as I know the SI ohm can be best realised by the Thompson-Lampard calculable capacitor which is basically a set very accurately machined stainless steel rods.

For most scientific and engineering purposes that is not a problem at all. First time in the history the ohm is the same everywhere in the world even though it is not the SI ohm.


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 01, 2015, 03:28:10 pm
Hello,

For those doubting the measurement results I recommend the reading of 2nd post on page 1 of the thread.
So battery powered devices, photocouplers and common mode noise filtering are all already done.
Otherwise I would have much noisier results as only the datasheet specs of the converter.
For the DUT I have done some improvements (thanks to Emanuels hint)
to keep the soldering junctions at same temperature (shown on later pages).

The absolute value of a resistor (or uncertainity) is not interesting me.
Neither for T.C. measurements nor drift observations.
I only need a refererence resistor that is stable during the measurement.
And why should the reference resistor (at constant regulated 27.5 deg C)
do any funny things even if its only a Z201? (Ok with drift this would be serveral days).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 01, 2015, 03:55:40 pm
Hello,

In the mean time I have done some measurements on 120 Ohms resistors.

Since my reference of the ADC cannot drive 2*120 Ohms = 21mA I have done a
voltage divider of the Z201#6 1K reference resistor against the DUT of 120 Ohms.
This also leads to a acceptable self heating of the resistors.

The drawback is that I have only around 0.5 V over the DUT.
And the noise level of 1uVpp from ADC over 1 minute integration time remains the same.
So ADC noise shows up as around 4 ppm (pp) related to resistance.

For the T.C. with LMS method many of these measurements are used.
And thus the noise cancels out mostly.

For the hysteresis, drift and T.C. with box method I am hard at the measurement limit.
So these values will be less reliable.

Attached the measurements of the first resistor

Z201 120R  #1 date code B0947-

19.04.2015: first measurement normal polarity
20.04.2015: 2nd measurement reversed polarity (check for thermal EMF)
21.04.2015: 3rd measurement normal polarity

LMS interpolation of 21.04.2015

A 0 =  5.47357940569535E-0001
A 1 =  7.98654305122316E-0001
A 2 = -1.46086280995968E-0002
A 3 = -4.29274706993186E-0004

So T.C. from LMS at 25 deg C is +0.80 ppm/K

The "box" T.C. is around 0.74 ppm/K including noise
and around 0.60 ppm/K from LMS interpolation

Relative large hysteresis on measurement visible.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 01, 2015, 04:02:48 pm
Hello,

further result Z201 120R #2 date code B0947-

23.04.2015: first measurement normal polarity
24.04.2015: 2nd measurement reversed polarity (check for thermal EMF)
25.04.2015: 3rd measurement normal polarity

LMS interpolation of 25.04.2015

A 0 = -7.64120247519562E-0001
A 1 =  6.22567646520884E-0001
A 2 = -5.21019214386767E-0003
A 3 = -2.94021440704217E-0004

So T.C. from LMS at 25 deg C is +0.62 ppm/K

The "box" T.C. is around 0.66 ppm/K including noise
and around 0.49 ppm/K from LMS interpolation

Obviously less hysteresis than on Z201_120 #1 on this resistor.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: HighVoltage on May 01, 2015, 05:00:34 pm
Hello Andreas,

You are doing an amazing job in educating us on this subject.
As a relative new voltnut, I really enjoyed this thread and just wanted to say thank you.
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 02, 2015, 12:46:52 am
I have had similar drifts on z-foild resistors. confirmed by Vishay AEs. For sure, a normal epoxy z-foild resistor cannot be used as a precision reference resistor. Too unstable; driven by moisture. Needs to be a hermetically sealed type.
Secondly, the question also is, what is the error budget of your setup per GUM. Have you ever done that analysis? Without, all measurements are meaningless.
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on May 02, 2015, 04:30:36 pm
Andreas,

Again, not trying to take away from your hard work, but here are some respectful observations why I do not agree with your numerical results.  All of the below comes from decades of experience in the lab:

Looking at your measurement device, I think you missed the point about optical coupling to get away from system noise - Yes you have an opto-coupler between the USB chip and the CPU, but that is not the requirement - You want anything that oscillates completely removed from the neighborhood of the test device altogether.  It looks like you have a CPU & USB chip on the same board as the ADC, with a noisy laptop and its internal switcher sitting very close to the resistor under test.  If that's what I'm seeing, that very easily will destroy measurements < ~5ppm if you're not very careful with your grounding setup.  I've never seen low-noise measurements at below 5ppm with such a setup with hobbyist boards.

What I'm taking about is that the DUT needs to be shielded and guarded completely. Without heavy shielding your test must be several feet away (at least) and isolated from any digital switch or oscillator of any kind (including un-isolated LTC2057 chopper amps) - and the only means of communication with test jig is over a fiber cable - usually in analog form telling us the resistance bridge is balanced.  It could be a digital signal on the fiber, but only if comm packets occur only when the device is not under active test.  We never use RF transmission in the area when testing DC components either.  Make sure all WiFi is turned off.  Its not even allowed in our building whatsoever as a security measure.

With a real standard reference resistor in its own shield / guarded metal box, then it is much more robust in the vicinity of digital switches, etc.

You do use a driven guard circuit, correct?  I don't see on one the schematic.  I have never been able to get good results on passive devices in the <5ppm uncertainty area without that.

Cheap quality test: If you are taking a resistance measurement, and you hold you hand close over the test jig - does the measurement change at all?  If so its probably not shielded & guarded enough. 

I know you are not interested about the absolute value of the reference resistor, but you are completely dependent on that reference resistor being -stable- during a test run, which it is probably not.  As DigilentMinds pointed out, that resistor value can be wobbling all over - and fairly quickly too - during your test.  Just the fact of your body and lungs exhaling (mostly water) coming within a few feet of that Vishay foil resistor can change the humidity enough to affect measurements. The PWW resistor won't care much, if at all.

I know you think that the Vref voltage isn't important during your test.  Be careful in assuming that.  The noise of your Vref still has a large effect with your measurement technique - and it has to be appropriate to the degree of uncertainty of your test jig.  Remember:  At the moment the ADC takes a measurement, it is using a roughly pure DC voltage on the sample & hold cap and comparing it to a shaky Vref.  The comparison is not instantaneous - and therefore any noise on Vref can affect any ratio measurement you are going after.  The noise may not be random in nature either - so trying to average ADC measurements in math can introduce weird effects on the results also.

Try this - If you think that Vref doesn't affect your ratio measurement, then add a sine or square wave to it and see what happens.  Say +- 1/2 volt (or whatever) @ 10Hz (sweep over a freq. range) and see what happens to your measurements.  Then add white noise to the Vref at a known level.  You can come up with a value in dB for the effect of both low frequency and higher freq wobbles on the Vref and how it affects the measurement outcome.

Other things to look at:
Study Linear Tech App note 86, and learn where you have to put in a solder compensation joint on your circuit.  You might have this already.

Then measure the effect of your soldered joints with a known, close-to-zero Ohms, gold wire (or gold-plated Tellurium) on your test jig and see what happens. Polarity reversals at least a few times per second.  You should be able to verify what effect your soldered joints have on the system, and then you'll learn why we use them as little as possible on the measurement path.  Depending on your solder technique, amount of solder, contaminates etc. you may also have introduced a small p-n junction diode into the test system.  When measuring under 5ppm, this is sometimes easy to have happen to you.  Don't forget the pathway from +Vref to your ADC and from ADC -Vref to your electron source.

Andreas: Back in early 2000's or so when the LTC2400 series came out, everybody got excited about doing resistance ratio measurements and comparisons like you are doing.  Nothing new.   For weigh scales etc. that don't need below say 10ppm accuracy, these ADC's are great and the ratio methods are sound especially in differential mode.  But trying to go after single-ended low noise measurements below 5ppm gets really tricky in a hurry.  Just because the datasheet says 24 bits in a large type face doesn't mean that your test jig is operating to that accuracy & certainty.  Just because the device says it self compensates for zero on the IC, doesn't mean you have zero compensation at the DUT - and that doesn't mean you are zero-compensating -during- the test at your DUT connections. 

Suggestion: Read up on how Wheatstone and Warshawsky bridges work.  Especially use the Warshawsky technique for resistances below 1k.  It will help you tremendously to learn how to calibrate your test jig and measure uncertainties at a low level and high accuracy.

For instance:  If you think you are measuring to 1ppm, then generally your system needs to do at least 10x better than that in meaningful resolution, even for ratio or TC measurements.  SO:  You want be able to take a resistance measurement to 1ppm.  On your test setup you purposely perturb the resistance under test by +0.1ppm, then by -0.1ppm, then nudge it by +- 0.2ppm, etc. In order to help validate your measurement technique, you need to accurately detect any sort of change at 0.1ppm steps, plus and minus.  Your reported results should be no lower than 10x the uncertainty of your test system capability; that's the rule that all labs I know of use.

And then test hundreds of devices that you have measured value and/or TC on other equipment.  Then I'll start to believe you when you say you saw 2ppm drift over a couple days. 

Even when you are measuring ratios against an unknown, uncalibrated "standard" resistor, this is how we do it.

Please consider learning how the bridge methods work, especially when measuring down into <10ppm area.  You will find out in a hurry why we don't use DVM's (and that especially means 3458's) without reference standard resistors attached in ratio mode.  Even then a quality resistance bridge will give you better results in the long run.

Bottom line: Bridges are really your best friend here for any serious discussion of resistor TC. 
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 02, 2015, 09:08:54 pm
Hello Ken,

for the drift over days you convinced me: It could be the reference resistor.
Although I have had only around up to 3% rH changes during
measurement of the same resistor over a couple of days since using Z201#6.
So I really doubt the 25 ppm from humidity (which is for 0% to 100%) in my case.

So for Edwins 120 Ohms resistors I will use the 1K UP805#3
resistor as reference.
So I will have "no significant effects" from humidity.
At least a [possible] unstable Z201 will not spoil the
result of a UP805 resistor.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: HighVoltage on May 03, 2015, 09:25:10 am
Bottom line: Bridges are really your best friend here for any serious discussion of resistor TC.

Are there any affordable bridges available?
Can you name a few models and brands?
Thanks
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on May 03, 2015, 10:24:51 am
https://www.hofstragroup.com/product/guildline-instruments-9975-direct-current-comparator-resistance-bridge/ (https://www.hofstragroup.com/product/guildline-instruments-9975-direct-current-comparator-resistance-bridge/)
Used :Guildline 9975(specs above,lots of knobs), esi242 different variants. Neither are very cheap
if you want new, ask guildline,measurement international...


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 03, 2015, 10:51:52 am
Hello
acbern, misterdiodes:

Of course a resistance bridge would give more accurate (absolute) readings.
The problem is: I will never buy one.

I would really appreciate if someone with a resistance bridge would do independant measurements.

But I fear that will never happen since it is very time consuming and
if you have a bridge I understand that you have to earn money with that
[or cannot disclose the secret details].

On my side I will continue with my ADCs and do my very best.
The target is to select among several resistors those that will fit best for my next LTZ1000 references.

I have made a evaluation on the measurements.
So typical spread is around 0.03 ppm/K for the T.C. at 25 deg from LMS interpolation for the 1K resistors with Z201#6 as reference.
With some exceptions for the drifting resistors up to 0.05 ppm/K. Over all standard deviation is around 0.013 ppm/K.

For the 120R resisitors the larger noise leads to about doubled uncertainities.
I have splitted the evaluation for UP805 and Z201 resistors.

The evaluation for Z201 120R resistors are still in progress.

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 03, 2015, 01:58:47 pm
Hello,

further result Z201 120R #3 date code B0947-

30.04.2015: first measurement normal polarity
01.05.2015: 2nd measurement reversed polarity (check for thermal EMF)
02.05.2015: 3rd measurement normal polarity

LMS interpolation of 02.05.2015

A 0 =  1.48285225770232E+0000
A 1 =  2.15243292958098E-0001
A 2 = -1.19791700742452E-0002
A 3 = -3.89171660289224E-0005

So T.C. from LMS at 25 deg C is +0.22 ppm/K

The "box" T.C. is around 0.40 ppm/K including noise
and around 0.17 ppm/K from LMS interpolation

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 03, 2015, 04:41:36 pm
Andreas,

the Guildline 9975 is really a precise bridge, however it is probably in the 4k range. New bridges from Guildline or MI (I have got qotes) is in the 30 to 40k range. They beat the 9975 in accuracy, these are secondary standard bridges, so the price is justified. The 242 is not as precise as the 9975, but cheaper. I do not want to go into the details, but the 242 has its challanges (temp drift...). The 9975 uses a more stable principle of measurment. Precision is achieved in comparison, not so much absolute (the 9975 is intended as a 10:1 bridge anyways).

However, there are other ways to do precision measurements. You could use a Fluke 720A as a bridge device. The 720a manual gives information about that. A 720A may be available for arround 500 bucks. Also, you need a lead compensator and a Null-Meter. Another 500. And a voltage source, that only needs to have good short term stability, could be a good DIY source (LTZ1000 based or something similar). Best would be a 5440A, another 1k probably.

I am doing precision resistance measurements using a 3458A in voltage divider mode (e.g. a 10k reference versus a 1k DUT resistor, kind of a ladder calibration), with a very stable voltage source. This is based on the excellent linearity of the 3458A. I have done a GUM analysis on the error on both methods, and the accuracy is pretty good. It gets harder as the extremes are reached (between 10Ohm/1MOhm acc. is better 5ppm, 1k-100k arround 1ppm, using the 3458A). Again, you have to have a very stable voltage source though, with the right voltage, to use the full range of the 3458A.

If you want to do precision measurements, there is no way arround good gear. And a GUM analysis, so you know what tolerances you are looking at.

If you want, and as you are in Germany too, I could e.g. measure  a 10k resistor for you. The standards I have are calibrated to <0.5ppm acc. Send me a PM if you are interested. The resistor needs to be stable though (normal Z-foil with epoxy are not usable, but hermetic ones are pretty good (I have had drifts of only 0.2ppm pa). And temperature drift needs to be low (<1ppm/K; respectively drift analyzed).
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 03, 2015, 06:36:05 pm
Not sure I understand what you mean. I have two sets of wires, one connected to the front voltage inputs of the 3458A, one connected to the rear (or one could also just connect the cables to the posts of the two different resistors to be measured one after another). Need to wait for voltage stabilization. So the sense versus drive voltage difference is not of concern (if i understand you correctly). The 10V range is best for accuracy, but for small values, one needs to go down (and live with the then worse linearity), too keep power loss in the resistors low. Voltage reversal is achieved by changing the polartity of the source (the Datron 4808 supports that, it also gives the best results in terms of stability, compared e.g. to a 5440, which I actually like much).
The limit is at 1M, a 10M ohm resistor cannot be validated that way (I calculated about 200ppm error budget), as the impact of the input impedance is too high. The 720A is better there.

The guildline, according to its manual, has an accuracy of 0.7ppm from 0.01 to 0.1 ohms, which is impressive (and 0.2ppm up to 100k, 0.7ppm up to 1M), while the 242D is about 1ppm at 100ohms, and even 100ppm (!) at 0.01ohms (substitution mode, 242D manual dated June 71). It is 1ppm up to 10M though, where the 9975 is 7ppm. The 242D needs cal. In a nutshell, the 9975 is good for low values, the 242D is good for high ohms resistors, I would say. None of them is worth the mone for me, unless I find a chep one somewhere.
Have you, btw, done anything/experience with the Guildline 9930?
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 03, 2015, 07:40:07 pm
I am not doing it this way, I am not using the sense terminals, just the normal voltage inputs. I am making two independent measurements, voltage drops accross the resistor sense terminals, force terminals driven by the Datron, resistors in series. 12V on the higher resistor, 1.2V on the lower. Voltage reversal. This gives the most precise results, as it is using the linearity spec of the 3458A. Also the error calculation is a little more complex if you do it according to GUM, but with K=2 the resulting accuracy is pretty good. I am intentionally not using a low voltage switch, as it introduces other errors, and does not add any benefit.
The 34420A unfortunatelly, although it has good low voltage capabilities, has no transfer accuracy specification, the 3458A is unbeaten in that respect. I had contacted Agilent/Keysight for this, but, to make a long story short, no outcome. So the 34420A is not usable for this.
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 03, 2015, 10:08:38 pm
Well, the transfer accuracy spec really is what you need when you do relative measurements. And that is not specified unfortunatelly with the 34420A.

Exchanging front and rear may be a good thing to do, I need to figure that (impact) out. The 3458A, in its cal prcedure, does diffferents offset cals with front and rear, so part of the impact is probably covered. Question is, what inaccuracies remain.

It is true, for really high resistances, a high voltage and current measurement is the way to go, the 3458A will not help here. For low impedances, I tend to live with the limited accuracy (still less than 20ppm at 0.1 ohms). Cannot get the data with a 34420A, although it is probably ok for that purpose. Question is, what (GUM compliant) error budget do you assume.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on May 04, 2015, 07:09:47 am
Hello,

for 10:1 resistance ratios, for example to check my Hammon precision divider, (250kOhm:25kOhm), I also use the 3458A, similar to acbern.

I use a 5442A as an ultra stable 10V reference, and measure input and output voltages (10V and 1V) simply on the fixed 10V range, using the 3458As 0.02ppm linearity.

This 10:1 transfer should be better (about 0.2.. 0.3ppm) than the specified 0.05 ppm of value +0.05 ppm of range (= 0.55ppm total), as latter includes linearity AND other instabilities of the LTZ and reference divider circuitry, and is barn door wide specified.

Last weekend, I also experimented on the Ratio mode, in comparison.
It can also deliver superb results for DCV : DCV, but you have to be careful, as its behaviour is not fully explained in the manual.

First, the Sense (-) and Input (-)  must not differ greater than 0.25 V.
Second, The Sense reference input accepts +/-12V DC only, and always autoranges!
That means, it will change range from 12V to 1.2V , if  the 1V output of my divider would be applied to the Sense jacks, and the 10V to the Input jacks.

That's no big deal, if the instrument has been properly autocaled just before, otherwise you will have inconsistent results due to range change errors.

But you have no control over offsets on the Sense input!

It's better for the ratio mode, if Sense and Input use the same, fixed  10V DCV range.

Therefore, for Ratio, always apply the higher voltage, i.e. 10V to the Sense input, and the lower voltage, i.e. 1V, to the Input jacks, but chose manual 10V DC for the Input first!
In advance, you may measure the offset of the 10:1 divider output first , and let the instrument calculate the Null, and then enter ratio mode.

The manual does not cover this special setting, but  I strongly assume, that the instrument will really behave like this:

Sense = 10V DC range, Input = 10V DC range (manual, fixed)
Ratio Result = (Input - Offset) / Sense

As Sense is 10V, its offset plays no role here.

It is also recommended, to use statistics on the ratio function, to improve result and to control the stability.


In comparison, I did the following measurements.
The Hammon divider was not calibrated before the measurements:

1. Consecutive measurement on fixed 10V DC range, with averaging and statistics.:
Offset = 0.000 000 0V
U1 = 1.000 000 68V, sigma = 133nV, NSamp= 10 (one additional digit by statistics function!)
U2 = 10.000 011 2V, sigma = 155nV, NSamp = 10

Ratio = U1 / U2 = 0.099999956

2. Ratio function, Offset used on the divider output, ranges and voltages as above

Ratio mode reading of 3458A = 99.9999573E-03, sigma= 5E-09, N=16

So I conclude, that both methods may deliver identical results up to 8 digits (1e-8!!), but I feel much more comfortable, NOT using the Ratio function, as I can better overview, what I'm doing.


I would also avoid usage of alternative front / rear jacks for ultra precise ratio measurements, as this does introduce hard to control thermal voltages... The Jack-to-cable voltages are not included in the offset compensation/calibration !

And also, the front/rear switch is not very reliable any more, after these years..

Frank
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 04, 2015, 04:35:27 pm
Dr Frank,

I am trying to avoid the ratio mode using the 4 front terminals too, as the - to - terminal voltage is limited and also, as you state, i kind of feel like things are out of control a little.

Using front and rear voltage inputs is convenient, so i changed to that some time ago, and I would think that the offset issue is taken care of by the separate calibration of both inputs which the meter requires. One does have to wait for thermal settling, but actually I do not see, in my setup, any change after connecting the meters and waiting 5 minutes. The best anyway is to just use bare copper wire (maybe plated), thats also what Guildline recommends for their DCC bridges. While I am still using gold plated copper spades cable or the low emf pomona cables, i will do some tests with dual twisted pair teflon cable. The alternative, hooking up the wires to one resistor first and then the other also bares the risk of thermal voltages. so I have mixed feelings about this too. the thermalvoltages should anyway be eliminated by reverse voltages. I would alos recommend to go to the limits, i.e. 12V and 1.2V applied, helps accuracy. if the switch is worn out, that should show as a fluctuating, unstable voltage.

On a slightly different subject, I calibrated one of my 3458As recently, and it starts up with an error message: 'calibration: secure required'. obviously i had to unsecure the meter by entering the pass code wehn doing the cal. but how do i set secure again. the manual is not very helpfull, and somehow i do not manage to get that resolved. Not the frirst 3458A I calibrate, but somehow I fixed that last times (year ago or so), and do not recall how. must have just pressed some buttons intuitively, does not work this time, issue drives me nuts.

Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on May 04, 2015, 05:33:27 pm


On a slightly different subject, I calibrated one of my 3458As recently, and it starts up with an error message: 'calibration: secure required'. obviously i had to unsecure the meter by entering the pass code wehn doing the cal. but how do i set secure again. the manual is not very helpfull, and somehow i do not manage to get that resolved. Not the frirst 3458A I calibrate, but somehow I fixed that last times (year ago or so), and do not recall how. must have just pressed some buttons intuitively, does not work this time, issue drives me nuts.

Dear acbern,

things you're doing not so often, can drive you crazy!

It's described in the calibration manual, and as I don't want to do a real calibration right now, I'm no 100% sure, that I'm telling it correctly, now.
(I'm also well over 50, therefore I remember the last CAL only dimly.)

Well, if I figure it out correctly, there is no explicit UNLOCK / LOCK process needed.

You simply type in CAL 10, 3458, and the 10V calibration will be done with assumed 10.000000V reference.
Without the default password '3458', this calibration will fail.

As said, an explicit LOCK command is not necessary afterwards, or can nowhere be applied.

SECURE 3458,1234 would only change the password to 1234, and would activate the password safety for the CAL command.
SECURE 3458,0 would simply deactivate the password for the CAL command, so only the default calibration value (10V or 10kOhm) would be needed.
SECURE XXX,YYYY, ON /OFF would also secure /unsecure the ACAL feature.

This simple mechanism is also, what I remember very dimly from the last CAL I did, after you kindly measured my VHP202Z resistor..

It's different to the LOCK switch at the rear of the 5440B, isn't it?

PS: Probably, this confusion is caused by the HP34401, or other SCPI instruments.
This instrument has an explicit UNSECURE and SECURE command, which you have to use before and after the 35 points (?) calibration.
As the 3458A only has 2 calibration points (nearly), such an elaborate process makes no sense, I think.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 04, 2015, 06:11:11 pm
thanks, and what you say is what I thought too. I did the cal as you describe, pretty straightforward. Command, voltage level, password (3458). I calibrated 0, voltage (10V), resistor (10k), and  ac (scal), the usual stuff (except maybe scal, but was necessary). all went ok, and meter works fine, but when I power up the meter, it always comes up with 'cal error, secure required'. so there must be something else...
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on May 04, 2015, 06:42:22 pm
What if you rewrite the security code? ie. SECURE 3458,3458

I remember getting this message after changing the NVRAMs. Once I did a SECURE 0,0 the message went away.

Syntax SECURE old_code, new_code [,acal_secure]
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 04, 2015, 07:24:39 pm
You made my day! I entered:

SECURE 0,0
SECURE 0,3458

Issue gone. Maybe just typing the second line (SECURE 0,3458) would have done it too.
That should now have set the cal security code back to 3458 after having removed the initial non-locked state.
Pretty logic overall, and many thanks
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 06, 2015, 09:25:17 pm
Next resistor Z201 120R #4 date code B0947-

this one came directly out of my long term ageing experiment (load life) with 100mW intermittend power at room temperature

03.05.2015: first measurement normal polarity
04.05.2015: 2nd measurement reversed polarity (check for thermal EMF)
05.05.2015: 3rd measurement normal polarity

LMS interpolation of 05.05.2015

A 0 =  4.60338549126336E+0000
A 1 =  6.92576655745229E-0001
A 2 = -8.92617577082719E-0003
A 3 = -7.33433859833421E-0005

So T.C. from LMS at 25 deg C is +0.69 ppm/K

The "box" T.C. is around 0.71 ppm/K including noise
and around 0.59 ppm/K from LMS interpolation

Together with the measurement of 06.05. total drift at 25 deg is around 6ppm through 4 days.
In the last days it was rainy so humidity increased from 50% rH to 58% rH.
I think I really am in urgent need of a humidity insensitive reference resistor
so that I can tell wether it is the DUT or the reference which is drifting.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Kleinstein on May 07, 2015, 06:28:05 pm
The high values of apparent drift tend to show that there is a problem with the measurement setup. Also the curves with reversed polarity are visibly different. So I think some more tests and improvements of the measurement setup are needed. The noise also looks quite large to me - I would expect less noise in a well shielded setup.

At least there is no more need for even more curves of questionable quality.

Humidity likely is one more parameter to influence the resistors, and possibly also the measurement setup, through leakage currents. So I might be helpful to read the humidity data as well.

From the pictures at the beginning of the thread, the rather long unshielded and dangling wires don't look very good. They may cause EMI problems, by operating as an antenna. Also they make the effect of reversing the polarity questionable, as moving the cables may also change thermal gradients. As a thermal cycle is rather slow, it might be better to do the polarity reversal more often within a cycle (e.g. every 30 seconds - to really separate possible thermal EMF problems.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 07, 2015, 07:57:03 pm
Hello,

Also the curves with reversed polarity are visibly different. So I think some more tests and improvements of the measurement setup are needed. The noise also looks quite large to me - I would expect less noise in a well shielded setup.

I do not find a 0.03ppm/K difference (for the 1K resistors) is a dramatically difference with reversed polarity.

Noise is at the datasheet spec of the used converter. With 1uVpp (9uVpp / sqrt(85 samples)) at ADC input at 500mV voltage over 120 Ohms gives around 4ppm pp noise when regarding that each measurement consists of 4 single voltages.
By the way the noise is still better than many 6.5 digit instruments

There are always 2 possibilites: shielding or (common mode) filtering.
I use filtering + galvanic isolation.
EMI effects are checked to be below noise level of 1uV.


So I might be helpful to read the humidity data as well.

From the pictures at the beginning of the thread, the rather long unshielded and dangling wires don't look very good. They may cause EMI problems, by operating as an antenna. Also they make the effect of reversing the polarity questionable, as moving the cables may also change thermal gradients. As a thermal cycle is rather slow, it might be better to do the polarity reversal more often within a cycle (e.g. every 30 seconds - to really separate possible thermal EMF problems.

Thats what I already did in the overview sheets last week.
The problem with humidity is that it needs several days of time constant until it gets fully visible.
I blame the drifts (> 1ppm @ 25deg for 1K resistors, or > 2-4ppm for the 120R resistors) on humidity.
I will see if I can get better with the PWW-resistor as reference resistor.
Otherwise I will have to use a hermetically resistor.

I also plan a polarity reversal every measurement cycle.  (< 1 second)
(just to check EMF: I do not expect changes on T.C.)
But this will take some time.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 10, 2015, 02:39:53 pm
Last resistor Z201 120R #5 date code B0947-

also came from ageing experiment but was switched off with Z201_120#4 on evening of 02.05.

07.05.2015: first measurement normal polarity
08.05.2015: 2nd measurement reversed polarity (check for thermal EMF)
09.05.2015: 3rd measurement normal polarity

LMS interpolation of 09.05.2015

A 0 =  1.47787395857863E+0000
A 1 =  6.39202276575331E-0001
A 2 = -9.37087943635444E-0003
A 3 = -9.72385941996082E-0005

So T.C. from LMS at 25 deg C is +0.64 ppm/K

The "box" T.C. is around 0.65 ppm/K including noise
and around 0.53 ppm/K from LMS interpolation

No significant drift (1ppm) during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on May 30, 2015, 06:28:22 pm
FYI I'm getting a pretty awesome 10K metrology grade transfer standard soon from Wekomm
2ppm tolerance
Long term stability better then 1ppm / year
Temperature stability better than 0.3 ppm / °C
And they are conservative specs, and they are working on producing 0.001ppm tolerance parts.

Teardown anytime soon...?


Now available from Datatec for 3750,00 EUR each

http://www.datatec.de/Labor-Messgeraete/Adapter-Kabel-Stecker/Wekomm-Widerstandsnormale/index.htm (http://www.datatec.de/Labor-Messgeraete/Adapter-Kabel-Stecker/Wekomm-Widerstandsnormale/index.htm)
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on May 30, 2015, 07:15:05 pm
@Itz2000,

"Teardown anytime soon", you are aware that opening the standard will not only invalidate the warranty but it will also invalidate the calibration certificate as well, not to mention any accidental damage possibly caused while opening it.  Otherwise, congratulations on the purchase of a primary standard, now what are you going to do with it besides a teardown?

Frankly, I would have purchased an SR-104 standard used, for around that price you would have gotten a resistance standard with very long term proven performance which the Wekomm has yet to prove.  While its stated specifications are very similar to the SR-104, it still must prove it to be considered a standard of the same calibre as the SR-104.

As to the claim of 0.001 PPM, pure nonsense, they can't measure to that accuracy, no national primary lab in the world can measure to that accuracy, the uncertainty will likely be reduced to 0.1PPM in the proposed 2018 standards but that is still a factor of 100 over 0.001 PPM.  Any one who makes such a silly claim is suspect and is not worthy of consideration.  Any serious metrologist would laugh at such a claim.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on May 30, 2015, 10:04:32 pm
"Teardown anytime soon", you are aware that opening the standard will not only invalidate the warranty but it will also invalidate the calibration certificate as well, not to mention any accidental damage possibly caused while opening it.

The same happens to a $10k spectrum analyzer or whatever the victim of the teardown is.

The manufacturers send their products to the blogger to get publicity. Which can be, and usually is, worth much much more than the cost of the instrument they give for free. Advertising in the traditional way is very expensive.

But there are always risks. In this case the manufacturer probably underestimated the possibility that someone points out that their product is basically a $50 component in a box with a $4000 price tag.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on May 31, 2015, 07:13:27 am
But there are always risks. In this case the manufacturer probably underestimated the possibility that someone points out that their product is basically a $50 component in a box with a $4000 price tag.

Hello,

the $4000 is not for the component inside the box. It is only for the box itself.
Of course including the sticker on the box (and the service from calibration).

I am shure that Dave is already doing a "extended shelf life test"   8)
until he gets a instrument which is better than his 34470A so that he can verify the result.

If I had such a stable device it would have to work hard.  >:D
(against the 12K or 12K5 resistors).

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on May 31, 2015, 04:30:18 pm
From what I've seen and read on this blog site, I very likely have the most accurate resistance measurement system of anybody here.  DVMs are NOT considered standards under any definition, just check with your National Primary Calibration Laboratory and they will tell you the same thing.  Even the 3458A is not considered a standard under any measurement conditions, they are not used as standards by any metrology lab I know of.

At 10K, my 242D system is accurate to 0.2 PPM as calibrated against an SR104, I also have the same 0.2 PPM uncertainty as everybody else.  That gives me a 5:1 accuracy ratio when comparing a resistor (standard or otherwise) over the range of 120R0 to 1Meg, accuracy begins to drop off above and below that range.  Therefore I am barely accurate enough to test the Wekomm standard under the required specifications.  While I have a resolution of  up to 0.0001PPM, it has no bearing on the accuracy.  A National Lab must be able to to maintain a 10:1 accuracy ratio.  Under these requirements I do not see anyone on this blog (unless unknown to me) that has the required measurement system to verify the Wikomm 10K standard or even make an accurate measurement of it.

I, for one, wouldn't mind seeing the inside of the Wikomm standard out of curiosity. If someone wants to open one up for all of to view, I certainly would not object.  I could say the same thing for an SR-104, however I am somewhat less curious about it as I know a fair amount about what is in there, just some minor details lacking in my knowledge of its innards.  I certainly wouldn't want to pop one of those open if it was still in working order, it would definitely destroy its value as a standard.

Title: Re: T.C. measurements on precision resistors
Post by: bingo600 on May 31, 2015, 04:59:34 pm
At 10K, my 242D system is accurate to 0.2 PPM as calibrated against an SR104, I also have the same 0.2 PPM uncertainty as everybody else. 

Edwin what's a 242D system (link please)

I think i read that Quarks have a SR104 (lucky him) , i could use it for cal'ing my 3458

/Bingo
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on May 31, 2015, 05:14:16 pm
Here is the manual, have fun...
http://www.ietlabs.com/pdf/Manuals/esi242D_im.pdf (http://www.ietlabs.com/pdf/Manuals/esi242D_im.pdf)
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on May 31, 2015, 05:35:53 pm
@bingo600,

The ESI 242D system is the best measurement system short of a primary lab, most secondary labs have them and any serious resistor manufacturer has one or an equivalent of it.  While the general specifications give up to 1 PPM measurement accuracy (at best), the instrument is capable of being calibrated to 0.2 PPM accuracy over the main ranges and only begins to lose accuracy above and below the range I specified earlier.  Since it is time consuming to calibrate to that level of accuracy, many metrology labs will settle for the 1 PPM level which is quite good to begin with.  A 242D new goes for somewhere around $35,000-$40,000 USD these days.  It also requires an SR104 and a large pile of transfer standards to achieve maximum accuracy.  They are still in high demand at secondary and lower level metrology labs around the world.  Even with an older 242D and 'used' standards, you are still looking at a good $20,000 USD plus traceable calibration requirements.

Below about 10K, the DCC bridge can achieve a higher accuracy but even used ones are quite expensive and few labs need such instrumentation (remember, the 0.2 PPM uncertainty still applies to this bridge as well).

Thank you Vgkid for posting the link to the current manual.
Title: Re: T.C. measurements on precision resistors
Post by: bingo600 on May 31, 2015, 06:43:31 pm
Thank you vgkid & Edwin

Manual study time (even though its 100% out of my budget)

/Bingo
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on May 31, 2015, 08:36:30 pm
...

As to the claim of 0.001 PPM, pure nonsense, they can't measure to that accuracy, no national primary lab in the world can measure to that accuracy, the uncertainty will likely be reduced to 0.1PPM in the proposed 2018 standards but that is still a factor of 100 over 0.001 PPM.  Any one who makes such a silly claim is suspect and is not worthy of consideration.  Any serious metrologist would laugh at such a claim.

That Wekomm standard is not serious, I agree.
Without a thermometer and a calibrated R(T) curve, it's nonsense to claim for a reasonable secondary standard.
Also, current SI Ohm is not better defined than 0.02ppm, 0.2ppm,  therefore no standard on the world can be less uncertain.

SR104 may be stable and uncertain to 0.1ppm  , and that's the limit of todays artefact standards.

If the SI will be redefined in 2018, Ohm will be uncertain to zero ppm, in definition.

The underlying quantum standard, the von Klitzing Hall effect resistor, is proven to be uncertain to 1e-19. (Definition equals mise en pratique)

Transferring this to any secondary standard, made of any metal alloy, is limited to about 1e-9 (0.001ppm), due to thermoelectric voltages of that order.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on May 31, 2015, 09:51:57 pm
From what I've seen and read on this blog site, I very likely have the most accurate resistance measurement system of anybody here.  DVMs are NOT considered standards under any definition, just check with your National Primary Calibration Laboratory and they will tell you the same thing.  Even the 3458A is not considered a standard under any measurement conditions, they are not used as standards by any metrology lab I know of.

At 10K, my 242D system is accurate to 0.2 PPM as calibrated against an SR104, I also have the same 0.2 PPM uncertainty as everybody else.  That gives me a 5:1 accuracy ratio when comparing a resistor (standard or otherwise) over the range of 120R0 to 1Meg, accuracy begins to drop off above and below that range.  Therefore I am barely accurate enough to test the Wekomm standard under the required specifications.  While I have a resolution of  up to 0.0001PPM, it has no bearing on the accuracy.  A National Lab must be able to to maintain a 10:1 accuracy ratio. Under these requirements I do not see anyone on this blog (unless unknown to me) that has the required measurement system to verify the Wikomm 10K standard or even make an accurate measurement of it.

The organisation I used to works for carried out a comparison of their resistance calibration systems at 10 kohm. The three competitors were an ESI 242D, a Guildline current comparator resistance bridge and an in-house built automated system made of 3458As measuring voltage, a nanovolt scanner and a stable current source.

The standards used were four stable SR104s with a long history and stored in an air bath. A set of ten cross measurements between the standards were made with each system during the day and another set a week later.

I couldn't find the paper so I am missing the exact data and the analysis. But from memory the 3458A system was the best. The results of the cross measurements were consistent within a few hundreths of ppm which was quite amazing. The current comparator was working at the high side of the measurement range but still performed much better than the specifications. The 242D was a more than a decade behind the current comparator and would have been even worse if used outside temperature stabilised lab.

Even with an older 242D and 'used' standards, you are still looking at a good $20,000 USD plus traceable calibration requirements.

Manual study time (even though its 100% out of my budget)

www.ebay.com/itm/221765322225 (http://www.ebay.com/itm/221765322225)

I got one for free and bought another from Ebay for $300 a few years ago. Both are the high accurary version 242D / SP3632 manufactured in the late 1980s.
Title: Re: T.C. measurements on precision resistors
Post by: acbern on May 31, 2015, 11:33:25 pm
If you look at the manual of the 242D, it is very apparent that this instrument is very high in resolution but relatively 'bad' in accuracy. An accuracy of 0.2ppm (in transfer mode) with the 242D in practical use is hard to guarantee. And in absolute measurement mode, it is anyway 'specified' much worse. E.g. the best drift rate of the 8 digit 925 resistance decade, which needs regular calibration, is 3ppm/K. In other words, at only 0.1K delta between the calibration temperature and the working temperature, some undefined number of resistor(s) within the instrument may (or may not) have drifted by 0.3ppm, other resistors even more (worst spec is 15ppm/K). And you cannot tell how that translates into the precision of a specific measurement and how it affects the above stated 0.2ppm accuracy, as you dont know what drifted how. A GUM error propagation calculation would essentially be impossible. And maintaing 0.1K stability over the whole decade is anyway merely impossible, so realistically we talk about several ppm of possible drift for the decade. Now you may say that this is a short term comparison measurement only, little temperature changes during that periode, but then the fact that between the reference measurement and the DUT measurement totally different decade resistors may be used (depending on the Standard to DUT ratio), is not taken into account. And these different sets of resistors may have drifted appart isnce last cal. Using the 240C deviation bridge, above 6ppm of difference (not  a lot), is only recise to 1ppm (para. 2.4.2). And it goes on with missing information about drifts for the resistors, stability of the voltage sources, trimming adjustment of resistors to 1,5ppm or worse and so on and so on.
So the 0.2ppm stated here, sounds just impossible to guarantee under standard conditions (even though of course, typically, it may be fine, but you cannot use this in a calibration).
242s sell at a relatively low prices used, see the link above, and I have always refrained from buying one, as using a 3458A in voltage comparison mode, has a well defined and much better accuracy, and has much less unknowns and allows to do a error propagation calculation. It is more work to do a resistance measurement though.
The highest accuracy devices today are still DCCs, see current data sheets of Measurements International for example or Guildline. They are pretty expensive though, and only are justified for primary labs or manufacturers of high precision resistors.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on May 31, 2015, 11:35:55 pm
@Frank,

The current standard SI uncertainty is quoted at 0.2PPM for a 'regular' calibration, but if the uncertainty is reduced to 0.1PPM in 2018 this only implies an improvement in the absolute measurement accuracy of the SI standard, it does not imply any improvement in the accuracy of the Quantum standard as it is already considerably higher than 10:1 accuracy.  Every standard of measure has some value of uncertainty in the measurement, that cannot be removed by any foreseeable technology.  I would anticipate that after 2018 my SR-104 may have an uncertainty of 0.1 PPM during that calibration in which case the uncertainty of my 242D will also be reduced by the same figure.

@Itz2000,

Technically, I can calibrate my 242D to have a zero PPM error as calibrated against my SR-104, however the limiting factor is still the 0.2PPM uncertainty, there is no way to know for sure what the exact value of the SR-104 is, that is why there is an uncertainty factor applied to the measurement.  My 242D can resolve 10K to well below 0.1PPM and it may even repeat but that doesn't eliminate the uncertainty of the measurement.  Now you could send the SR-104 to a National Metrology lab, such as NIST and request that they provide a measurement which exceeds the 0.2 PPM uncertainty.  There are two problems with that, one is that they are going to charge you several thousand dollars to do the measurements IF they agree to do it at all and secondly, they may question whether or not the standard could survive shipping without any change in the calibrated value.  Unless your SR-104s were calibrated to beyond the 0.2PPM standard uncertainty, your measurements cannot be any better than that.   You may have been able to measure stability to better than 0.2PPM but that does not remove the uncertainty.  You can indeed quote better measurements, however the uncertainty of the standard still takes precedence over those measurements, therefore you certainly can say that the repeatability of your system was within ~0.05PPM but the accuracy of the measurement is not the same.  So the automated system, in the end, has no better accuracy than the 242D (apparently that system was not as well calibrated as mine).

Considering that you say they achieved better than 0.1PPM, I'm betting that system cost a whole lot more than a 242D and a SR-104 plus with so many more instruments and connections involved, keeping thermals and errors well below 0.1PPM would be nothing short of a primary lab capability.  I do not know why you are complaining about a 242D in a temperature stabilized room, that is what a metrology lab is by definition and I rather doubt the rest of your equipment would have performed as well outside of a controlled lab environment either so your comment is rather mute on the subject.

The Guildline has about the same accuracy as the 242D at 10K, where it really shines is with low ohm resistors where the 242D has more limited resolution (this was a design decision not so much a limitation of technology), the Guild line can exceed <0.05PPM, I have heard of 0.01 PPM with careful calibration and depending on the value.  Again, the Guildline is still limited by the 0.2PPM uncertainty even though it can measure to ten times better than that.

Finally, my original comment was that I do not believe that anyone on this blog has the required measurement system, such as I do to verify a Wikomm standard.  The fact that a large company may have the where with all to spend tens of thousands of dollars on a near useless exercise does does meet the stated parameters I put forth.  I am certainly not in competition with NIST over resistance accuracy, for that matter, very few companies have the same resources that NIST has nor are silly enough to claim so.  0.2PPM uncertainty is certainly good enough for any of my customers unless they are in the market for an SR-104 (same uncertainty) and I am not ready to take on that as yet.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 01, 2015, 12:50:24 am
@acbern,

The manual (any manual) is not going to tell you what the capability of an instrument is in actual practice.  I have used and calibrated 242Ds since 1973 and they are quite capable of exceeding manual values.  Since it appears you have not had any experience with an 242D, you are merely speculating on what may or may not be its capabilities.  Direct measurement mode wholly depends on how good the calibration of the machine is and if you are depending on another cal lab to do it, I guarantee they will absolutely not take the time to calibrate it to capability only to 'spec' and yes that includes your 3458As as well.  I've been in calibration for over 40 years so unless you manual readers have any hands on experience, you do not qualify as a accurate source.  The uncertainty which I specify is the uncertainty of my SR-104, I can calibrate my 242D to practically the exact same value and in-turn calibrate the rest of the 242D to nearly the exact same accuracy on the main decades, I am currently within 3 PPM at 10Megohms despite the fact the manual says 10 PPM.  Since I cannot adjust the lowest decades, the accuracy does indeed fall off, I do have other methods of greater accuracy including direct comparison against low ohm standards improving accuracy considerably.  Over the main adjustable decades, I am within the uncertainty limit.

Your drift speculation is only valid if the room temperature varies significantly (yes, I can indeed measure this) that is why thermometers were invented, I can check any calibration point on the 242D quite quickly if need be and if needed, make the appropriate adjustment if needed, it is exceedingly rare than any customer would need resistors made to even PPM levels let alone sub PPM levels.  Unless you are a Metrology lab or a manufacturer making very precision measuring equipment, there is absolutely no reason for PPM resistors, even I rarely need resistors like that and I have business reasons for them.

The 242D can indeed hold tight accuracy over more than just a few hours since I have thousands of hours on 242D, I can be considered an expert on the subject.

As to the 3458A accuracy, just where does it say in the manual that it has PPM or sub-PPM accuracy except on possibly the 10V DC range.  The linearity of the ADC is not the same as accuracy, in using the 3458A in ratio mode to compare two components, you still must have a reference component of sufficient accuracy and stability for the measurement to compare to.  If you are using a SR104 as the reference, that also means that the voltage across the unknown is limited as is the ratio mode voltage range of the 3458A.  Even if you are using the internal 3458A reference you still must provide a reference component to compare to the unknown.  In the example of an SR-104 compared to an unknown 10K, you've already lost half of the adc range on the resistor divider.  Linearity is not the only specification in play here, check your manual and you will find several other specifications in play as well, that possible 0.02PPM linearity is likely swamped out by the other factors and lets not forget, DVMs also have uncertainties just like everything else.  It is also temperature sensitive just like most other instruments.  All in all, I think I'll trust my 242D system over your 3458A.

You mentioned stability of the voltage source for a bridge????  A resistance bridge does not rely on voltage source stability, it only needs to be reasonably clean (not too much ripple or large amounts of noise), the measurement is differential, the PSRR is very high, much higher than a DVM.  The main concern is keeping the wattage down to reasonably low values in the resistors.  Your 3458A may have more problems with dirty or unstable voltage sources.  The use of math to reduce errors is questionable, particularly if it is not used with care, just because a DVM has math doesn't mean it makes your measurements any better.  There are limitations to reducing errors with math, statistics can easily lie, I don't need math with the 242D and an accurate measurement with the 242D does not take that long, I don't know why you might think that except that you haven't used one.  Just how long does it take to set up an accurate measurement with a 3458A in ratio mode?  Now be careful here, I know people and metrology labs that do use 3458As so no fudging. <grinning>

For those who have a 3458A, it is convenient to use it for various measurements, nothing wrong with that, just so long as you know the limitations.  Primary cal labs (such as NIST) do not and will not consider a DVM as a standard, not even the 3458A, it can be used as a monitor for instance, NIST has a number of Fluke and HP DVMs in use but not in any capacity as standards, they do not qualify as such.  I know of 3458As which are used to monitor LTZ1000A voltage references in laboratories and when they go in for calibration, they get the 'deluxe' calibration which costs a lot, they are handled with kid glove care, HP actually picks them up, transports them to the lab, calibrates them and transports them back and you folks (me included) aren't in that league of calibration but they still aren't standards even though they are cycled every 6 months.

 Again, DCCs can out perform the 242Ds on low ohms, by the time they are up to 10K, they are about the same accuracy as the 242D (never mind the stupid manual), a DCC can be justified if you need the low ohm accuracy, otherwise you are better off with a 242D.  DCCs do not measure resistance by the same method as the 242D, they have a very intricately wound set of ratio transformers that do the real work.

Let's say one of you gets a Wikomm 10K and you just happen to have a calibrated 3458A (but you don't know what it is actually performing to on the various ranges, it is just within specs), what are you going to use to compare the Wikomm to using the 3458A's ratio mode?  You already have some unknowns in the 3458A so you're going to need another standard such as an SR104 (that is calibrated of course) for the ratio comparison otherwise you're dead in the water before you even got started.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 01, 2015, 12:52:10 am
Curious, I did not underline all of that!

In the modify window, the wayward underlining is not showing up at all.  Just a minor annoyance, sorry folks.
Title: Re: T.C. measurements on precision resistors
Post by: acbern on June 01, 2015, 12:26:20 pm
Edwin,

obviously, neither a 3458A nor a 242D are standards, they are meters. Standards, by definition, are e.g. a SR104 standard resistor or 732 voltage reference or a rubidium clock. Meters are used to compare standards with DUTs. This works the better (precision-wise), in a nutshell, the lower the transfer inaccuracies of the meter are (such as temperature drift, applicable aging, non-linearity...; and not to forget other gear involved). Under GUM, you cannot use the 3458A for direct resistance comparision, unless you have done a lengthy, and pretty limited (to essentially only the condition you validated) statistical analysis for your specific instrument. The 8508A or even the 7081 are better spec'ed here. The resistor transfer accuracy unfortunatelly is not specified by Keysight, although of course, when you try, and many here have, you are somewhere in the 0.1ppm region. Still, you cannot use this accuracy for an error budget statement under GUM.
To do exact resistance measurements, you would do that in voltage divider mode. Here, the 3458A has very precise transfer specifications, which actually are independent of wether you do a gold standard cal or a normal cal. You do a comparison, comparing (voltage drops across) e.g. a SR104 and a DUT. This is essentially a linear relationship (the SR104 being the standard), under mathematical consideration of the various error contributions and weighing factors. You can do repeatable measurements to say 0.1ppm ball park, but when you do the math, depending on the other gear you use, you rather are in the 1ppm ball park (even with very stable voltage references). It may not sound very exciting (or be considered conservative) to have a factor of say appr. ten between how precise you think you measure and what you can prove mathematically with k=2, especially as sub ppm accuracies seems to be kind of en vogue here. More details are in the 3458A spec and related historical HP/Agilent design background data, frequently discussed here.
And yes, unfortunatelly, we manual readers do rely on what data we get from gear vendors, thats how the procedures work, and how the cal labs work, and unless we want to do a research exercise specifying a gear ourselves, as per GUM rules, and with potentially unclear results, we use what we get specified to establish a error calculation. If no data is available, it is in most cases more efficient to use something that has appropriate data. The 242 has not.

It may well be that your 242 typically is more precise than spec in some (or many) ranges (you do not know for sure, unless you have a more precise instrument of which we dont know yet), but you cannot prove this in compliance with GUM error budget guidelines. Maybe you have done that, let us know, we would certainly be interested to see it, maybe you also have specs other than what is in the manual. Or, maybe, your statement that you 'dont need math when using the 242' says it all.
After having had a look at the 242 manual long ago, I decided for me it is not an exercise that is worth it, given all the missing data and (specified and unspecified) drifts. This is a pitty, as the 242 is well priced in used condition.
With respect to accuracy data of DCCs, I would recommend to consult MI's data sheets, they are factors more precise than the 242, yet at a different price tag too (50k region), allowing use in national labs. The only option worth it, for me, would be an old 9975.

Title: Re: T.C. measurements on precision resistors
Post by: engiadina on June 01, 2015, 04:25:04 pm
@Edwin G. Pettis

So you are telling us, that this document

http://www.kalibriercentrum.de/pdf/DAkkS_Urkunde_Bayern.pdf (http://www.kalibriercentrum.de/pdf/DAkkS_Urkunde_Bayern.pdf)

is basically rubbish ?

Wow.
Title: Re: T.C. measurements on precision resistors
Post by: mikerj on June 01, 2015, 04:30:52 pm
Curious, I did not underline all of that!

In the modify window, the wayward underlining is not showing up at all.  Just a minor annoyance, sorry folks.

You have three opening underline tags around the word "accurate", and the closing underline tag at the end of your post.

e.g.
Code: [Select]
[u]accurate[u] [u]

...
...
...
[/u]
Title: Re: T.C. measurements on precision resistors
Post by: TimFox on June 01, 2015, 08:12:22 pm
@Edwin G. Pettis

So you are telling us, that this document

http://www.kalibriercentrum.de/pdf/DAkkS_Urkunde_Bayern.pdf (http://www.kalibriercentrum.de/pdf/DAkkS_Urkunde_Bayern.pdf)

is basically rubbish ?

Wow.

My German isn't as good as it used to be, but which items on the calibration facility list disagree with E G Pettis' posting?
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on June 01, 2015, 09:04:30 pm
@engiadina,

While I do not read German, I do understand the listings for the various measurement types, I do not understand why you think what I said negates this document.  While a few of the resistor limits (if I understand it correctly) are a bit different than some of the ones I stated, that does not in anyway negate the document.  My calibration is in compliance with our NIST standards and uncertainties which may be better than in some other labs.  I see nothing wrong with the document you presented.  It is nothing more than a statement of capabilities of this particular calibration lab which most labs compile for their customers.

@acbern,

You are half correct, DVMs are not considered standards period, ask a metrology lab and they will tell you so, the 242D is indeed a standard in its own right and by definition is a standard.  It is fully calibrated and traceable to which ever national metrology lab your country maintains, here it is the NIST.  I see no contrary explanation for that.  It is certainly not a 'meter', while it has a null detector, that in no way makes the instrument a meter, that is just plain silly.

As to the unnecessary underlining in my earlier post, it is of no consequence so it does not warrant any further effort.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 07, 2015, 01:28:37 pm
Dear Mr. Pettis,

I would like make some interesting clarifications about the Ohm uncertainty:
The BIPM lists on its KCDB page all National Metrology Institutes (NMI), i.e. their capabilities and uncertainties: http://kcdb.bipm.org/ (http://kcdb.bipm.org/)
When you search for the German PTB, or the US NIST, you will find the hint, that the Ohm is relative to R(k-90), that is the von Klitzing / Quantum Hall Effect (QHE) standard, not relative to S.I. Ohm!!

These NMIs either use a DCC, Direct Current Comparator, which gives 0.2ppm or less uncertainty, or the CCC, Cryogenic Current Calibrator, which allows comparisons as low as 0.004ppm uncertainty, directly to the QHE! See listing for the NIST in the attached document!

Explanation of the DCC and CCC bridges used, can be found here: http://www.nist.gov/calibrations/upload/tn1458.pdf (http://www.nist.gov/calibrations/upload/tn1458.pdf)

According to that, it's very well possible to make measurements /comparisons with much lower uncertainty than 0.1ppm, even outside an NMI, and without direct access to a QHE.

Therefore, this Wekomm resistor, which Dave received, and which is a prototype only, really had been calibrated to 0.1ppm uncertainty, by this Bavarian calibration lab! Dave published the Wekomm calibration document, where you can find further details:

The capability of the lab relies on their (decades old) GenRad 1444A-10kOhm standard, which is the predecessor of the IET / ESI SR-104.
This is an ultra stable artefact, much more stable than the usual SR-104, and which has been calibrated many times directly at the PTB, so its stability of about 0.01ppm/yr. has been characterized since a long time.

They also use an ultra precise bridge, an MI 6010B, which obviously allows direct 1:1 comparisons below 0.1ppm level.
(Neither do I know that bridge, nor how it compares to the 242D bridge of Mr. Pettis.)

There is obviously a demand by the NMIs, especially our PTB, for more stable working-standard resistors, than these Thomas type 1 Ohm and the SR-104, which are mostly too unstable to reach the 1e-9.. 1e-10 comparison region of the QHE and CCC bridge.

The Wekomm resistor, when a precise thermometer and T.C. characterisation will be implemented in the future, is a candidate to improve these standard resistors by orders of magnitude regarding stability. Dave already mentioned, that our German PTB is involved somehow.

I had an interesting discussion with 'engiadina', who is the design engineer of this Wekomm resistor, who confirmed that in detail.

It was a big surprise for me, what are the weak points of the current Ohm metrology, and what seems to be possible with these upcoming Wekomm resistors, implementing  Vishay BMF technology.

Anyhow, in 2018 the difference between the QHE and the S.I. will probably vanish, so that the S.I. Ohm really will be precise to exactly zero ppm. requiring also better working standards.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on June 07, 2015, 06:17:27 pm

The capability of the lab relies on their (decades old) GenRad 1444A-10kOhm standard, which is the predecessor of the IET / ESI SR-104. This is an ultra stable artefact, much more stable than the usual SR-104, and which has been calibrated many times directly at the PTB, so its stability of about 0.01ppm/yr. has been characterized since a long time.

The GR 1444-A was introduced 5 years later as a competitor to the ESI SR104. It generally turned out to be less stable and never became popular. But there still can be (and in this case obviously is) very very good individual units.

There is obviously a demand by the NMIs, especially our PTB, for more stable working-standard resistors, than these Thomas type 1 Ohm and the SR-104, which are mostly too unstable to reach the 1e-9.. 1e-10 comparison region of the QHE and CCC bridge.
The Wekomm resistor, when a precise thermometer and T.C. characterisation will be implemented in the future, is a candidate to improve these standard resistors by orders of magnitude regarding stability.

A typical SR104 drift rate is approximately +0.07 ppm per year. Magnitudes better would really be something!

In a document I posted earlier the VSL measured their Vishay VHA518-11 resistors against the Quantum Hall. The Wekomm uses the VHA518-7 which is the same type except there are only 7 bulk metal foil chips instead of 11 inside the hermetic can.

The two resistors measured by the VSL showed drift rates of 0.4 and 1.9 ppm/year. And most importantly the drift rate it is not (yet) predictable.

Even if Vishay had some magic to improve the stability that much, why would they share that secret only with a small German toy train manufacturer?
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on June 07, 2015, 08:33:46 pm



A typical SR104 drift rate is approximately +0.07 ppm per year. Magnitudes better would really be something!

In a document I posted earlier the VSL measured their Vishay VHA518-11 resistors against the Quantum Hall. The Wekomm uses the VHA518-7 which is the same type except there are only 7 bulk metal foil chips instead of 11 inside the hermetic can.

The two resistors measured by the VSL showed drift rates of 0.4 and 1.9 ppm/year. And most importantly the drift rate it is not (yet) predictable.

Even if Vishay had some magic to improve the stability that much, why would they share that secret only with a small German toy train manufacturer?

Obviously, it's not the standard VHA518-11, Mr. Everybody would get.

It was a very interesting story, and Dave already made some hints about cooperation between Wekomm and PTB.

Maybe, engiadina may tell some background info on his own, I dare to do so.


Frank
Title: Re: T.C. measurements on precision resistors
Post by: engiadina on June 08, 2015, 07:08:08 pm
Let me give some answers:

Quote
why would they share that secret only with a small German toy train manufacturer

Well, we are not exactly a toy train manufacturer. We develop and produce electronics, and yes, some of our customers are toy train manufacturers. On the other hand, developing electronics for those applications does not qualify or disqualify a company in any way. We have car manufacturers, big stock industries and high technology companies as our customers. Our WEB-Site could be improved, there I have to agree totally. Will do that !

Regarding the relationship towards Vishay. First of all, you should know that VPGSensors is now seperate from Vishay. VPGSensors make their own business and focus on precision resistors and strain gauges. So the company itself is much smaller.
Second, it is a question of demands. We did for some years research with standard VPG Resistors and identified some areas where the performance could be optimized, let us call it this way. Then we addressed VPG with our results and with a bunch of proposals how to implement those optimizations. We were extremely lucky to find some open ears at VPG. They realized that our proposals could improve their products as well so a very open and friendly cooperation developed.

Some ideas are much too expensive to put it in series production but their facility in Israel is able and willing to implement them in the resistors we get. They are surely extremely selected, not by nude virgins of course but rather by men with the grey beard. During our research we found a lot of issues which can happen if someone does not treat the resistors in a specific way. Of course, if you take a VHA518-11 and have it specified at 0.01% (10ppm) then nothing special has to be regarded. But do not forget, we take the drift up to 1ppm and the uncertainty to 0.1ppm.

One key lies in the ominous PMO, mentioned in the datasheet. This "Post Manufacturing Operations" help stabilize the resistors quite a lot if done in the right way. As we found out, some effects of the PMO can be reverted, if the resistor is mounted afterwards. So we developed together with VPG a distributed PMO process, which is partly done in the factory of VPG and partly done at our place. By this way we can guarantee the properties of the resistor much better.

It is not only the resistor itself which matters. It is every piece of the construction which has to be engineered properly. We had our own cable type manufactured for example, which provides lowest thermal voltages and best isolation. The mount of the resistor itself keeps it from every strain of outside impact, etc.

Every step we took was checked and verified with our own 242D Bridge. But this is only an indication if the direction is correct. Often we have put the resistor to a standards lab which is using a MI bridge and some Thomas resistors in an oil bath. They could verify the success or in many cases showed us some weak points.

So developing a resistor with that precision is not a fast task. Certainly not going to Vishay, asking them for a precision resistor and putting a box around. There is much more behind it, using years of experience and is hidden sometimes in very small details. The reason why we are now on the market is that we verified, that we can produce reliably those resistors according to our specifications, which are mostly much better that a competitor product from F...e.

Of course we showed the resistors to the german national standards lab, the "Physikalisch Technische Bundesanstalt" or PTB where they have a Cryo Comparator to measure and define the "technical" Ohm down 1E-10. What I can say for now is that they checked our resistors against that Cryo Comparator and are extremely happy with the results.

So I think we are on a good path for now. We have a second type of resistor in development which has included temperature sensors and features some much more rigid specifications in term of stability. Even if this takes time, we even now are sure that this goal can be achieved. There is still some potential in those Vishay Foils which has to be activated by the right techniques.

But for now our actual resistor series are already better (and cheaper) than most of the so called resistance standards. Only ESI 104 and GenRad 1444A have better specs, but they are way more expensive.

Oh, before I forget. The mentioned VHA518-7 resistor mentioned in the marketing paper from VPG refers to a previous, much earlier development status. Now we use a wekomm resistor, manufactured by Vishay Precision Group. This is probably the best way to describe it.
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on June 08, 2015, 07:47:14 pm
Thanks for the response.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 08, 2015, 09:24:35 pm
Hello,

After the Z201 120R resistors as comparison some PWW 120R resistors from Ultron plus (UP805_120)
Setup has changed:
I now use a PWW resistor with 1K (UP805#3) as reference resistor.

Edit:
And I had to move to the basement with the whole setup since
otherwise it gets too warm for the reference resistor during summer.
(The 27.5 deg setpoint was close to run out of regulation during the last measurements).

UP805_120#3 delivery date 1510
T.C. 3ppm/K from datasheet

17.05.2015: first measurement normal polarity
18.05.2015: 2nd measurement reversed polarity (check for thermal EMF)
19.05.2015: 3rd measurement normal polarity

no significant hysteresis

LMS interpolation of 19.05.2015

A 0 =  1.06481425389144E+0000
A 1 = -1.60431994557356E-0001
A 2 = -2.25165649662830E-0002
A 3 =  1.44281949166390E-0004

So T.C. from LMS at 25 deg C is -0.16 ppm/K 
-> not bad compared to datasheet and even better than most of my Z201_120 samples

The "box" T.C. is around -0.45 ppm/K including noise
and around 0.30 ppm/K from LMS interpolation (without noise)

No significant drift (1.15 ppm = within noise level) during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 08, 2015, 09:30:30 pm
Hello,

Next PWW 120R resistors from Ultron plus (UP805_120)
Reference resistor is again UP805 1K #3

UP805_120#4 delivery date 1510
T.C. 3ppm/K from datasheet

21.05.2015: first measurement normal polarity
22.05.2015: 2nd measurement reversed polarity (check for thermal EMF)
23.05.2015: 3rd measurement normal polarity

no significant hysteresis

LMS interpolation of 23.05.2015

A 0 =  3.80606747298013E-0001
A 1 = -4.11315154136265E-0001
A 2 = -2.05365763017324E-0002
A 3 =  3.33819468006340E-0004

So T.C. from LMS at 25 deg C is -0.41 ppm/K 

The "box" T.C. is around -0.58 ppm/K including noise
and around 0.43 ppm/K from LMS interpolation (without noise)

No significant drift (1.09 ppm = within noise level) during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 08, 2015, 09:41:45 pm
Hello,

Next PWW 120R resistors from Ultron plus (UP805_120)
Reference resistor is again UP805 1K #3

UP805_120#5 delivery date 1510
T.C. 3ppm/K from datasheet

25.05.2015: first measurement normal polarity
26.05.2015: 2nd measurement reversed polarity (check for thermal EMF)
27.05.2015: 3rd measurement normal polarity

no significant hysteresis

LMS interpolation of 27.05.2015

A 0 =  3.84755349289595E-0002
A 1 = -3.24576744031759E-0001
A 2 = -2.12792039565395E-0002
A 3 =  1.94117540677847E-0004

So T.C. from LMS at 25 deg C is -0.32 ppm/K

The "box" T.C. is around -0.53 ppm/K including noise
and around 0.39 ppm/K from LMS interpolation (without noise)

No significant drift (0.03 ppm = within noise level) during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 09, 2015, 06:46:55 pm
Hello,

and again a overview.
This time for the up to now measured 120 Ohms resistors.

So Z201 metal foil against UP805 PWW resistors.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 14, 2015, 09:41:20 pm
Hello,

I have built a new toy to measure the resistors.
Similar to AN96 figure 8.
http://www.linear.com/docs/6637 (http://www.linear.com/docs/6637)

To eliminate the thermocouple voltages I make a AC excitation of the resistor divider.
All is supplied from the voltage reference.
As the H-bridge does only switch between the measurements the switching noise does not affect measurement.

By adding the normal and the inverse difference voltages over the resistors
the thermocouple voltages are canceled out without losing signal to noise ratio.
(this would be the fact if I simply would switch off the reference voltage
each 2nd measurement to read the thermocouple voltage).

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on June 15, 2015, 11:34:19 am
To eliminate the thermocouple voltages I make a AC excitation of the resistor divider.

Good idea. Many metrology grade platinum thermometers use the same technique.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 16, 2015, 05:45:07 am
Many metrology grade platinum thermometers use the same technique.

Hello,

thanks for the hint:
do you have a schematics of (at least the analog frontend including ADC) of such a thermometer?

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 17, 2015, 08:56:26 pm
Hello,

what does the AC-excitation bring in the end?
A measurement of the Z201_1K#2 with UP805_1K#3 as reference gives following results.

30.05.2015 normal multiplexer reverse polarity
31.05.2015 normal multiplexer normal polarity
06.06.2015 AC excitation (both polarities)

If there were significant thermocouple voltages I would see differences in hysteresis openings
(by different temperature direction up/down) between the diagrams.

Each +/- 1uV thermocouple voltage would give +/-0.4ppm difference.
Obviously I have no significant thermocouple voltages since I use the thermal
coupling of the wires with aluminium sheets (isolated with thermal silicone stripes)
according to the suggestion of Emanuel.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 18, 2015, 08:52:55 pm
Hello Emanuel,

you are right the cirquit in the paper seems to be really not the best.
3 different amplifiers used with 3 different drifts and offsets.
And also the LTC2442 has 10 fold temperature drift compared to the LTC2400 that I use.

The CN0155 cirquit is really the best (lowest component count) for ratiometric measurements.

Only the gain drift of the AD7195 is rather high with 1 ppm/K typ.
The LTC2400 is specced with 0.02ppm/K typ.
In my cirquit I also sense the resistors directly (4 wire) so that the RDSon does not influence accuracy.
In this case the RDSon only slightly reduces signal to noise ratio by some ppm.
Unfortunately the LTC2400 has rather large noise. (so I need to average many samples).
I am still looking for a ADC which has low gain drift and low noise.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: branadic on June 19, 2015, 07:58:35 am
Hi Andreas,

what about using something like picostrain instead of an ADC? They are used for weightscales measurements and have high resolution, ENOB up to 18.9bit. I was thinking of PS081. Maybe you want to check this way to measrure your resistors?

branadic
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 19, 2015, 09:37:14 pm
Do you use a simple voltage divider or a wheatstone bridge?

Its a voltage divider with (pseudo differential) 4-wire measurement.
Schematics of the resistors is on the first page of thread:
With the AC-Multiplexer GND and VRef are interchanged after each measurement cycle.

(https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=97776)

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 19, 2015, 09:58:24 pm
what about using something like picostrain instead of an ADC? They are used for weightscales measurements and have high resolution, ENOB up to 18.9bit. I was thinking of PS081. Maybe you want to check this way to measrure your resistors?

Hello Branadic,

perhaps it would be worth a try.

But I do not see how I can do a 4 wire measurement with such a device.
In my case the wiring resistance (and T.C. of wiring) cannot be neglected.

Do you have soldered break out boards for these devices?
(my hand soldering will not create reliable connections for the QFN package)

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on June 20, 2015, 01:03:48 am
If you want to read up an an AC resistance bridge here is a manual of a LR400
http://www.lakeshore.com/ObsoleteAndResearchDocs/LR400.pdf (http://www.lakeshore.com/ObsoleteAndResearchDocs/LR400.pdf)
no schematic is given, but a rather detailed description is there.
I was debating on bidding on one earlier in the year, it was a parts unit that appears to have been in a barn.
I did not bid, it was sold.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 20, 2015, 06:47:12 am
4 channels? Don't you use a LTC2400? (Several ones maybe?)

Of course I use a LTC2400 with a external multiplexer like MAX4052A or MAX4051A
and a buffer (ADA4538-1) between multiplexer and ADC.

With different ADCs you would have different gains and offsets within the signal chain.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: branadic on June 20, 2015, 11:21:06 am
Quote

Hello Branadic,

perhaps it would be worth a try.

But I do not see how I can do a 4 wire measurement with such a device.
In my case the wiring resistance (and T.C. of wiring) cannot be neglected.

Do you have soldered break out boards for these devices?
(my hand soldering will not create reliable connections for the QFN package)

With best regards

Andreas

Hi Andreas,

there is no possibility for 4 wire measurement, because the measurement principle is that one after another resistor is used to form a RC circuit with the same capacitor and discharge time is measured with a Time-to-Digital Converter with ps resolution.
I have the evalkit available at work with serveral measurement modules (high resolution module, standard module, wheatstone module), no seperate breakout board.
If you are able to fabricate a pcb I could solder the QFN package for you.

Regards, branadic
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 20, 2015, 03:35:51 pm
Aaaaaaah... Ok! Now I understand why you said your measurements take long. I'm convinced this is a great idea to measure on the 4 channels to compensate for various wires losses and drifts, but you miss the possibility of true ratiometric operation (With excitation/output voltage being measured synchronously and not sequentially) that can be brought by a single differential input + reference ADC.

Hello,

I call the measurement "pseudo differential" because I do not measure the high and low voltage over the resistor at the same time.
Since I use the reference voltage as supply for the resistors directly. Each measurement is ratiometric.
I do not measure absolute voltages but only "voltage ratios" between the point of interest and the reference voltage.
So even reference voltage changes between the different measurements will not affect the ratio. (only the signal to noise ratio).
The long time is needed to average several measurement cycles. A single measurement has around 10uVpp noise.
So a single measurement would have around 4ppm uncertainity.
By averaging 85 measurement cycles the noise is reduced to about 1uVpp or 0.4ppm for a "1 minute measurement point".

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 20, 2015, 03:39:53 pm

Hi Andreas,

there is no possibility for 4 wire measurement, because the measurement principle is that one after another resistor is used to form a RC circuit with the same capacitor and discharge time is measured with a Time-to-Digital Converter with ps resolution.
I have the evalkit available at work with serveral measurement modules (high resolution module, standard module, wheatstone module), no seperate breakout board.
If you are able to fabricate a pcb I could solder the QFN package for you.

Regards, branadic

Hello branadic,

so this measument system is only usable if all resistors are nearby and
 do not have different (exactly controllable) temperatures (which will need longer wires).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Kleinstein on June 20, 2015, 04:48:19 pm
The LTC2410 might be a slightly better ADC. At such a slow temperature cycle, I don't thing the rate of measurement is already critical - for a faster data rate and thus less final noise, one could in principle use 2 ADCs to measure both voltages (differential) simultaneously with periodic swapping of the ADCs - that is having two max4052.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 20, 2015, 07:29:40 pm
The LTC2410 might be a slightly better ADC. At such a slow temperature cycle, I don't thing the rate of measurement is already critical - for a faster data rate and thus less final noise, one could in principle use 2 ADCs to measure both voltages (differential) simultaneously with periodic swapping of the ADCs - that is having two max4052.

After looking at the data sheet:

On the first view the noise is halved at same speed. And by differential measuring the resistors you can double the measurement rate.

But the disadvantage is that the measurement range is limited to -Vref/2 ... +Vref/2 without over range.
So you have carefully to select the reference resistor so that both voltages over the resistors are below half of the Vref supply. So it will not be possible to use a 12K5 reference resistor to measure a 12K DUT.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 20, 2015, 07:48:31 pm
If you want to read up an an AC resistance bridge here is a manual of a LR400
http://www.lakeshore.com/ObsoleteAndResearchDocs/LR400.pdf (http://www.lakeshore.com/ObsoleteAndResearchDocs/LR400.pdf)
no schematic is given, but a rather detailed description is there.
I was debating on bidding on one earlier in the year, it was a parts unit that appears to have been in a barn.
I did not bid, it was sold.

Hello,

AC excitation: yes. But not sinusoidal. I want to have a ratiometric measurement. So during measurement the voltage has to be stable. Sinusoidal AC would complicate the evaluation with lesser accuracy. -> I need rectangular AC with stable amplitudes.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 20, 2015, 09:40:40 pm
Hello,

Next resistors: will be all 12K5 resistors measured with AC excitation.

First measurement:
Z201_12K5#1 as DUT with UP805_12K5#1 as reference resistor.

Z201_12K5#1 date code B1142-
T.C. +/-0.2ppm/K typ +/-0.6ppm/K  from datasheet

15.06.2015: first measurement AC
16.06.2015: 2nd measurement AC
17.06.2015: 3rd measurement AC

relative large hysteresis about +/-3ppm

LMS interpolation of 17.06.2015

A 0 =  3.97841696233471E+0000
A 1 =  5.80125106318059E-0001
A 2 = -7.92049996619505E-0003
A 3 = -4.17013633414582E-0005

So T.C. from LMS at 25 deg C is +0.58 ppm/K 

The "box" T.C. is around 0.59 ppm/K including noise
and around 0.55 ppm/K from LMS interpolation (without noise)

Visible drift (4.13 ppm) during the 3 days
So either reference resistor or DUT drifts.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 20, 2015, 10:01:19 pm
Hello,

2nd measurement: (the counterpart: both resistors exchanged)
UP805_12K5#1 as DUT with Z201_12K5#1 as reference resistor.

UP805_12K5#1 delivery date 1510
T.C. +/-3 ppm max. from datasheet (typical +/-1 ppm/K)

18.06.2015: first measurement AC
19.06.2015: 2nd measurement AC
20.06.2015: 3rd measurement AC

hysteresis about +/-1.5ppm

LMS interpolation of 20.06.2015

A 0 =  2.89372285322921E+0000
A 1 =  7.31179052260287E-0001
A 2 = -2.89803039080773E-0002
A 3 =  6.27147475010951E-0005

So T.C. from LMS at 25 deg C is +0.73 ppm/K 

The "box" T.C. is around 0.69 ppm/K including noise
and around 0.65 ppm/K from LMS interpolation (without noise)

So again the T.C. for the PWW resistor seems to be specced very conservative.

Visible drift (2.59 ppm) during the 3 days
So either reference resistor or DUT drifts.
I will exchange the reference resistor (Z201) with UP805_12K5#2 to see if the drift gets better.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 21, 2015, 07:18:19 am

Who said that? :)

http://www.linear.com/solutions/1365 (http://www.linear.com/solutions/1365)

Hello,

I do not know what you want to say with that link.
The LTC2402 is more a dual channel LTC2400
(with the same overrange but noise doubled against the later)
and has no real differential measurement like the LTC2410

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Kleinstein on June 26, 2015, 07:43:53 am
This simple circuit will not fully compensate for the wire resistance. So it may work for an RTD but not for very precise measurements.

The way to go would be using a ref. resistor that is smaller (slightly less than 50%) of the DUT and use only the voltage at the reference resistor as ADC reference. So the reference resistor may need to be something like 10 K , 10 K and 470 K in parallel.

The alternative would be having 2 channels and measure the voltage on both resistors one after the other. This could be just one AD and a MUX (e.g. max4052) or 2 ADCs that are exchanges by two multiplexers. For testing drift the limited normal mode rejection should not be a problem, as the average voltage does not change much.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 27, 2015, 07:25:28 pm
I don't understand why it won't work with a 12.5k reference resistor and a 10k resistor under test. A LTC2440 (And others ADCs from the same family) accept 0...5V differential on the reference pins. Only the input is limited to -Vref/2 to +Vref/2.

In this case yes. But the other way round or when you have nearly equally resistors (with one resistor changing by +/-10 ppm) it would not work. (The measured value is limited to 2500mV).

This simple circuit will not fully compensate for the wire resistance. So it may work for an RTD but not for very precise measurements.

Thats what I have overlooked on the first view.

The only annoying wire resistance comes from the wire between the reference resistor and the resistor under test. The others are fully compensated if you connect the IN and Ref pins close enough to the resistors.

I have to keep the reference resistor on constant temperature and the DUT on variable temperature.
With 12K5 resistors the voltage drop between reference and resistor under test is 0.12 mV.
(With 1K0 resistors its even more).
0.12 mV against 2500 mV over the resistor is  around 50 ppm.
Copper wire has around 3850 ppm/K. With a temperature difference of 30 deg C
I would have up to 6ppm error by the wiring T.C.
-> 4 wire measurement is absolutely necessary.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 27, 2015, 08:54:48 pm
Hello,

as announced: UP805#1 as DUT with UP805#2 as reference resistor
I have also increased setpoint temperature of the reference resistor from 27.5 to 30 deg C
So temperature readings are more stable (within 0.01 K instead of 0.05 K) at the reference resistor.

UP805_12K5#1 delivery date 1510
T.C. +/-3 ppm max. from datasheet (typical +/-1 ppm/K)

21.06.2015: first measurement AC
22.06.2015: 2nd measurement AC
23.06.2015: 3rd measurement AC

hysteresis about +/-1.5ppm

LMS interpolation of 23.06.2015

A 0 =  4.03980761381591E+0000
A 1 =  7.41270259831732E-0001
A 2 = -2.85343759498466E-0002
A 3 =  3.63096053524801E-0005

So T.C. from LMS at 25 deg C is +0.74 ppm/K 

The "box" T.C. is around 0.71 ppm/K including noise
and around 0.68 ppm/K from LMS interpolation (without noise)

Again visible drift (3.99 ppm) during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 27, 2015, 09:03:07 pm
Hello,

this time reversed: UP805#2 as DUT with UP805#1 as reference resistor (with 30 deg C setpoint).

UP805_12K5#2 delivery date 1510
T.C. +/-3 ppm max. from datasheet (typical +/-1 ppm/K)

24.06.2015: first measurement AC
25.06.2015: 2nd measurement AC
26.06.2015: 3rd measurement AC

hysteresis about +/-2 ppm

LMS interpolation of 26.06.2015

A 0 =  4.93434506507624E+0000
A 1 =  9.53465670990234E-0001
A 2 = -2.70096487231644E-0002
A 3 =  2.19612533635238E-0004

So T.C. from LMS at 25 deg C is +0.95 ppm/K  (slightly higher than #1)

The "box" T.C. is around 0.94 ppm/K including noise
and around 0.92 ppm/K from LMS interpolation (without noise)

Again visible drift (5.56 ppm) during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on June 27, 2015, 09:15:32 pm

Absolutely not, this is the same problem for strain gauges for example. You can lower the excitation voltage using a little series resistor, see my schematic (This resistor = 1k).

Hello

sorry but in your schematic the 1K resistor also lowers the reference voltage.
So this resistor only reduces self heating of the resistors but will not be helpful since you cannot measure ratios above 0.5 with the LTC2440.
(you would have to put it in series to the reference resistor -> or increase the value of the reference resistor.)

Strain gauges measure only around 20mV of the 5V range so you cannot compare that.
Of course cou could use 3 reference resistors to measure one DUT to create a real bridge.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on July 02, 2015, 06:10:18 pm
Andreas,
I have never, ever measured drift like that on any UltraOhm PWW (measured using a real bridge) over a year, let alone a few days.  That doesn't mean it couldn't happen, but you may still have something a bit off in your setup. 

Respectfully: You seem to be adding a lot of "nonsense digits" in your math (which can cause errors as well)- and remember that the ADC noise on the LTC2404 part you are using is not always perfectly random white noise - as you will soon be learning.  Also- watch your Vref.  It seems the drift data noise has gone up with increasing resistance - highly suspicious.  That means something is probably wrong in measuring technique in your measuring jig itself. 

For example, I don't take out a 25 foot carpenter's tape measure to measure a steel block to 0.01 micron accurately.  Which is kind of what you're doing here.  You want to keep your math resolution only to the accuracy of your measurement methods.  It is very, very hard to build a voltage-mode measuring jig that is stable & accurate to < 10ppm, let alone 1ppm.

Another item: you are not sure how well your reference resistor is temperature stabilized.  You think it is but it is very easy to be fooled by this one.  In fact we aren't absolutely 100% sure your test jig can even measure to some ppm accuracy to begin with.

Suggestion: You probably want to start using a resistance bridge for these measurements - looking for a "null" on a balanced bridge will give you much more stable results over time, and the measuring technique is much more suited to some x ppm measurements.  I know bridges are expensive but I know here in the states they are fairly inexpensive to rent for a few month's time.  Even a 3458a is really not the best tool for resistance measurements.  The best tool is a quality resistance bridge for what you are doing.

This doesn't cost a lot: I would suggest you pause and really look at your measurement technique accuracy and real resolution (accuracy and resolution are NOT the same thing) - maybe measure a 10k/ 10k LTC5400 ratio part for a reality check (these will get you a known ratio in ppm area guaranteed). LTC5400's will have more shot noise than PWW but are a cheap way to do a basic reality check on your system.

Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on July 03, 2015, 09:06:08 pm
Andreas: You might have considered this already but -

Another "Gotcha!" to watch out for when using LTC24xx parts - remember your leakage current on the ADC inputs if you're not using a buffer.  That's going to sneak up on you if you start testing 12.5k resistors instead on 1k resisters.

See LTC2404 datasheet page 12 - even a 5k series impedance with the ADC input circuits  is going to start throwing off your measurements at ppm values with "Very strong temperature dependency".

Not sure if you have a buffer amp between your measuring jig test setup and the ADC, but this is yet another reason to be using a quality resistance bridge to investigate the stability of a resistor.

Just something to check.

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 05, 2015, 04:24:23 pm
Andreas: You might have considered this already but - Another "Gotcha!"
Just something to check.

Hello MisterDiodes:

You are right I started with the 1K resistors without buffer.
(with 500 ohms resulting input impedance this is not a issue with the LTC2400)
But since I had some unclear offset issues due to rectification
of the input noise of the LTC2400 at the protection diodes
of the MAX4052A multiplexer I decided to use a ADA4538-1 buffer
between multiplexer and LTC2400. Of course the ADA4538-1 buffer
has a output filter 825R + some pF (carefully adjusted for linearity)
to prevent rectification of the LTC2400 noise on the output of the ADA4538.

So since 03.02.2015 I am using the ADA buffer.

I have never, ever measured drift like that on any UltraOhm PWW (measured using a real bridge) over a year,

So we have to regard the differences in measurement conditions:
You are using a clamp to contact the resistors.
I am soldering. I try to keep away the soldering heat from the body of the resistor
with 2 crocodile clips, but I cannot guarantee that there is no influence at all.

see also:
https://www.eevblog.com/forum/projects/vishay-bulk-foil-drift-after-soldering/msg445297/#msg445297 (https://www.eevblog.com/forum/projects/vishay-bulk-foil-drift-after-soldering/msg445297/#msg445297)

I do not know wether you have a room with controlled humidity.
I have none. So humidity changes from around 40-50% in winter to 60-70% in summer.
With several % change possible over a few days.

Perhaps with "precision lab conditions" the devices would perform better.
But since I intend to solder the resistors in the final cirquit
(because most users of resistors will not use any clamps in their cirquits) I will not change this.

By the way: where are your measurent results on T.C. and drift?
Up to now I have seen none from your side.

ADC noise on the LTC2404 part you are using is not always perfectly random white noise - as you will soon be learning.  Also- watch your Vref.  It seems the drift data noise has gone up with increasing resistance - highly suspicious.  That means something is probably wrong in measuring technique in your measuring jig itself. 
By the way I am using the LTC2400 (not the LTC2404 which relative similar) together with a MAX4052A/MAX4051A multiplexer.
Noise picture I have added below.
VREF is irrelevant in ratio measurements.
Ratio stability at half input voltage over a >8 hour period is shown to be around 0.1uV (giving 0.04 ppm error at 2500 mV).

Another item: you are not sure how well your reference resistor is temperature stabilized.
Stay on the carpet:
A few days before I wrote that I have improved temperature stability from below 0.05 deg C to below 0.01 deg C.
I monitor each resistor with 2 temperature sensors.
Please read more carefully.

maybe measure a 10k/ 10k LTC5400 ratio part for a reality check (these will get you a known ratio in ppm area guaranteed). LTC5400's will have more shot noise than PWW but are a cheap way to do a basic reality check on your system.

Don´t make yourself ridiculous the LTC5400 has worse specs in datasheet than a pair of Z201
or a typical pair of UP805 PWW resistors from the same batch.
And the HP3458A is still one of the best multimeter.

The only part that I trust on around ppm ratio stability is a LTC1043 capacitive divider.

Sanity check data attached:
Ratio check VREF/VIN at VREF divided by buffered LTC1043 2:1 divider.
1) Raw data of 8 hours = 170000 values (in mV)

2) Averaged values 1 minute as I use them for measuring to reduce noise. And 25 minute averages to show drift.

3) Classified raw data showing nearly perfect gaussian distribution.

4) Allan deviation showing stability over time in ratio mode. (better than 0.1 mV for large averaging periods)

compare that with the absolute voltage mode of a HP3458A:
https://www.eevblog.com/forum/projects/project-kx-diy-calibrator-reference-sourcemeter/msg592144/#msg592144 (https://www.eevblog.com/forum/projects/project-kx-diy-calibrator-reference-sourcemeter/msg592144/#msg592144)
By the way there you can see: my ADCs (including compensated VREF) are even in absolute voltage mode
only around factor 5 less stable than a HP3458A if the 1 minute averaged values are used.
(please note that the measurements of 3458A are in "volt" whereas I measure in "milli volt").


With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 05, 2015, 05:39:04 pm
Andreas,

I'm working on an order so I'll keep it short at the moment.  Normal soldering will have no long term effect on my resistors but overheating can cause damage to the epoxy which should be quite visible, this could cause problems particularly if the lead assembly's anchor into the epoxy bobbin is damaged sufficiently.  Unlike the film/foil resistors which will can have permanent changes in resistance from soldering, mine do not under normal conditions, they will withstand over 150°C long term.  None-the-less, it is always good practice to keep high heat sources from precision components whether or not they can withstand such temperatures.

Humidity is another non-issue, under MIL-STD-202 humidity testing, there is no discernible change in resistance, the fact that film/foil resistors are made from very similar alloys is not the issue at all, it is the form of the resistor element that produces the sensitivity, the flat film/foil surface allows condensation on it and forms a parallel leakage resistance while the polyimide coating used on my wire prevents any condensation to form a parallel leakage path like the film/foil resistors.  In a PWW resistor, the leakage path would have to exist between the lead assemblies, many millimeters in length plus internal barriers, in the film/foil resistor the leakage path is in sub-millimeter lengths making humidity a much more significant problem (except for hermetics of course).  In PWW resistors that are not welded, humidity is an issue in the mechanical joint.

You are both correct and incorrect about the LTC5400 matched resistor chip.  True the absolute TCR is definitely worse than mine but their ratio performance (which is the main specification) is about the same as mine.  In a ratio setup, my resistors can outperform the LTC5400 but not by a lot, we are talking sub-PPM for both technologies here.  I also outperform them in lower noise significantly and on a price/performance comparison, a set of four of my resistors can be a little higher in cost but not a lot, The LTC5400 is a good bargain for certain requirements although the higher performing 'B' version is harder to come by and can be slightly higher in cost than mine, just depends on the required specs.  I believe MisterDiodes was suggesting the use of the LTC5400 as a ratio check since its ratio parameters are well known and guaranteed and the 'A' version is not difficult to obtain.

Since I am not familiar with all of the details of the various measurement setups that MisterDiodes is using, I will give him the floor for any details he wishes to give.

While you have posted photos of your test setups and schematics, it is difficult at best to 'trouble shoot' from a distance, there very well may be some very low level errors creeping into the measurements which are not consistent or vary with DUT value.  MisterDiodes is attempting the difficult task of accounting for possible errors given the measurements of the 12.5Ks (and others) you have posted, I can only say what characteristics my resistors have exhibited over years of production.  I am not saying it is impossible for a resistor to get through QC with some very small 'glitches', after all they are made by human hands but the manufacturing processes have been designed to eliminate resistors that are problematic before they are shipped and this has been exceedingly successful.

Again, there is no intent to take potshots at your work, we are merely suggesting that from the data you have posted, there is something apparently amiss, you are working in a difficult area of measurement.  What may have worked well at one resistance may have a problem at another resistance.  From time to time, I find errors have crept into my measurements and it can be difficult flushing out the culprit(s), it happens even in the best of cal labs.

Best regards,

Edwin
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on July 06, 2015, 12:55:11 am
Andreas,

Edwin is correct, humidity effect should be minimal on PWW resistor.

I do not publish data on measurements with suspect results, and I can't publish data for my employers since I don't own the data that was collected at their expense.  The bottom line is that after hundreds of measurements on hundreds of different resistors, on a real resistance bridge (typically MI 6600 or ESI /SR '242 or similar) - if we saw a drift of over 5ppm in a few days on a PWW that would be considered a very, very suspect measurement, and probably something else wrong beside the resistor under test.  On a quality PWW like Edwin's, we wouldn't see that kind of drift in a year maybe.  Vishay's are a different story, but I'll save that for another day.

The 3458a is an OK instrument for voltage (if it is recent -adjusted- calibration), not the best for direct reading resistance UNLESS you are operating with a good reference standard.  Look at the specs.  Even then, a quality resistance bridge will have much, much better accuracy and repeatability than any 3458a, in any mode.  That's why you want to use a real bridge.

An LTC2400 is not a metrology grade instrument either.  Even with 17 million measurements, you haven't gotten a true average value yet - because the noise data is not white, nor is it random.  You'll find this out in time.

SUGGESTION:  Order an LT5400-1.  That will give you a couple pairs of 10k / 10k to work with with a guaranteed matching ratio of 1ppm TC (1ppm is an outlier, typical we see is maybe 0.2 to 0.3ppm TC).  Set it up in your jig.  Now take some measurements - you will see higher shot noise in diffused resistors but the matching is guaranteed.  On another day, take measurements again and this time adjust your Vref up or down a volt and watch what happens.

I know you think Vref doesn't matter, but you'll see why it does when you investigate closely.  You are comparing a S & H voltage on ADC inputs to the noise in Vref....and you're not even close yet.

Once you use a bridge, and see much more stable data, you'll see why 3458a's aren't used in calibration rooms to check resistance, even for relative values.  In fact at some places an old  '3456 is used for better accuracy (at less resolution ) than of 3458a...

You will certainly find 3458a's out on the semiconductor process lines for sure, but that's just used as a "daily driver" to get a solid 20 to 50 ppm accuracy that is dependable and fast on the testing platforms. For resistor checks in the ppm range, the instrument of choice and is almost universally used is a real, quality bridge.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 12, 2015, 01:03:13 pm
SUGGESTION:  Order an LT5400-1.  That will give you a couple pairs of 10k / 10k to work with with a guaranteed matching ratio of 1ppm TC (1ppm is an outlier, typical we see is maybe 0.2 to 0.3ppm TC). 

You are comparing a S & H voltage on ADC inputs to the noise in Vref....and you're not even close yet.
Hello,

I already have a couple of  LT5400-4 here. But I will not use them as a sanity check. Since they would have to be at least a factor 3 better than my instrument. I will only do a comparison to the DMSZ-Measurements of last year.

You seem really have no clue: A sigma delta converter does not have a S & H.
There is a continuously integration of input and VREF or GND.
The noise of the VREF (LTC6655 on ADC14) is a magnitude order lower than that of the LTC2400.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: bktemp on July 12, 2015, 01:17:06 pm
SUGGESTION:  Order an LT5400-1.  That will give you a couple pairs of 10k / 10k to work with with a guaranteed matching ratio of 1ppm TC (1ppm is an outlier, typical we see is maybe 0.2 to 0.3ppm TC). 

You are comparing a S & H voltage on ADC inputs to the noise in Vref....and you're not even close yet.
You seem really have no clue: A sigma delta converter does not have a S & H.
There is a continuously integration of input and VREF or GND.
Andreas, you are wrong:
See LTC2400 datasheet:
Quote
Driving the Input and Reference
The analog input and reference of the typical delta-sigma
analog-to-digital converter are applied to a switched capacitor
network. This network consists of capacitors
switching between the analog input (VIN), ground (Pin 4)
and the reference (VREF). The result is small current spikes
seen at both VIN and VREF. A simplified input equivalent
circuit is shown in Figure 15.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 12, 2015, 01:18:58 pm
Hello,

UP805_12K5#3 as DUT with UP805_12K5#1 as reference resistor (with 30 deg C setpoint).

UP805_12K5#3 delivery date 1510
T.C. +/-3 ppm max. from datasheet (typical +/-1 ppm/K)

29.06.2015: first measurement AC
30.06.2015: 2nd measurement AC
01.07.2015: 3rd measurement AC

hysteresis about +/-1.5 .. 2 ppm

LMS interpolation of 01.07.2015

A 0 =  1.63629077103983E+0000
A 1 =  8.91505751881022E-0001
A 2 = -3.19358747995385E-0002
A 3 =  2.80114154455740E-0004

So T.C. from LMS at 25 deg C is +0.89 ppm/K (again within the typical spec of 1 ppm/K)

The "box" T.C. is around 0.83 ppm/K including noise
and around 0.80 ppm/K from LMS interpolation (without noise)

no significant drift (1.56 ppm) during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 12, 2015, 01:23:04 pm
Andreas, you are wrong:
See LTC2400 datasheet:
Quote
Driving the Input and Reference
The analog input and reference of the typical delta-sigma
analog-to-digital converter are applied to a switched capacitor
network. This network consists of capacitors
switching between the analog input (VIN), ground (Pin 4)
and the reference (VREF). The result is small current spikes
seen at both VIN and VREF. A simplified input equivalent
circuit is shown in Figure 15.

And what has this to do with a S&H cirquit?
Title: Re: T.C. measurements on precision resistors
Post by: bktemp on July 12, 2015, 01:26:31 pm
Andreas, you are wrong:
See LTC2400 datasheet:
Quote
Driving the Input and Reference
The analog input and reference of the typical delta-sigma
analog-to-digital converter are applied to a switched capacitor
network. This network consists of capacitors
switching between the analog input (VIN), ground (Pin 4)
and the reference (VREF). The result is small current spikes
seen at both VIN and VREF. A simplified input equivalent
circuit is shown in Figure 15.

And what has this to do with a S&H cirquit?
switched capacitor = S&H
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 12, 2015, 01:40:35 pm
This is no sample & hold.
This is a parasytic capacitor resulting from the input switches.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 12, 2015, 01:57:07 pm
Hello,

UP805_12K5#4 as DUT with UP805_12K5#1 as reference resistor (with 30 deg C setpoint).

UP805_12K5#4 delivery date 1510
T.C. +/-3 ppm max. from datasheet (typical +/-1 ppm/K)

02.07.2015: first measurement AC
03.07.2015: 2nd measurement AC
04.07.2015: 3rd measurement AC

hysteresis about +/-1.5 .. 2 ppm

LMS interpolation of 04.07.2015

A 0 = -1.52641348208928E+0000
A 1 =  9.16919772523320E-0001
A 2 = -2.54624953729614E-0002
A 3 =  5.40008996144842E-0005

So T.C. from LMS at 25 deg C is +0.92 ppm/K (again within the typical spec of 1 ppm/K)

The "box" T.C. is around 0.77 ppm/K including noise
and around 0.74 ppm/K from LMS interpolation (without noise)

drift -1.76 ppm during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 12, 2015, 02:29:58 pm
Hello,

UP805_12K5#5 as DUT with UP805_12K5#1 as reference resistor (with 30 deg C setpoint).

UP805_12K5#5 delivery date 1510
T.C. +/-3 ppm max. from datasheet (typical +/-1 ppm/K)

07.07.2015: first measurement AC
08.07.2015: 2nd measurement AC
09.07.2015: 3rd measurement AC

hysteresis about +/-1.5 .. 2 ppm

LMS interpolation of 09.07.2015

A 0 =  3.13651864602711E+0000
A 1 =  9.47633786432885E-0001
A 2 = -2.89619803639599E-0002
A 3 =  3.58284202483480E-0004

So T.C. from LMS at 25 deg C is +0.95 ppm/K (again within the typical spec of 1 ppm/K)

The "box" T.C. is around 0.89 ppm/K including noise
and around 0.87 ppm/K from LMS interpolation (without noise)

drift 3.76 ppm during the 3 days

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 12, 2015, 05:08:13 pm
Andreas, #570

Referring to Fig. 15 in the LTC2400 data sheet:

The Ceq referred to in the figure is not a parasitic capacitor, it is the equivalent of the distributed capacitors in the "internal switched capacitor network".

Quoting from the data sheet:  The analog input and reference of the typical delta-sigma
analog-to-digital converter are applied to a switched capacitor
network. This network consists of capacitors
switching between the analog input (VIN), ground (Pin 4)
and the reference (VREF). The result is small current spikes
seen at both VIN and VREF. A simplified input equivalent
circuit is shown in Figure 15.

The 'schematic' of Fig. 15 is an equivalent, not actual schematic of the switched capacitor network.  Internal capacitors on dies are very small by physical requirements, even a few pF takes up a lot of room on a die, therefore they must be small.  In this case, the sampling capacitor, identified as Ceq, is only 10pF where as on the LTC1043, the sampling capacitors are external (allowing for much lower clocking frequencies) and much larger values, i.e 1.0uF, obviously a 1.0uF capacitor cannot be integrated onto an IC die.

Again, quoting from the data sheet:  The key to understanding the effects of this dynamic input
current is based on a simple first order RC time constant
model. Using the internal oscillator, the LTC2400’s internal
switched capacitor network is clocked at 153,600Hz
corresponding to a 6.5µs sampling period. Fourteen time
constants are required each time a capacitor is switched in
order to achieve 1ppm settling accuracy.  Therefore, the equivalent time constant at VIN and VREF
should be less than 6.5µs/14 = 460ns in order to achieve 1ppm accuracy.

Please note the use of the term 'sampling' in the above paragraph, by definition, a switched capacitor network is a sample and hold circuit without doubt.  Much older integrating ADCs also used switched circuits but the signal was integrated for a complete measurement cycle, not truly sampled even though the input was not being integrated constantly because of the auto-zero cycle, it did not constitute a sample/hold circuit.
Title: Re: T.C. measurements on precision resistors
Post by: MisterDiodes on July 12, 2015, 05:41:34 pm
Andreas,
As others and I have pointed out, please see LTC2400 data sheet pg 22. You have a sample and hold as 5k switch feeding 10pF cap, as well as Vref.  That 10pF is not a "Parasitic" on inputs, that is a purpose-built capacitor on the die and functions a "hold" capacitor during conversion.  It is temperature sensitive as well as the input switch network (Noted in Linear Documentation).

You are assuming VRef doesn't matter, but it does very much matter:  You are comparing a voltage (+ noise) out of your resistor divider at time t1, to a voltage Vref (+ noise) at time t2.  Two different voltages measured at two different times. At ppm levels, this is comparing apples to oranges.   That's why your data will have non-random artifacts generated by switching freq, a 5k and 10pF RC, and the difference of Vin vs VRef.  Those all play together to generate pesky math averaging errors.  Yes the data looks like it has a Gaussian noise curve, but its not truly random.  Watch what happens when you start your Vref at 3V and raise it to 3.5V.  Then start at 4V and lower it to 3.5V.  You will soon learn what happens in your averaging.   

If VRef weren't important, you should be able to use a slow sine wave for VRef.  That's another interesting experiment for another day.

Also notice the INL and data noise values changes with your VRef.

That 5k resistance in 2400 and 10pF hold cap are very temperature sensitive as you'll notice in datasheet.  And humidity sensitive as well, except that isn't explained nearly as well.  Test that yourself and you'll see.  The LTC2400 was designed long ago primarily for weigh scales / temp sensors etc and is very good to say 20~15ppm.  Using it to divine voltages in 1ppm area is theoretically possible, but very hard to achieve in real life as you are finding out.

If you're not using proper guarding, you might want to read up on that also.

SUGGESTION:  Please try a LT5400-1 in ratio mode to verify your test setup.  And then try a 3V Vref vs 5V Vref and watch what happens.  Any LT5400-1 A or B grade will get you in the world of <1ppm TC ratios right off the shelf (guaranteed), and then you can work on your test setup to see what is going wrong.

While it is possible that your PWW resistors are drifting huge amounts, Long Experience tells me you probably have something else wrong.  We use UltraOhm resistors here, and 13k values, and have never seen that kind of drift in a few days.  Maybe over the course of a year maybe 3~5ppm maybe.  Not that much in a few days...never-have we seen that.

In the end, if your intention is to use these resistors with LTZ1000, it hardly will make any difference anyway if the heater ratio resistors have similar TC and in the same direction.  The LTZ does a very good job at attenuating any resistor drift - down to the point you can't measure it with any equipment owned by average hobbyist or even industrial equipment - at least not within the range of drift inherent in the measuring instrument itself.

Again:  In the ppm world of resistance measurement, the method universally used is a resistance bridge for best accuracy - even when comparing relative resistor values.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 13, 2015, 08:58:21 pm
The Ceq referred to in the figure is not a parasitic capacitor, it is the equivalent of the distributed capacitors in the "internal switched capacitor network".

Please note the use of the term 'sampling' in the above paragraph, by definition, a switched capacitor network is a sample and hold circuit without doubt.  Much older integrating ADCs also used switched circuits but the signal was integrated for a complete measurement cycle, not truly sampled even though the input was not being integrated constantly because of the auto-zero cycle, it did not constitute a sample/hold circuit.

Hello,

ok so I obviously missed something from the data sheet.
Sorry for inconvenience.

But what is the consequence:
I have either to use low ohmic resistors (< 1K)
or I have to use a precision buffer which:
- has low output impedance
- is not disturbed by the "switching noise" of the LTC input.

And that´s exactly what I do with my ADA4538 buffer (with filtered output).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 14, 2015, 01:44:13 am
No inconvenience at all, it is just another name for the same function done in a different manner than a 'classic' sample and hold circuit.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on July 14, 2015, 01:54:36 am
Andreas,

I can't find anything on a ADA4538 except that it is an alternator and I don't think that is what you are using.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on July 14, 2015, 04:55:03 am
Andreas,

I can't find anything on a ADA4538 except that it is an alternator and I don't think that is what you are using.

Hello,

of course it is the ADA4638-1
http://www.analog.com/en/products/amplifiers/operational-amplifiers/zero-drift-amplifiers/ada4638-1.html (http://www.analog.com/en/products/amplifiers/operational-amplifiers/zero-drift-amplifiers/ada4638-1.html)

with best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on November 29, 2015, 12:50:42 pm
Maybe the internal layout is similar to the Fluke 742A.

There are high resolution internal photos of the Wekomm resistor in this Daves video, from 5:17.

https://www.youtube.com/watch?v=UYGl0vebWN8 (https://www.youtube.com/watch?v=UYGl0vebWN8)

I don't know how to extract the photos from the video...
Title: Re: T.C. measurements on precision resistors
Post by: plesa on November 29, 2015, 07:21:56 pm
Maybe the internal layout is similar to the Fluke 742A.

There are high resolution internal photos of the Wekomm resistor in this Daves video, from 5:17.

I don't know how to extract the photos from the video...

The enclosure is Rolec ALuPlus 100 http://www.rolec.de/en/aluPLUS/196.100.000/AP100.pdf (http://www.rolec.de/en/aluPLUS/196.100.000/AP100.pdf)
And resistors are VHA518-7   http://www.vishaypg.com/docs/63625/63625.pdf (http://www.vishaypg.com/docs/63625/63625.pdf)
VHA518 datasheet http://www.vishaypg.com/docs/63120/hzseries.pdf (http://www.vishaypg.com/docs/63120/hzseries.pdf)
According to information from Vishay datasheet VHA518-11 with PMO seems to be better option. Maybe Wekomm needs to to make some selection from Vishay batches...
What is interesting, that on first photo are wires and resistor soldered and on third photo wires are crimped to resistor.
Wire seems to be PTFE insulated and stranded silver coated. I have no idea why they used stranded wire instead solid copper. Any ideas?
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on November 29, 2015, 10:10:54 pm


And resistors are VHA518-7   http://www.vishaypg.com/docs/63625/63625.pdf (http://www.vishaypg.com/docs/63625/63625.pdf)
VHA518 datasheet http://www.vishaypg.com/docs/63120/hzseries.pdf (http://www.vishaypg.com/docs/63120/hzseries.pdf)
According to information from Vishay datasheet VHA518-11 with PMO seems to be better option. Maybe Wekomm needs to to make some selection from Vishay batches...
What is interesting, that on first photo are wires and resistor soldered and on third photo wires are crimped to resistor.
Wire seems to be PTFE insulated and stranded silver coated. I have no idea why they used stranded wire instead solid copper. Any ideas?

According to the designer of these standards, the resistive element is a special construction, no VHA518, although it might be in the case of this type.
PTFE insulation is very important for low leakage, as are the special and very expensive banana jacks.

I found my notes of that talk, so silver coated wire should be better than pure copper because of thermal voltages.
Anyhow, as you usually use Offset Compensation, this effect can be mitigated in any case.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: TiN on November 30, 2015, 07:01:43 am
VPG on their case study paper (http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/) state it's VHA518-7, with having Dipl.-Ing Guido Weckwerth, CEO, wekomm engineering GmbH as author :)
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on November 30, 2015, 08:52:58 am
VPG on their case study paper (http://www.vishaypg.com/foil-resistors/case-studies/study/wekomm_1/) state it's VHA518-7, with having Dipl.-Ing Guido Weckwerth, CEO, wekomm engineering GmbH as author :)

If original VHA518 would have these stability figures (in practice, not on paper), then there would not have been any reason for Vishay to get involved with Wekomm (or vice versa).
If you read the study more carefully, these VHA518 were the starting point / basis of the WEKOMM standard only.

Guido Weckwerth really told me on phone, that he made many special improvements together with Vishay on their MBF technology to overcome the known drawbacks.
Our German standards institute - PTB - already acquired several WEKOMM standards, as these are presumably more stable than the usual ones. On a 10k resistor @ 23.0°C, short and mid term stabilities on the order of 1E-8 were demonstrated (versus QHE standard).

Especially 1A shunts (1 Ohm , 0.1Ohm) showed much better retrace behaviour after power burden, on the order of 1ppm, which was not possible with former standards, or required much higher effort.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on November 30, 2015, 10:23:17 am
PTFE insulation is very important for low leakage

Actually no, because the insulation resistance of the binding posts dominates. And there is no real need for the the on board PTFE insulators either. The printed circuit board could have been simply divided into two pieces and mounted directly to the binding posts. Air is an excellent insulator. And that way you also get rid of the wire inductance and a number metal-metal connections (source of thermal EMF).

I found my notes of that talk, so silver coated wire should be better than pure copper because of thermal voltages.

Copper-Copper <0.3 µV/°C, Copper-Silver 0.5 µV/°C. But most important is to minimize the number of joints and make them thermally equal.

I think in this case using exotic materials has been more important than thinking the whole picture and doing the math.

Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on November 30, 2015, 11:05:58 am
Actually no, because the insulation resistance of the binding posts dominates. And there is no real need for the the on board PTFE insulators either. The printed circuit board could have been simply divided into two pieces and mounted directly to the binding posts. Air is an excellent insulator. And that way you also get rid of the wire inductance and a number metal-metal connections (source of thermal EMF).


Copper-Copper <0.3 µV/°C, Copper-Silver 0.5 µV/°C. But most important is to minimize the number of joints and make them thermally equal.

I think in this case using exotic materials has been more important than thinking the whole picture and doing the math.

You are right, but these binding posts are also highly insulating, and extremely expensive, as Guido told me.

If you chose a solution with cables which might touch each other, you must use PTFE, or maybe Kapton which is also highly isolating, but may have better dielectric behaviour.
In my lab, I use relatively cheap PVC mantled cables, which have a resistance of 1E10..1E11 "only". This already creates 1ppm error on 10k, if you drill plus and minus cables to reduce noise pickup.

A yes, one important feature of these cables was the thermal resistance between the resistor element and the binding posts.
That also balances the temperature distribution.
This is important, if you use these devices as working standards, instead of leaving them for days in an oven.

Also don't forget, that Dave has got a prototype, only WEKOMM knows how the devices were built nowadays.


Wire inductance and capacitance may be another aspect, but these are generally so low that they do not play an important role during measurements of the resistor. The time constants involved with these parasitics usually are mostly in the µs range.
AC resistance bridges usually are also capable to measure the ohmic part only.

There are other, more disturbing effects, similar category as L,C parasitic, which I will describe later on.. I'm still collecting information & measurements.

Copper easily oxidizes, giving many 100µV/K thermo offset instead.
The junctions were crimped, not soldered, so this problem applies.
Don't know, how silver coating and its oxides behave, maybe better.

But anyhow, if the whole assembly is well thermally stabilized, these offsets vanish, and will also be eliminated by Offset Compensation techniques.

Frank 
Title: Re: T.C. measurements on precision resistors
Post by: TiN on November 30, 2015, 11:08:09 am
Mounting directly at binding posts would also introduce all mechanical stresses directly to resistor as well. That is what they seem trying to avoid with all that springy crimping and spongy holder.
And they make different value resistors, up to 100Meg in same case, so perhaps PTFE wire is due to that (save cost and unify BOM for all resistors).

Silver surface in air form sulfide layer as well, by turning black, but slower than copper oxidation if I remember correctly.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on November 30, 2015, 11:51:33 am
You are right, but these binding posts are also highly insulating, and extremely expensive, as Guido told me.
Also don't forget, that Dave has got a prototype, only WEKOMM knows how the devices were built nowadays.
I'm still collecting information & measurements.

Sorry to say, but when we talk about this specific product you sound more like a business partner rather than the scientist familiar to all of us.

So far this is just a $50 resistor in a box with a $5000 price tag. No data of any kind, just sellers vague promises of the exceptional performance. Somehow reminds me of the high-end audio business.

BMF resistors have been used as transfer standards in primary metrology for decades. Especially in AC/DC metrology because of their excellent AC behaviour compared with wire wound resistors. The 10^-8 short term stability mentioned is achievable using off the self BMF resistors without any special treatment or black magic.

My point is that at the moment I see nothing to justify the astronomical price tag. The metrology business is very conservative and for a reason. The equipment are expensive not because of the material cost, but because someone has done the hard, expensive and time consuming work for you. The scientific proof of performance is what you pay for, not some technology or fancy look. From that perspective I think that Wekomm with their marketing first approach has started from the wrong end.

Title: Re: T.C. measurements on precision resistors
Post by: Theboel on November 30, 2015, 12:19:34 pm
I really don't know if the resistor cost only 50 USD but I will not judge it before put it side by side to some proven standard,
I understand they have calibrated with with a very good standard but the main concern are reliability how long they will kept it.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on November 30, 2015, 01:45:41 pm
Sorry to say, but when we talk about this specific product you sound more like a business partner rather than the scientist familiar to all of us.

So far this is just a $50 resistor in a box with a $5000 price tag. No data of any kind, just sellers vague promises of the exceptional performance. Somehow reminds me of the high-end audio business.

BMF resistors have been used as transfer standards in primary metrology for decades. Especially in AC/DC metrology because of their excellent AC behaviour compared with wire wound resistors. The 10^-8 short term stability mentioned is achievable using off the self BMF resistors without any special treatment or black magic.

My point is that at the moment I see nothing to justify the astronomical price tag. The metrology business is very conservative and for a reason. The equipment are expensive not because of the material cost, but because someone has done the hard, expensive and time consuming work for you. The scientific proof of performance is what you pay for, not some technology or fancy look. From that perspective I think that Wekomm with their marketing first approach has started from the wrong end.

Your statement is very insulting!
You even impute to me that I have a business interest towards WEKOMM.

I don't tolerate that at all, and expect your apologies!


In the beginning, I have also been very sceptical about their claims.
At this instance though, I'm only reproducing information, I got from WEKOMM directly, and these sounded very reasonable and serious to me, also the document from PTB.
This in fact made me think different.

You obviously make some wrong assumptions about WEKOMM and their resistors, even so imputing dubious business behaviour to them.
Instead of also bringing them in discredit w/o good reasons, I propose that you contact Guido Weckwerth directly for details.


I also think, that your claims are wrong, that it is really that easy to create and even measure 1E-8 stability, short term and especially also mid term, i.e. over several days,  on 'analogue' resistors (in contrast to cryogenic QHE ones).
You also have to accept, that 1E-8.. 1E-9 is on the absolute edge of precision measurements when comparing to a QHE standard, due to thermal voltages, noise , and so on.

Standard MBF resistors have many drawbacks, as we as 'amateurs' already discussed in this thread, like e.g. hysteresis effects, which normally limit their stability figures to 1e-6 .. 1e-7.

Please give a valid reference of the MBF standard resistor you mentioned, which is assumed to have 1E-8 stability, on what scale ever.

Maybe some very good SR104 or similar PWW based standards may have this capability, on the order of < 1e-7 maybe, but these are also extremely costly, although their obvious BOM cost may be a fraction of that only.


Frank
Title: Re: T.C. measurements on precision resistors
Post by: engiadina on November 30, 2015, 02:39:19 pm
@Itz2000:
No claims. Just facts.

I attached some documents from our german National Standard Institute PTB. They qualify our resistors as primary transfer standards.


@Theobel:
Long term drift if measured by comparing a resistor directly against the Cryogenic Hall Standard for some days. That tells us the drift with a resolution of 10^-10
Dr. Schumacher from PTB developed main parts of the Cryogenic Hall Standard and some mathematical methods for deriving some quite reliable long term drift from these measurements. We specify a max yearly drift of 1ppm, but up to now all resistors were way better than that.

We did not yet reach the good behaviour of an old SR104, they are extremely good. But we are working on that.

At the time being our resistor is better than most of those of our competitors.
Oh, and for example try to load a Fluke 421A-1? with 1A (makes 1Watt power) and do the same with our resistor. Ours drifts max 10ppm and comes back to it's original value better than 10^-8. That lead to the construction of a second resistor type, being capable of heavy loads. That one drifts about 0.8ppm at this load.

Guido
Title: Re: T.C. measurements on precision resistors
Post by: engiadina on November 30, 2015, 02:40:04 pm
Sorry, PDF files are big ....
Title: Re: T.C. measurements on precision resistors
Post by: engiadina on November 30, 2015, 02:41:00 pm
Second Cal Certificate ...
Title: Re: T.C. measurements on precision resistors
Post by: engiadina on November 30, 2015, 02:43:17 pm
And this is the "high load" resistor, being tortured in the PTB lab ....
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on November 30, 2015, 03:22:01 pm
I don't tolerate that at all, and expect your apologies!

Sorry, no intention to be impolite. Not at all.

You obviously make some wrong assumptions about WEKOMM and their resistors, even so imputing dubious business behaviour to them. Instead of also bringing them in discredit w/o good reasons, I propose that you contact Guido Weckwerth directly for details.

They voluntarily came out at the very early stage by sending a free sample unit to Dave. Obviously the idea was to get free publicity for their product among the possible customers. You must admit that the claims were quite exceptional, so it was not a surprise that they were not accepted straight away just because the manufacturer says so. From the publicity management point of view it would have been a good idea to be prepared to show at least some proof for their claims.

Please give a valid reference of the MBF standard resistor you mentioned, which is assumed to have 1E-8 stability, on what scale ever.

Almost any type will be work. The main benefit of the BMF resistor is the small size, which allows it to be temperature stabilized extremely accurately. A common solution is an oil filled solid machined aluminium block. The block can be made large compared with the size of the resistor cavity providing excellent temperature uniformity. The small size also makes the insulation and heater arrangements much much easier (compare with the LTZ1000!). But because of the small size you also need to be very carefull with the power coefficient, no free lunch.

In case of the Wekomm resistor the size benefit is lost. It is a large box mostly empty space inside, which brings in the air convection problems. The only way to temperature stabilize is an air bath providing quite limited performance. 10^-8 is not practical.


(https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=142344;image)

Maybe some very good SR104 or similar PWW based standards may have this capability, on the order of < 1e-7 maybe, but these are also extremely costly, although their obvious BOM cost may be a fraction of that only.

A new SR104 costs about the same as the Wekomm. A second-hand unit much less.

Again the limiting factor of the short term stability with the SR104 is the temperature stabilization arrangements. It can and has been modified for oil bath though.

Title: Re: T.C. measurements on precision resistors
Post by: splin on November 30, 2015, 03:29:08 pm
@Itz2000:
No claims. Just facts.

I attached some documents from our german National Standard Institute PTB. They qualify our resistors as primary transfer standards.


Perhaps I'm missing something but as far as I can tell, all those documents tell us is the resistor values as measured by the PTB  at 23C +/- 20mC which is of no interest whatsoever. Might as well have provided a certificate telling us what colour it is to within +/-10^-8 nm. There is nothing about stability with temperature or time which is what is interesting. What is your point?

Quote
@Theobel:
Long term drift if measured by comparing a resistor directly against the Cryogenic Hall Standard for some days. That tells us the drift with a resolution of 10^-10
Dr. Schumacher from PTB developed main parts of the Cryogenic Hall Standard and some mathematical methods for deriving some quite reliable long term drift from these measurements. We specify a max yearly drift of 1ppm, but up to now all resistors were way better than that.

We did not yet reach the good behaviour of an old SR104, they are extremely good. But we are working on that.

I'm sure that your products are very good quality, but where is the evidence that it performs any better than an off the shelf Vishay hermetic foil resistor which is specified to drift less than 2ppm in 6 years, or .3ppm/year for <100 euros?

Quote
At the time being our resistor is better than most of those of our competitors.
Oh, and for example try to load a Fluke 421A-1? with 1A (makes 1Watt power) and do the same with our resistor. Ours drifts max 10ppm and comes back to it's original value better than 10^-8. That lead to the construction of a second resistor type, being capable of heavy loads. That one drifts about 0.8ppm at this load.

Guido

For 3700 euros I reckon I could put a lot of Vishay foil resistors in parallel and get much, much less than 10ppm shift under load.
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on November 30, 2015, 03:31:56 pm
No claims. Just facts. I attached some documents from our german National Standard Institute PTB.

Still no data of the stability, which is probably the most important parameter of a standard resistor.

They qualify our resistors as primary transfer standards.

?

EDIT: splin was faster...
Title: Re: T.C. measurements on precision resistors
Post by: engiadina on November 30, 2015, 03:46:26 pm
Quote
... off the shelf Vishay hermetic foil resistor which is specified to drift less than 2ppm in 6 years, or .3ppm/year for <100 euros?

Have you ever checked that ???

Why then would Fluke sell their 742A resistors if those off the shelf resistors are way better in all respect ?
Title: Re: T.C. measurements on precision resistors
Post by: engiadina on November 30, 2015, 04:02:37 pm
Quote
A new SR104 costs about the same as the Wekomm.

Yes ... and is specified with a drift of 2ppm/year. Have you ever bought one the last five years and verified that drift ?


I respect, that most of you are extremely critical, especially towards a new manufacturer.

Well, in public we are rather new in that business. In fact we are working on that subject for more than three years now. And I know very well that the metrology business is a business of trust. Not claims or buzz marketing.

Do you really think you could start a business in this field just by claiming?
How many resistors were you going to sell before the first customer starts complaining? Those guys from cal labs meet regulary with the PTB guys and talk. I know that two month ago many cal lab engineers met at METAS for a special measuement course. They talked as well.

Actually I believe just ONE bad product sold will kick you out of business in metrology if you are in a newcomer position like us.



Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on November 30, 2015, 04:04:14 pm
Why then would Fluke sell their 421A resistors if those off the shelf resistors are way better in all respect ?

Because they can and customers buy.

The Fluke 742A is far from perfect. Especially the long term stability is poor. But on the other other hand, there is no remarkable hysteresis, the temparature coefficient is very low (matched resistors of opposite tempco) and the power coefficient is acceptable.
Title: Re: T.C. measurements on precision resistors
Post by: ManateeMafia on November 30, 2015, 04:12:37 pm
The IET SR-104 datasheet is here ...

http://www.ietlabs.com/pdf/Manuals/SR102-104_im.pdf (http://www.ietlabs.com/pdf/Manuals/SR102-104_im.pdf)

Drift rate 1ppm first yr with 0.5ppm thereafter.
Title: Re: T.C. measurements on precision resistors
Post by: HighVoltage on November 30, 2015, 04:16:07 pm
And this is the "high load" resistor, being tortured in the PTB lab ....
Do you also have some pictures of the PTB lab equipment, measuring this reference resistor?
What equipment are they using?
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on November 30, 2015, 04:29:43 pm
Quote
A new SR104 costs about the same as the Wekomm.
Yes ... and is specified with a drift of 2ppm/year. Have you ever bought one the last five years and verified that drift ?

The specification is 0.5 ppm/year and it is very conservative. The actual drifts of the four SR104s in the lab I worked for were between 0.06 and 0.08 ppm/year.

Do you really think you could start a business in this field just by claiming?

No, absolutely not. But based on the limited and possibly inaccurate information I have got from this thread, that is what you are doing.
Title: Re: T.C. measurements on precision resistors
Post by: splin on November 30, 2015, 05:07:01 pm
Quote
... off the shelf Vishay hermetic foil resistor which is specified to drift less than 2ppm in 6 years, or .3ppm/year for <100 euros?

Have you ever checked that ???

Ooh now your teasing! No I haven't but it would hardly be unusual to find out that Vishay, once again, are being economical with the truth. I'm sure we're all on the edge of our seats waiting for you to tell us some of your findings if it's possible?

Dr Frank was certainly convinced that his own 5 VHP202Zs were drifting much less than Vishay's long term stability spec and I have no reason not to believe him. https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/360/ (https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/360/)

On the other hand, rather bizarrely, Vishay actually publish this http://www.vishaypg.com/docs/63620/63620.pdf (http://www.vishaypg.com/docs/63620/63620.pdf) showing a 6K45 VHA518-11 drifting 11ppm over 5.5 years, directly contradicting their 2ppm/6 year claim.

Even the 12k9 in that test didn't meet the spec at just over 2ppm in 5.5 years - not far off but the graph shows the drift accelerating over time to .58ppm/year after 2000 days (3.5ppm in 6 years). They even have the cheek to use a truncated version of that 12K9 graph in the H-series datasheet - truncated no doubt because the slope of the drift at 1000 days is only .27ppm/year (1.62ppm in 6 years), or less than half that at 2000 days.

So perhaps Dr Frank got lucky and the 2ppm/6 year claim is actually worthless - if Vishay had any better test results than the above I've no doubt they would have used them in the datasheet rather than one showing that it didn't meet spec!
Title: Re: T.C. measurements on precision resistors
Post by: engiadina on November 30, 2015, 05:32:39 pm
And this is the "high load" resistor, being tortured in the PTB lab ....
Do you also have some pictures of the PTB lab equipment, measuring this reference resistor?
What equipment are they using?

Sure ... basically the PTB are using a Fluke Transconductance Amplifier controlled by a Wavetek calibrator to generate the power. Then they have a range extender from MI (that blue box you can spot) to measure the current via a reference resistor (in the silver box). That is compared to the current through the bigger resistor.
The smaller 1Ohm resistor is used as a short to keep the current flow constant, when the bigger resistor is not loaded. That just helps stabilize the transconductance amplifier.

Title: Re: T.C. measurements on precision resistors
Post by: Theboel on November 30, 2015, 09:30:06 pm
I Thought the 2 ppm / 6 years are the claim for shelf life and for load life are 20 ppm / 2000 hours at rated power ?
http://www.vishaypg.com/docs/63120/hzseries.pdf (http://www.vishaypg.com/docs/63120/hzseries.pdf)  (Pages 3)

so the claim from vishay and the information from DR Frank is Valid for me.

Honestly I don't interested in this area of discussion but I think its far better for WEEKOM to ask PTB to do a test like this to support their claim. 
http://www.vishaypg.com/docs/63620/63620.pdf (http://www.vishaypg.com/docs/63620/63620.pdf)
Title: Re: T.C. measurements on precision resistors
Post by: ltz2000 on December 01, 2015, 09:37:52 pm

Off-topic, but the first Transmille calibrator that I have seen in a primary lab.

The details are not visible but could be
3010 Calibrator and
EA3012 Transconductance Amplifier

(https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=184688;image)
Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on December 01, 2015, 10:20:35 pm
I completely missed the Fluke 8508 in the stack, and immediately looked at the rack. With the 2 displays, they always remend me of a power supply.
Title: Re: T.C. measurements on precision resistors
Post by: HighVoltage on December 02, 2015, 09:54:53 am

Sure ... basically the PTB are using a Fluke Transconductance Amplifier controlled by a Wavetek calibrator to generate the power. Then they have a range extender from MI (that blue box you can spot) to measure the current via a reference resistor (in the silver box). That is compared to the current through the bigger resistor.
The smaller 1Ohm resistor is used as a short to keep the current flow constant, when the bigger resistor is not loaded. That just helps stabilize the transconductance amplifier.
Thanks for the high resolutions pictures, it is really a pleasure, looking at them.
Interesting things to notice:
Fluke 732A, marked "BUND" (German military) but label partially covered.
Nice copper bar for ground connections, sitting on Styrofoam.
May be the 34401A is used to measure the temperature through resistance?
Some of the 4 mm banana plugs / cables do not look like metrology grade.

Great opportunity for you to go to their lab.




 

Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on December 09, 2015, 05:58:58 pm
To bring up an earlier discussion on this thread about ratio measurements using the 3458A (or any other DVM for that matter), this paper from Agilent/Keysight;

http://literature.cdn.keysight.com/litweb/pdf/5992-1058EN.pdf?id=2643219 (http://literature.cdn.keysight.com/litweb/pdf/5992-1058EN.pdf?id=2643219)

lends some information about the accuracy and uncertainties of ratio measurements.  Two very important pieces of information about this type of measurement, the reference must be a known, accurate quantity including its uncertainty of accuracy and secondly ratio measurements consist of two sequential measurements, thus the effects of time and environment enter into the measurements, plus ratio measurements are very sensitive to noise.  While the filter function can reduce this some, it cannot eliminate it, particularly when the noise is varying.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on December 09, 2015, 08:00:24 pm
If you have a known accurate reference (and its uncertainty), within the parameters that apply to your DVM, you can still get a ratio that will be a bit more accurate than your DVM's actual accuracy as set out in the Keysight paper.  Just be very careful of noise pickup, ratio modes are quite sensitive to it and even with a filter on, you can expect an additional drop in actual ratio accuracy because of it.  A ratio is made up of two independent readings made at two different times (sequentially), even if the linearity of the ratio mode were perfect, the readings would still have error attached to them.  According to the engineers at Keysight, that could double the error of the readings.
Title: Re: T.C. measurements on precision resistors
Post by: 3roomlab on December 09, 2015, 10:28:01 pm
If you have a known accurate reference (and its uncertainty), within the parameters that apply to your DVM, you can still get a ratio that will be a bit more accurate than your DVM's actual accuracy as set out in the Keysight paper.  Just be very careful of noise pickup, ratio modes are quite sensitive to it and even with a filter on, you can expect an additional drop in actual ratio accuracy because of it.  A ratio is made up of two independent readings made at two different times (sequentially), even if the linearity of the ratio mode were perfect, the readings would still have error attached to them.  According to the engineers at Keysight, that could double the error of the readings.
sorry i deleted the question
the question was : it seems using ratio method, an uncalibrated DMM can also make useful measurements?
then i thought, no way, impossible (deleted the previous post  :-DD)

now this is getting more interesting for me as i dont intend to calibrate my used DMM (as in send it to the fluke shop)
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on December 10, 2015, 02:34:49 am
In a manner of speaking, it does improve on the uncalibrated accuracy of your DVM, within limits, but it really isn't a substitute for a calibrated instrument.
Title: Re: T.C. measurements on precision resistors
Post by: 3roomlab on December 10, 2015, 02:54:37 am
well i have nothing back in working condition to do any measurement
i still have a few more things to try/mod to reduce noise :D
just curious what kind of noise level are we looking at when we say a good external ref and a DMM to use for this ratio thingy?
Title: Re: T.C. measurements on precision resistors
Post by: acbern on December 11, 2015, 02:13:31 pm
You need to have min 100NPLCs and average the result of a few measurements (or do more NPLCs, because the 3458A avarages lower count NPLC mesurements when you choose high NPLC numbers, details see manual). Typically I see the last digit jump on ma 3458As. No EMI surces arround, thermally stabilized cabling... Of course, any other meter has its own noise specs, so noise realy depends on meter used.
And yes, an uncalibrated 3458A can be used as a precise meter if you have a precise reference as the basis. I am aways doing this even with my calibrated 3458As because it leads to lower inaccuracies (a 732A/B is way more stable than a 3458A).
Now the problem is that few meters have transfer accuracies specified. So you need to use the 24h value then, which immediatelly makes uncertainties worse. Even the 34420 does not have it specified.

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 22, 2016, 10:50:25 pm
Hello,

again some overview sheets of measurements.

For the 12K5 and 12K resistors I recognized that the AC excitation of the AC-Multiplexer
which I introduced on the 1K resistors to avoid thermal EMF voltages gave strange results.
(see also the discussion of Frank on his HP3458A).
https://www.eevblog.com/forum/testgear/precise-offset-compensation-ohm-measurements-and-validation-of-dmms/msg834537/#msg834537 (https://www.eevblog.com/forum/testgear/precise-offset-compensation-ohm-measurements-and-validation-of-dmms/msg834537/#msg834537)

So I did the measurements again with the DC-Multiplexer.
Due to the mounting of the resistors onto a metal sheet the thermal EMFs play no role during measurement.

From the 12K5 resistors all UP805 resistors specced <=3ppm/K were below 1ppm/K.
The Z201 resistors (typical spec 0.2ppm/K) play in the same ball park.

From the 12K0 resistors (2 different batches) the second batch are all below 1ppm/K.
#6 shows a "open hysteresis curve" (does not return to the old value during measurement).
It needs over night to return to the old value.

The 2 resistors from the first batch have higher values of T.C:

With best regards

Andreas




Title: Re: T.C. measurements on precision resistors
Post by: Andreas on January 23, 2016, 06:18:39 pm
Hello,

further measurements: this time on 70K resistors.

Measurements get more difficult.
With my standard setup (1.5m long twin lines) the noise increased from 10-15 uVpp to more than 40uVpp.
(10uVpp is the noise floor of my ADC for a single measurement, with averaging over 1 minute I go below 1uVpp).

So I decided to use shielded cables to go back to around 15 uVpp.

The Z201 70K resistors all measured below 0.5ppm/K
The UP805 70K resistors with shipped 1510 were between 1-2 ppm/K
and the 1532 shipped below 0.8 ppm/K

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 19, 2016, 10:51:29 pm
Hello,

measured the 2nd batch of UP805 1K resistors with datecode 1532.
T.C. is around -1.2 ppm/K.

Overview attached.
next will be this undressed guy.
Lets see if the missing epoxy has a influence on hysteresis.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: Vgkid on February 19, 2016, 11:52:03 pm
The Vishay VAR, this will be interesting.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 26, 2016, 10:35:06 pm
Hello,

and here the Vishay VAR results of my 2 samples.
Datecode B1532

Even from the same batch the stray of VAR is rather large compared to PWW-resistors UP805.

When regarding the drift over temperature there are 2 special things.
The hysteresis curve behaves like a "eight" and is not smaller than on other resistors.
(I hoped that the hysteresis on this resistors is near zero.)

The drift over one day (on the cold side) is around 5 ppm even at the same temperature.
Over night the drift restores almost fully so that the game at the next day is the same.
At sample 2 there is a remaining drift of nearly 2 ppm.

As comparison the hysteresis of a UP805 1K (#15) resistor.
and one of a Z201 1K (#5).

With best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 27, 2016, 10:39:37 am
Hello Ken,

I am shure that it makes a difference between continous ramping and only measuring the 3 interesting temperature points.
The reason is: it takes some time until the epoxy creeps (due to stress) and absorbs / is freeing the humiditiy.

For me the question is: what is more realistic for the application.
For that I would have to even ramp more slowly. (minimum a factor of 5-10).
But then the testing time gets longer than one day. (Which I do avoid).

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: acbern on February 27, 2016, 10:43:17 am
Re. Z-foil humidity drifts, I had initially used standard (non hermetic) as references, and have seen drifts of about 5-10ppm over the year. That was the end of these references... The hermetic ones were up to Vishays spec (2 ppm in 6 years, low load, iirc).
So if you want to use standard z-foils in the voltage divider (multiplier) of an LTZ design to step up to 10V, they are probably not worth it. I have not done any tests with dissicators or putting them into oil, which my be worth a try, so cannot comment.
The other option /besides hermetic resistors/dividers) that may be more stable is dividers in one package. This way the humidity impact affects both resistors on the substrate (made from the same material) the same way and one would expect a more symmetrical behaviour.
Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 08, 2016, 04:19:44 am
I have a lot of foil resistors to test their TC, I'd like the connections to be reliable, repeatable, and no soldering involved.
I have a 16 by 4 scanner that is very good to test 15 resistors at the one time, speeding up the process quickly.
However, how to connect resistors at the chamber has been the trouble for me.

While surfing this thread about 'ugly' wires, 'ugly' heat, I thought there may exist a 2-wire socket for one lead. I then went thru my stuff and found out a kind of socket that allow the leads go through and insert to another socket at the bottom as well. This will make 4 wire connection required.
The bottom socket will soldered onto a PCB and I'll find some way to firmly attach the top one before making fly wires to the side of the PCB. ^-^
Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 08, 2016, 04:26:36 am
The other option /besides hermetic resistors/dividers) that may be more stable is dividers in one package. This way the humidity impact affects both resistors on the substrate (made from the same material) the same way and one would expect a more symmetrical behaviour.
That's the idea. I have ordered some VHD200 from Vishay that both hermetic and in the same package.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 08, 2016, 05:42:13 am
The bottom socket will soldered onto a PCB and I'll find some way to firmly attach the top one before making fly wires to the side of the PCB. ^-^

Hello,

Perhaps you can reverse orientation (upside down) for the upper connector and use a 2nd PCB.

Measurement may work if you have a instrument that compensates for thermocouple voltages.
The 2nd problem is to sense the resistor temperature exactly.

With best regards

Andreas

Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 08, 2016, 06:20:18 am
Hello,

Perhaps you can reverse orientation (upside down) for the upper connector and use a 2nd PCB.

The reversal seems to be a good idea but it doesn't work in reality, they allow one way insertion only.
Title: Re: T.C. measurements on precision resistors
Post by: plesa on March 08, 2016, 09:21:33 pm
While surfing this thread about 'ugly' wires, 'ugly' heat, I thought there may exist a 2-wire socket for one lead. I then went thru my stuff and found out a kind of socket that allow the leads go through and insert to another socket at the bottom as well. This will make 4 wire connection required.
The bottom socket will soldered onto a PCB and I'll find some way to firmly attach the top one before making fly wires to the side of the PCB. ^-^

For similar connection I'm using the thru-hole MicroJack connectors made by Keystone
http://keyelco.com/userAssets/file/M65p102.pdf (http://keyelco.com/userAssets/file/M65p102.pdf)

The plastic in the connectors which you would like to use can be source of error due to the poor insulation resistance and humidity sensitivity.
Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 09, 2016, 12:52:16 am
The other option /besides hermetic resistors/dividers) that may be more stable is dividers in one package. This way the humidity impact affects both resistors on the substrate (made from the same material) the same way and one would expect a more symmetrical behaviour.
That's the idea. I have ordered some VHD200 from Vishay that both hermetic and in the same package.

Hate to ruin your day, but these don't seem to be consistent in production in regards to TCR tracking.  If you order a big batch, you may find that some of them TCR track very well, while the others maybe not so much.  In addition, there is no guarantee that they will age together.

For a much better ratio resistor set that actually will track, contact Edwin Pettis-- he can make you a set that are made from the same resistance wire that comes off of the supply reel one after another.  This will ensure that the two resistors track very well, and they should age together.  I suggest placing several wraps of copper tape around each resistor individually, then wrap those together with several more turns of copper tape.  This will help keep the resistors at the same local temperature.  If one of the resistors ends up being much smaller than the other, Edwin may suggest that the smaller one be placed in a smaller package-- this will help with tracking.  With this method, it is possible to get near zero TCR tracking coefficient.
Although Vishay is notorious deceiving people in the datasheets for tempco, they do have guarantees for some of their hermetic packages for shell-life of 2ppm/6yr, which is THE most important factor for voltage standards that worth trying out, and seems no other WW manufactures say something about the aging even close.

As for the tempco, foil resistors show much better straight-line curve with typical beta 0.01ppm/C2 or better in room temperature, once compensated(which is easy), the curve will be more flat than even the best standard resistors ever made.

I have two Fluke 731A transfer standards where they suppose to use ratio resistor set with the same technique as you described(same supply reel on one mica stripe), but the performance is not up to expectation. Fluke changed to hermetic WW of individuals in all their later versions(731B, 732A, 732B),.

Furthermore, I have a plan to put more than a dozen voltage references in a small traveling case and Vishay VHD resistors are perfect for that because of their small physical dimension.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 13, 2016, 01:54:00 pm
It would be interesting to put your same experiment inside of another box that has a few large bags of desiccant to see if you get the same overnight shift in value.

Hello Ken,

same experiment (12.03.) with a 1 kg (2 lbs) dry pack on ZVAR #2
As comparison the measurement of 25.02. without dry pack.

In the warm phase the hysteresis has changed to smaller values.
But no significant change during cold phase.

Unfortunately due to the large thermal "mass" I do not reach the same minimum temperature.
(only around 15 deg C instead of 13 deg C).

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on March 13, 2016, 08:41:48 pm
Interesting.  I wonder if the greater thermal mass of the desiccant packs was responsible for the difference, and not humidity.  How do we test this?

Hello Ken,

I do not think so. The only difference is on the warm side where the heater is well in regulation so there is no difference in ramp speed. On the cold side the ramp speed is slowed down dramatically. This should give also a dramatically smaller Hysteresis on the cold side which is not the case.

On the other side: for me the pin pointing is not essential. The ZVAR has from my side of view no advantage over a Z201 resistor in precision application (which was my hope). The hysteresis behaviour is even worse. And they are very fragile. The ageing is undefined. -> I will not use them for my purposes.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: branadic on March 13, 2016, 09:00:12 pm
Quote
The ZVAR has from my side of view no advantage over a Z201 resistor in precision application (which was my hope). The hysteresis behaviour is even worse. And they are very fragile. The ageing is undefined.

Well, putting the Vishay VAR into a bath of Galden such as HT110, ZT180 or similar to prevent humidity effects could be a benefit over the possible mechanical stress Z201 could be influenced by.
Some effects that still haven't been investigated yet is barometric pressure. As was found Fluke 732 showed an influence due to barometric pressure. As far as I remember the paper did not identify where this influence came from, resistor or voltage reference.

However, I would expect a bigger influence with molded package (Z201) compared to raw and unpackaged resistor (VAR). So putting the single resistor in a brass case with feedtrough capacitors and filling the case with Galden could make a good improvement here.

The better solution would be to have a vacuum inside the case. This way there would be no pressure conducting material between case and resistor.

EDIT: Don't we have guys over here that are able to put a VAR inside a glas tube "filled" with vaccum? That would be awesome.

It's not that vacuum resistors have not been existing, but none of the current manufactors had the idea to revive this technic.

(http://www.radiomuseum.org/forumdata/users/217/Loewe_Vakuumwiderstand_rm40.jpg)
Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 23, 2016, 03:59:56 pm
The precision film resistors also have their place-- they have relatively low current noise [than carbon or carbon-film] and are good at the higher frequencies-- even better at high frequencies than VPG foils because the resistors are so thin.  But they are at the bottom of the list if you are looking for the best long-term stability [Exception: Ta2N [Tantalum Nitride] film resistors, which can have excellent long term stability, but only "good" TCR unless highly selected]. 
That's very informative. One thing I'd like to ask is base on this:
J. Pickering must be the master mind behind the Wavetek 7000 voltage reference and he hold several US patents for that as well.
There was an article "A solid state DC reference system"(in NCSL Conf. Dig., 1995, p. 369, he was the co-author), which I cannot find but cited by this article:
"Design of a 10mA DC Current Reference Standard"
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1202069 (http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1202069)
(I attached an excerpt)

Obviously this is refereed to the Wavetek 7000, and he meant that they use TaN film resistor for the step up from 7V to 10V.

This also supported by his article "Setting new standards in DC voltage maintenance systems"
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=771967 (http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=771967)
A practical approach to maintaining DC reference standards
www.elcal.ch/files/11749-eng-01-a.pdf (http://www.elcal.ch/files/11749-eng-01-a.pdf)

My question is:
Did 7000 systems really use TaN film resistors?
Are Vishay TDP resistor(that 7000 use)  TaN film resistors?
www.vishay.com/docs/60045/tdp.pdf (http://www.vishay.com/docs/60045/tdp.pdf)
(They say it is Passivated nichrome)
Title: Re: T.C. measurements on precision resistors
Post by: plesa on March 23, 2016, 06:31:15 pm
TaN resistors are for example Vishay NOMCA ,are you sure they are using nichrome ones?
http://www.vishay.com/docs/60117/nomca.pdf (http://www.vishay.com/docs/60117/nomca.pdf)
Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 24, 2016, 02:09:31 am
Thanks @DiligentMinds.com for the reply, seems their claims were not entirely true, who ever 'they' might be.

Also, there is this thing puzzling me:
One Wavetek 7001 specified as 1.8ppm/year for long term drift, consisting of one LTZ1000 plus one set of resistor network(what ever the network may made of).
On that 10mA current source paper, they claimed the annual drift is 1ppm. How a LTZ1000 plus one resistor network of the same type, plus four Z-foils, plus another resistor network for 1mA to 10mA ratio, achieve better drift than a 7001 with fewer variables?
(attached two excerpts from that 10mA article)
Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 24, 2016, 02:57:43 am
TaN resistors are for example Vishay NOMCA ,are you sure they are using nichrome ones?
http://www.vishay.com/docs/60117/nomca.pdf (http://www.vishay.com/docs/60117/nomca.pdf)
Thanks for the info. The only thing I'm sure of is 7001 were using Vishay TDP1603 resistor networks. There are a lot of TDP1603 resistor networks in Fluke 8508A as well, but, Fluke seems using better resistor networks(Vishay 1446) around the reference board:
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/184/ (https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/184/)
(https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/?action=dlattach;attach=51699;image)
 
Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 24, 2016, 03:03:25 am
I'm pretty sure that both the 7001 and the CERN reference use the 'A' version of the LTZ...
Nope.
Title: Re: T.C. measurements on precision resistors
Post by: zlymex on March 24, 2016, 03:12:19 am
Well on the CERN reference, there is a 10V output and a 10mA output.  Obviously, the 10mA is derived from the 10V, and so the 10V will have less than 1ppm/year drift, and the voltage to current converter will add a small amount of drift, totaling up to <= 1ppm/year.

The CERN reference was designed much later than the 7001, so I'm assuming that the designer learned some things along the way to make it better. 
And now is much later than the CERN reference, no one has designed a 10V reference with specification better than 1ppm/year.
Title: Re: T.C. measurements on precision resistors
Post by: quarks on March 24, 2016, 07:45:28 am
according to att. CERN schematic, they use a LTZ1000ACH
Title: Re: T.C. measurements on precision resistors
Post by: quarks on March 24, 2016, 08:08:02 am
here is a Fluke comparison spec information
Title: Re: T.C. measurements on precision resistors
Post by: Kleinstein on March 24, 2016, 09:49:14 am
Interesting to see a rather different circuit to do the temperature regulation at the LTZ1000. 
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on April 21, 2016, 09:23:56 pm
Hello,

finally I have measured the last 120 Ohm PWW (UP805) resistor.
So its time to show the overview sheet:

The measured values of the UP805 are well within spec (3 ppm/K).
So I can now build a 2nd set of LTZ references with selected sets of UP805 resistors.
This will be mostly LTZ1000 (non A) version and perhaps 1-2 LTZ1000A.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Andreas on October 30, 2016, 10:05:45 pm
Hello,

this article

https://www.eevblog.com/forum/metrology/prototyping-my-at-home-resistance-standards/msg1015157/#msg1015157 (https://www.eevblog.com/forum/metrology/prototyping-my-at-home-resistance-standards/msg1015157/#msg1015157)

brought me to the idea: perhaps there is also a even cheaper possibility?
Those Dale RH25 have a aluminium case. But the sealing is simple epoxy with all probabilities for humidity influence.
What about simple cemented wire wound resistors?

The datasheet looks promising: +/- 10 ppm/K for the Vitrohm KH208-8 resistor.
http://www.vitrohm.com/content/files/vitrohm_series_kh_-_201501.pdf (http://www.vitrohm.com/content/files/vitrohm_series_kh_-_201501.pdf)

So I ordered some 1K and 12K resistors (perhaps for a cheap LTZ module?).
I did a quick measurement of T.C. with a peltier.
So perhaps the temperature sensor has a slightly different temperature than the resistor.
But for a first impression this should be sufficient.

First I measured the 12K 10% resistor.
Around 400 ppm change over 23 deg C temperature
-> -17 ppm/K. (ok they say +/- 10 ppm/K in datasheet).

The 1K resistor has 870 ppm over 37 deg C temperature change.
-> -23 ppm/K

So the T.C. is obviously too high for a LTZ reference.
And obviously the resistors also need some thermal treatment before
they could be used as "precision" devices, since the drift is also rather high.

With best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Nuno_pt on October 30, 2016, 10:14:35 pm
Andreas did you find something useful on the VitrOhm website, for resistors to try?
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on October 30, 2016, 10:21:20 pm
Hello,

I only took the resistors which where in stock by the local catalog distributor. (Reichelt).
The datasheet of the resistors is linked in previous post.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Nuno_pt on October 30, 2016, 10:27:27 pm
Hi Andreas, yes I see it, I was asking if you look at VitrOhm website to see if there is some resistors worth to try.

I'm looking and only see CR series can be in 0.5% and till 3k9 ~10ppm/K.

You can look here - http://www.vitrohm.com/ (http://www.vitrohm.com/)

Let me know if you find something that is worth to try, to see if I can get them, the Portuguese factory is about 1km from my house.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on October 31, 2016, 10:25:19 am
Hello Nuno_pt,

I was not shure if it makes sense to use power resistors which are trimmed for cost in a precision application.
In my "lab" (with 18-32 deg C) I will need resistors which have a maximum T.C. of around 1 ppm/K.
So the 0 +/-10 ppm/K spec is already high.
Another thing to consider is the stability spec. 3% for ageing drift is also much more as on precision resistors which are usually specced below 0.05%.
The absolute tolerance for the 12:1 or 13:1 ratio is not so relevant. As long as it stays over temperature and time.

After looking at the catalog: Yes it seems that the CR series would be the best for (semi) precision applications.
It is the only resistor with a welded construction. (The others are crimped).

From size the 3K9 values are rather bulky compared to other precision resistors (8E16, UPW50 or Edwins 805 style).
So the range of resistor values (with CR 254 maximum size) is further limited.

So it was worth a try to test resistors which are a factor of 10-100 cheaper than PWW. But I think it does not make sense for me to do further tests.

with best regards

Andreas


Title: Re: T.C. measurements on precision resistors
Post by: Nuno_pt on October 31, 2016, 10:45:49 am
Andreas, that's what I was thinking also, my workshop also varies alot in temp range, and I can't find anything stable from VitrOhm.
If there was something that was worth to try I could ask a friend of mine that is working there to bring me a couple of them, but with at least the 10ppm/K for the CR series, and like you said with good PWW resistors, it makes no sense to ask him to bring a couple of them.

I can ask him if they have anything better then 10ppm/K.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on October 31, 2016, 03:46:38 pm
Power resistors are not 'precision' by nature or construction.  In the case of these particular power resistors, they made the obvious decision to switch to Evanohm instead of Cupron (most commonly used) for some of the value ranges, they managed to reduce the usually higher TCRs by this simple change of alloy but this does not make a 'precision' out of a power although they have managed to tighten up the tolerance a bit.  Many of the smaller power resistors are/have been welded for many years but the 'weld' isn't the problem here as it is in most precision resistors.  The problem is in the alloy used for the end caps to which the wire is welded, in the case of the CR series (and other similar parts), changing the alloy to Evanohm managed to bring the TCR down somewhat but the end caps have a fairly high TCR, the combination results in a lower TCR but also a nonlinear TCR, notice that the TCR increases with lower values and that they are still using the same old alloy on the lowest values.  In power resistors a crimped joint is not necessarily bad unlike precisions, the conditions present in a 'mechanical joint' do not harm power resistors like they do in true precision resistors.

If you need precision, you have to use precision resistors, they are designed for that use (well at least some are), power resistors are designed just for that...power not precision and unless the circuitry design can tolerate the 'sloppier' performance of powers, stick with precisions to do the job right.  If you are going to insist on PPM performance you aren't going to get it with dirt cheap resistors.
Title: Re: T.C. measurements on precision resistors
Post by: Kleinstein on October 31, 2016, 05:40:24 pm
Some of the digital pot might be an option for a relatively stable and cheap resistive divider. Overall resistance is not that good, but the ration can be reasonable good (e.g. 5 ppm/K for the ratio in AD5260) but the choice of higher voltage types is not that large.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on November 03, 2016, 04:59:16 pm
Digital pots are not intended for 'precision' use, while the main divider ratio has a relatively low TCR around 5 PPM/°C typical at half scale only, the TCR varies as it approaches either extreme.  There are no limits on the TCR specifications, only typical and they will vary with temperature.  You will also find that the noise is going to be significantly higher than precision wire wound resistors or foils.....you are wasting your time with parts that will not perform as you need them to.

As I've said before, if you want precision you are going to have to pay for it, there are no 'cheap' short cuts when it comes to precision, you want PPM performance it is going to cost and there is no cheap way around it unless you accept less performance.
Title: Re: T.C. measurements on precision resistors
Post by: acbern on November 03, 2016, 05:34:35 pm
Edwin, looking at precision resistors (say in the 5ppm/C, 0.01% range) what is your experience re. ball park figures for the annual drifts per year for these technologies:
-metal film
-metal foil
-wirewound
all related to non-hermetic versions. I am referring to drifts not caused by temperature (so humidity, general aging). This is a broad question, sure, and depends on many influences, but you, or someone else, may have some experience. Seems this is nowhere specified (only load life changes, which are very high; only Caddock has some data with its ceramic type resistors)
All I can contribute is metal foil, non-hermetic, in the 5-10ppm range pa (controlled lab environment; set of 5 resistors tested). Wirewound is certainly better, more material; and no clue about standard metal film resistors.
Title: Re: T.C. measurements on precision resistors
Post by: Edwin G. Pettis on November 03, 2016, 06:32:07 pm
Shelf drift, just laying around in a regular room environment, tends to have the lowest drift numbers because it is the easiest environment to live in for a resistor, no power, relatively small temperature range, no on/off cycles.  This figure varies a lot depending on resistor technology and to some degree the value.  For film and foil, they tend to have a bit more effect from humidity than wire wounds due to their construction and very fine circuit paths internally.  Approximate shelf drift for film usually runs in the range of 20-50 PPM/year, sometimes a bit lower depending on the variables.  Foil drift may be as low as 10PPM/year but can also be as high as 35 PPM/year approximately.  Precision wire wounds can vary as well depending on construction, as a general range many are in the 20-35 PPM/year but some can be lower.  As a general rule Ultrohm Plus are spec'd at <10 PPM/year but can achieve much lower drift rates.  I have some 200 ohm resistors that I made for Bob Pease several years ago which I have been keeping an eye on (spares), they have drifted less than 5 PPM over the years (that was about 7 years ago).  This drift can be influenced by value and environment to some small degree, I usually spec a conservative <10 PPM/year in most cases, it is usually less but most of the data I have is just feedback from customers who do track such data.

These are just general figures, the specific resistor manufacturer and type of part/construction determines the actual drift value along with environment, the figures I have given should be taken mainly as guide lines rather than absolutes for any given resistor type and should apply only to shelf 'life', other drift figures are specific to given conditions such as humidity, power cycling and thermal shock for example.

I might add that TCR and tolerance have no effect on drift figures, it is the same resistor no matter what the actual TCR or tolerance is.  TCR is a characteristic of the given alloy and construction; tolerance is merely the limits on the nominal value.
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on November 03, 2016, 06:41:41 pm
For Thin Film resistors, Beyschlag created the best (most serious) specifications.

The drift depends on the resistance value, i.e. the lower, the thicker the Thin Film, the more stable, and mainly on  the outer passivation / lacquer, if it's tight, or not.
But it depends lesser / not at all on the T.C.

As a ballpark, humidity and temperature variations create about 100 ..500 ppm of change.
Usually, Thin Film were not tested and specified for Shelf Drift, as this technology makes no sense for longterm stability.

A good example for such a specification of Beyschlags ultra precision series UXA can be found here:

http://www.vishay.com/docs/28726/uxa0204.pdf (http://www.vishay.com/docs/28726/uxa0204.pdf)


Foil resistors have a drift comparable to PWW, I'd guess, and as specified,  as these are real bulk material type resistors, not comparable to thin or thick film technology.


Frank
Title: Re: T.C. measurements on precision resistors
Post by: acbern on November 04, 2016, 03:01:19 pm
As a ballpark, humidity and temperature variations create about 100 ..500 ppm of change.
Usually, Thin Film were not tested and specified for Shelf Drift, as this technology makes no sense for longterm stability.

Foil resistors have a drift comparable to PWW, I'd guess, and as specified,  as these are real bulk material type resistors, not comparable to thin or thick film technology.

Thanks, I would think though that foil resistors are also some type of metal film, so as long as they are not hermetic I would expect a similar aging drift behaviour as with metal film. And I would expect wirewound to be lower. Also, I would rather expect 10 to 50ppm/year drift for metal film/foil (shelf life/low power; not temperature drift/extremes), otherwise a 0.05% better resistor would not make much sense. Do you have any specific data supporting 500ppm/year?
Title: Re: T.C. measurements on precision resistors
Post by: Dr. Frank on November 04, 2016, 03:29:37 pm
As a ballpark, humidity and temperature variations create about 100 ..500 ppm of change.
Usually, Thin Film were not tested and specified for Shelf Drift, as this technology makes no sense for longterm stability.

Foil resistors have a drift comparable to PWW, I'd guess, and as specified,  as these are real bulk material type resistors, not comparable to thin or thick film technology.

Thanks, I would think though that foil resistors are also some type of metal film, so as long as they are not hermetic I would expect a similar aging drift behaviour as with metal film. And I would expect wirewound to be lower. Also, I would rather expect 10 to 50ppm/year drift for metal film/foil (shelf life/low power; not temperature drift/extremes), otherwise a 0.05% better resistor would not make much sense. Do you have any specific data supporting 500ppm/year?

Usually, metal film is identical to Thin Film technology, with film thickness on the order of several hundred nm.

Metal Foil has several µm thickness.

I made no statement about the annual drift of TF resistors, as I've never seen any shelf stability data.

These 100..500ppm are absolute drifts after humidity and temperature / power dissipation stress.

It's correct, anyhow, that < 0.1% trimming accuracy for TF makes no big sense, as the stress induced drifts cause changes of the same order of magnitude.

Frank
Title: Re: T.C. measurements on precision resistors
Post by: Alex Nikitin on November 04, 2016, 05:40:07 pm
Shelf drift, just laying around in a regular room environment, tends to have the lowest drift numbers because it is the easiest environment to live in for a resistor, no power, relatively small temperature range, no on/off cycles. ... Precision wire wounds can vary as well depending on construction, as a general range many are in the 20-35 PPM/year but some can be lower.  As a general rule Ultrohm Plus are spec'd at <10 PPM/year but can achieve much lower drift rates.  I have some 200 ohm resistors that I made for Bob Pease several years ago which I have been keeping an eye on (spares), they have drifted less than 5 PPM over the years (that was about 7 years ago).  This drift can be influenced by value and environment to some small degree, I usually spec a conservative <10 PPM/year in most cases, it is usually less but most of the data I have is just feedback from customers who do track such data.

As an example, I've bought a set of LT450C PWW 0.002% tolerance 3ppm/C max resistors, made in 1989. All of these drifted up less than 50ppm from the nominal value (so less than 70ppm max in 27 years). Two 10K resistors I've measured came up as +30ppm one (I'm using it now as a reference, after trimming it down with 300M in parallel, so far it is almost scary stable) and +15ppm another.

Cheers

Alex
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 15, 2016, 02:39:05 pm
Hi everybody.

I come out with my solution to do TC of resistors.
My method to TC of resistors basically consists in heating this resistors to 25°C, keep this temperature for some minutes, than heat up to 55°C and calculate TC.
I actually do no do negative temperature sweep and I'm thinking in doing some more complicated thermal profiles ...

The "thermal chamber" is realised by few simple components:

  - 2 power wirewound resistors
  - the resistor/dut
  - 2 pieces of Teflon tubes where the resistor leads are inserted
  - a temperature sensor based on the DS18B20
  - some aluminium foil

I found this setup very convenient, easy to realise and with very good thermal properties.
Despite having a big chunk of metal, aluminium foil still has good thermal conductivity but very low thermal capacity.
This means it react fast to the heat produced by the power resistor, and power is immediately transferred to the DUT without having the thermal inertia of the big chunk of metal.
Thermal transfer is also optimal because the resistor in encapsulated in the albumin foil leaving no space around it, even 70% of the leads are encapsulated into this aluminium foil.

The temperature sensor is inside the aluminium foil too, just near the resistor. This means the sensor is thermally in contact with the resistor ... with some more aluminium foil acting as thermal dumper.
Resistor and temperature sensor can have different temperature, but not too much because everything inside the assembly have equalised temperature.
All this assembly if fully encapsulated inside other aluminium foil that works as temperature equaliser/dumper.

The tricky thing was to develop a software that is able to counterbalance heat loss with heat produced by the power resistor to keep a relatively stable temperature ... but this is another episode.

I attach some photos of the assembly so if someone want's to replicate what I did will be more easier to do it.

PS: I added also a thermal switch that cut off power if temperature goes over 70°C ... don't want to set on fire my lab

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on December 15, 2016, 09:07:20 pm
  - 2 pieces of Teflon tubes where the resistor leads are inserted

Hello,

from where did you get this?

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 15, 2016, 09:32:27 pm
  - 2 pieces of Teflon tubes where the resistor leads are inserted

Hello,

from where did you get this?

With best regards

Andreas
I get from eBay. They are used somewhere in 3D printers. If you search for Teflon tubes you should find it.

Inviato dal mio Nexus 6P utilizzando Tapatalk

Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 17, 2016, 10:43:48 pm
STRANGE RESISTOR

Well on the quest for finding a 1K resistor with +2ppm/°K TC for my LTZ1000 reference I found this one?

This resistor has quite good TC measured at +0.3ppm/°K circa and it has gone a dozen of thermal cycles on it in the past few days. If you look at fist cycle of today it almost recovered all resistance variation when it was at 65°C (30°C to 65°C was only 2ppm apart).

On this UPW50 resistors a recovery after a positive thermal shocks always cause the resistor to drift down if TC is positive or drift up if TC is negative. I think this drift is caused by temperature stress on the resistor ... can be this drift used to evaluate stability of the resistor?

PS: first hour on chart it seems the resistor has negative TC but this is just the meter heating up.
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 20, 2016, 01:14:48 pm
Another UPW50 resistor, this 120 ohm give me more consistent result.
Still you can see a 17 ppm change from fist and last reading at 30°C, but curve is less pronuced.
At least it seems drift/hysteresis is less evident after fist thermal cycle.

This TC includes also TC of the two leads (8 mOhm total) that was calculated to influence total TC with +5% circa.

   step:temp 000:25 - avg.: 120.059312 ohm (1473  samples)
   step:temp 001:30 - avg.: 120.062963 ohm (1414  samples) total +30.40 ppm - +6.08 ppm/K°
   step:temp 002:65 - avg.: 120.083843 ohm (729  samples) total +173.91 ppm - +4.97 ppm/K°
   step:temp 003:30 - avg.: 120.062053 ohm (1437  samples) total -181.45 ppm - +5.18 ppm/K°
   step:temp 004:65 - avg.: 120.083235 ohm (1193  samples) total +176.43 ppm - +5.04 ppm/K°
   step:temp 005:30 - avg.: 120.061325 ohm (1361  samples) total -182.46 ppm - +5.21 ppm/K°
   step:temp 006:65 - avg.: 120.082729 ohm (684  samples) total +178.28 ppm - +5.09 ppm/K°
   step:temp 007:30 - avg.: 120.061072 ohm (1393  samples) total -180.35 ppm - +5.15 ppm/K°
   step:temp 008:65 - avg.: 120.082460 ohm (758  samples) total +178.14 ppm - +5.09 ppm/K°
   step:temp 009:30 - avg.: 120.060839 ohm (1135  samples) total -180.05 ppm - +5.14 ppm/K°

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on December 20, 2016, 08:02:35 pm
When I see this:

how is the drift of your K2001 during measurement?
What happens if you leave the resistor temperature slightly over room temperature?

Just to distinguish if the drift comes from some creeping epoxy or from some warm-up effects on the K2001.
I also think you would get different results if you would cycle temperature around room temperature.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 21, 2016, 05:59:33 pm
Well I tested 10k range and it was stable ...  but I didn't tested yet every range. Resistors generally recover most of the drift after one day resting at ambient temperature.

Warm up effects? I don't think so. During week end I leave meter turned on and the room has AC to keep temperature in 1°C. Resistors measured during week end shows the same behaviour.

I measured one of this Epoxy/not hermetically sealed 10K Vishay metal foil 0.005 percent resistor and I have seen no histeresys there. Only the TC curve was a little bit wonky but it's seems normal with metal foils resistors.

More investigation needed. I will receive some resistors from Edwin in a few days. I'm very curious to see what I will find.

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Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 24, 2016, 12:33:22 pm
Just finished the software for automatic TC ... bad me now I have to port it form K2001 to 3458a.

(http://uploads.tapatalk-cdn.com/20161224/c0ae8aea4df862614e3a54a4f6c051cc.jpg)

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Title: Re: T.C. measurements on precision resistors
Post by: MasterTech on December 24, 2016, 12:57:20 pm
wow, this is a hell of a thread and I just discovered it today, time to start reading
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 25, 2016, 08:41:25 am
An this is another UPW50 measured with a 3458a ... drifting effects after power cycle is still there and are identical to what measured with K2001.
So it's almost impossible that this effect is due to meter heating or drifting.
Title: Re: T.C. measurements on precision resistors
Post by: acbern on December 25, 2016, 04:53:51 pm
It would be interesting if you repeated this with less of a cycle with the exact same device, say only +/-15K. This way one could see if the hysteresis is linear with delta T or if there is some acceleration at higher delta T.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on December 25, 2016, 05:18:19 pm
Hello,

when I compare this with my own UPW50 measurements the behaviour is quite different:
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg462300/#msg462300 (https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg462300/#msg462300)

I usually have a "lag" between temperature change and resistance. This gives the typical hysteresis opening around the middle of the temperature extremes. Over night the resistance returned around to the middle of the opening.

On the other side I never had temperatures above 45 deg C.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 26, 2016, 08:23:24 am
So this is another experiment with this resistor.
This experiment was done 24h after the previous. In this first experiment the resistor has drifted 12ppm down. After 24h of resting it has recovered 11ppm returning back almost at the same point of start.

The second experiment consists in three thermal cycles with low temperature delta, 20°K from t.amb and two with 40°C of delta.

Results are with first three thermal cycle, the ones with limited temperature delta there was only a drift down on 1ppm in first cycles.
With the high temp cycle the drift was perfectly the same as with the previous day.

EDIT:
I just noticed the temperature of the room changed a little bit during the night ... even if the measurements are consistent this test must be repeated.

READINGS:

  step 00 - temp. 25 °C - avg. res. 10005.033472 ohm (0122 samples)
  step 01 - temp. 30 °C - avg. res. 10005.236821 ohm (0098 samples) total +20.32 ppm - +4.06 ppm/K°
  step 02 - temp. 40 °C - avg. res. 10005.597550 ohm (0100 samples) total +36.05 ppm - +3.61 ppm/K°
  step 03 - temp. 30 °C - avg. res. 10005.223934 ohm (0084 samples) total -37.34 ppm - +3.73 ppm/K°
  step 04 - temp. 40 °C - avg. res. 10005.601928 ohm (0065 samples) total +37.78 ppm - +3.78 ppm/K°
  step 05 - temp. 30 °C - avg. res. 10005.225218 ohm (0080 samples) total -37.65 ppm - +3.76 ppm/K°
  step 06 - temp. 40 °C - avg. res. 10005.602340 ohm (0110 samples) total +37.69 ppm - +3.77 ppm/K°
  step 07 - temp. 30 °C - avg. res. 10005.225215 ohm (0036 samples) total -37.69 ppm - +3.77 ppm/K°
  step 08 - temp. 65 °C - avg. res. 10006.284930 ohm (0081 samples) total +105.92 ppm - +3.03 ppm/K°
  step 09 - temp. 30 °C - avg. res. 10005.190542 ohm (0026 samples) total -109.37 ppm - +3.12 ppm/K°
  step 10 - temp. 65 °C - avg. res. 10006.281207 ohm (0036 samples) total +109.01 ppm - +3.11 ppm/K°
  step 11 - temp. 30 °C - avg. res. 10005.181792 ohm (0043 samples) total -109.87 ppm - +3.14 ppm/K°
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 26, 2016, 08:36:39 am
when I compare this with my own UPW50 measurements the behaviour is quite different:
I usually have a "lag" between temperature change and resistance. This gives the typical hysteresis opening around the middle of the temperature extremes. Over night the resistance returned around to the middle of the opening.
On the other side I never had temperatures above 45 deg C.

This was partially confirmed with my last experiment. At 40° drift was of just 1ppm. When temperature was raised at 65° I had the other 4ppm.
Anyway just noticed the temperature of the room changed a little bit during the night ... I will repeat this test one more time.
Title: Re: T.C. measurements on precision resistors
Post by: d-smes on December 26, 2016, 11:44:16 pm
When temperature is stepped to the higher set-point, there is a downward drift in resistance.  This is especially noticeable when stepped to 65 deg C.   What would happen if resistance was allowed to stabilize at 65C before the next step down in temperature?
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 27, 2016, 10:06:54 pm
What would happen if resistance was allowed to stabilise at 65C before the next step down in temperature?
As far as i understood the most of the drift happens in the first 30 minutes at high temperature ... sub sequential thermal cycle cause drift but this became less evident cycle by cycle.

Need to do another experiment with long resting times. Something like this should be appropriate:


Up to now I have measured this things:

 1. when the temperature is stabilised after a temperature change there is always some drift
 2. this "drift" is proportional the the dt
 3. drift is negative when temperature dt is positive (and viceversa)
 4. when temperature goes back to "ambient temperature" this drift is still there and measurable
 5. resistor with lower TC generally drift lesser than the ones with higher TC
 6. most of the drift is recovered (or it drift back) after many hours of resting at "ambient temperature"
 7. this drift effect reduces much with many therm cycles

Points 6 and 7 are known facts and observable in data but I still need to confirm with appropriate experiment that is not a multimeter drift or some changes in the measurement methods I made in last days (changed leads and probes from cheap mini grabber to Pomona golden plated ones).
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on December 27, 2016, 10:21:05 pm
Hello,

I think it is better to solder the connections instead of using grabbers.

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: EmmanuelFaure on December 28, 2016, 07:38:28 am
*ugly grabbers 8)
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 28, 2016, 09:08:24 am
Hello,
I think it is better to solder the connections instead of using grabbers.
with best regards
Andreas

OK I can give it a go ... may I solder 4 copper wires to the leads and than grab in the outside box where temperature is more stable?


Just realised in 3458A you can screw wires directly to the banana plug ... still have to get accustomed to it.
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 29, 2016, 12:46:22 pm
Another day of 3458a logging resistor.

As by Andreas suggestions this time I used copper wire (from ethernet cable) soldered to the end of resistor leads. I also used a clamp to reduce overheat of the resistor.

Measured resistance of the DUT in free air before to start tests. Temperate 21°C +/- 2°C circa (no way to measure it).

   - DUT R with pomona leads 12K00596
   - DUT R directly to binding posts 12K00592 (observed heating/cooling process when in the binding posts)
   - DUT R with soldered ethernet copper leads 12K00597

I also measured the 10K resistor inside my MF calibrator before to start test and at the end of the test to be sure multimeter doesn't drift.

   - start of test: 10.000061 ohm
   - end of test: 10.000059 ohm

At the end of cycle I added also a log period at 25°C (steps 10 and above). Every step has same temperature and lasts 15 minutes, by this I can get resistance value averaged every 15 minutes. If you really force a bit readings you can notice a slight drift down because 25°C is still higher than normal temperature.

I give it also a run 30/65/30 without offset compensation just to check if anything changes but also this tests give me the same results.

Here are all data and charts.

READINGS - 4W - SOLDERED COPPER ETH LEADS - OFFSET COMP ON (DELAY 0.5s):
===================================================================================================

  step 00 - temp. 25 °C - avg. res. 12005.837343 ohm (0026 samples)
  step 01 - temp. 30 °C - avg. res. 12005.656371 ohm (0088 samples) total -15.07 ppm - -3.01 ppm/K°
  step 02 - temp. 40 °C - avg. res. 12005.198936 ohm (0101 samples) total -38.10 ppm - -3.81 ppm/K°
  step 03 - temp. 30 °C - avg. res. 12005.612139 ohm (0054 samples) total +34.42 ppm - -3.44 ppm/K°
  step 04 - temp. 40 °C - avg. res. 12005.195729 ohm (0102 samples) total -34.68 ppm - -3.47 ppm/K°
  step 05 - temp. 30 °C - avg. res. 12005.607506 ohm (0042 samples) total +34.30 ppm - -3.43 ppm/K°
  step 06 - temp. 65 °C - avg. res. 12003.806769 ohm (0076 samples) total -149.99 ppm - -4.29 ppm/K°
  step 07 - temp. 30 °C - avg. res. 12005.559735 ohm (0076 samples) total +146.03 ppm - -4.17 ppm/K°
  step 08 - temp. 65 °C - avg. res. 12003.794508 ohm (0065 samples) total -147.03 ppm - -4.20 ppm/K°
  step 09 - temp. 30 °C - avg. res. 12005.531690 ohm (0026 samples) total +144.72 ppm - -4.13 ppm/K°
  step 10 - temp. 25 °C - avg. res. 12005.740309 ohm (0018 samples) total +17.38 ppm - -3.48 ppm/K°
  step 11 - temp. 25 °C - avg. res. 12005.743185 ohm (0056 samples) total +0.24 ppm - +0.24 ppm/K°
  step 12 - temp. 25 °C - avg. res. 12005.742822 ohm (0051 samples) total -0.03 ppm - -0.03 ppm/K°
  step 13 - temp. 25 °C - avg. res. 12005.742047 ohm (0051 samples) total -0.06 ppm - -0.06 ppm/K°
  step 14 - temp. 25 °C - avg. res. 12005.743028 ohm (0050 samples) total +0.08 ppm - +0.08 ppm/K°
  step 15 - temp. 25 °C - avg. res. 12005.742188 ohm (0051 samples) total -0.07 ppm - -0.07 ppm/K°
  step 16 - temp. 25 °C - avg. res. 12005.742950 ohm (0051 samples) total +0.06 ppm - +0.06 ppm/K°
  step 17 - temp. 25 °C - avg. res. 12005.741966 ohm (0049 samples) total -0.08 ppm - -0.08 ppm/K°
  step 18 - temp. 25 °C - avg. res. 12005.740597 ohm (0050 samples) total -0.11 ppm - -0.11 ppm/K°
  step 19 - temp. 25 °C - avg. res. 12005.740345 ohm (0051 samples) total -0.02 ppm - -0.02 ppm/K°
  step 20 - temp. 25 °C - avg. res. 12005.740984 ohm (0051 samples) total +0.05 ppm - +0.05 ppm/K°
  step 21 - temp. 25 °C - avg. res. 12005.740076 ohm (0050 samples) total -0.08 ppm - -0.08 ppm/K°
  step 22 - temp. 25 °C - avg. res. 12005.740010 ohm (0050 samples) total -0.01 ppm - -0.01 ppm/K°
  step 23 - temp. 25 °C - avg. res. 12005.738172 ohm (0051 samples) total -0.15 ppm - -0.15 ppm/K°
  step 24 - temp. 25 °C - avg. res. 12005.739758 ohm (0050 samples) total +0.13 ppm - +0.13 ppm/K°
  step 25 - temp. 25 °C - avg. res. 12005.739483 ohm (0050 samples) total -0.02 ppm - -0.02 ppm/K°
  step 26 - temp. 25 °C - avg. res. 12005.739246 ohm (0051 samples) total -0.02 ppm - -0.02 ppm/K°
  step 27 - temp. 25 °C - avg. res. 12005.739165 ohm (0051 samples) total -0.01 ppm - -0.01 ppm/K°
  step 28 - temp. 25 °C - avg. res. 12005.738020 ohm (0051 samples) total -0.10 ppm - -0.10 ppm/K°
  step 29 - temp. 25 °C - avg. res. 12005.737698 ohm (0050 samples) total -0.03 ppm - -0.03 ppm/K°
  step 30 - temp. 25 °C - avg. res. 12005.738319 ohm (0051 samples) total +0.05 ppm - +0.05 ppm/K°
  step 31 - temp. 25 °C - avg. res. 12005.738173 ohm (0051 samples) total -0.01 ppm - -0.01 ppm/K°
  step 32 - temp. 25 °C - avg. res. 12005.738530 ohm (0051 samples) total +0.03 ppm - +0.03 ppm/K°
  step 33 - temp. 25 °C - avg. res. 12005.738807 ohm (0050 samples) total +0.02 ppm - +0.02 ppm/K°
  step 34 - temp. 25 °C - avg. res. 12005.738572 ohm (0051 samples) total -0.02 ppm - -0.02 ppm/K°
  step 35 - temp. 25 °C - avg. res. 12005.737621 ohm (0051 samples) total -0.08 ppm - -0.08 ppm/K°
  step 36 - temp. 25 °C - avg. res. 12005.735601 ohm (0051 samples) total -0.17 ppm - -0.17 ppm/K°
  step 37 - temp. 25 °C - avg. res. 12005.715372 ohm (0067 samples) total -1.68 ppm - -1.68 ppm/K°

READINGS - 4W - SOLDERED COPPER ETH LEADS - OFFSET COMP OFF:
===================================================================================================

  step 05 - temp. 30 °C - avg. res. 12005.513999 ohm (0160 samples)
  step 06 - temp. 65 °C - avg. res. 12003.726170 ohm (0119 samples) total -148.92 ppm - -4.25 ppm/K°
  step 07 - temp. 30 °C - avg. res. 12005.495170 ohm (0093 samples) total +147.37 ppm - -4.21 ppm/K°
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on December 29, 2016, 01:44:55 pm
Hello,

so I see no more overshoot with soldered wires.
Is that correct? (althought a different resistor value).

with best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on December 29, 2016, 02:59:21 pm
In reality I cannot say this 100% ... my idea is that using Pomona leads was enough to solve this
error. A couple of resistor when I was using Pomona leads also didn't showed overshoot.

But in last tests I also changed resistor from UPW50 to ULTROHM from Edwin. Maybe it is just
Edwin's ones that does not have this effect ...

I need to test a couple more resistor from Edwin to build first LTZ1000 reference and than I will
make another RUN with the most overshooting resistor in the UPW50 pool. Only this can confirm
if Pomona leads or soldered leads solved this problem or not.

This stuff really takes forever ...
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on February 03, 2017, 02:24:39 pm
Many resistors later ...

Now I can say Edwin resistors doesn't have this overshoot effect. I tested a total of 15 resistors from Edwin and none of them showed this strange overshoot.

As soon as I back to UPW50 I have this overshoot effect again. I'm now sure it's just resistors type and construction causing it. Also a Vishay foil resistor doesn't have this overshoot.

I speak with Edwin and he find strange this end cycle hysteresis. I will make some more tests in next days as  I'd like to find how many days it takes to recovery to initial value and try to isolate what is causing this.

Inviato dal mio Nexus 6P utilizzando Tapatalk

Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 03, 2017, 07:37:25 pm
Hello,

For me its not clear: what effect do you mean with "end cycle hysteresis"?

For some effects it would be also better to have the temperature as X-axis additional to a time diagram.
And a interesting question would be if the behaviour would be the same if you also had a
"lower than room temperature / 30 deg C" point in your measurement.

I personally blame a part of all hysteresis effects to the viscosity of the epoxy used to seal the resistors.
And another part on air humidity effects to the epoxy. (the epoxy is swelling when taking humidity from environment).
Air humidity is very tricky as the time constants for rH changes are usually long.
I have observed 3-7 days time constant for changes around 55% rH on a DIP8-package.
The viscosity seems to be more a effect "over night" or several hours.
On resistor measurements I have often restarting the measurement value somewhere in the "middle" of the hysteresis opening when repeating the measurement next day.

with best regards

Andreas



Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on February 03, 2017, 09:59:15 pm
>> For me its not clear: what effect do you mean with "end cycle hysteresis"?

"end cycle hysteresis" is the resistance change measured after one or more thermal cycles.

If you get the following example you can see that at beginning (step 2) resistance was 20011.70 ohm circa.
After the temperature going up and down two times resistance was 20011.39 ohm circa (step 47).
So the resistor has lost 15ppm after thermal cycling.

Code: [Select]
|  # | °C | avg. resist. - sam. | abs ppm - dt  | ppm/°K - dt  |
| -- | -- | ------------------- | ------------- | ------------ |
| 00 | 25 |  20011.229886 ? 021 |               |              |
| 01 | 25 |  20011.236979 ? 019 | +000.35   +0  |              |
| 02 | 30 |  20011.703395 ? 022 | +023.31   +5  | +04.66    +5 |
| 03 | 30 |  20011.703168 ? 019 | -000.01   +0  | +04.66    +5 |
| 04 | 35 |  20012.125043 ? 021 | +021.08   +5  | +04.22    +5 |
| 05 | 35 |  20012.120768 ? 019 | -000.21   +0  | +04.17    +5 |
| 06 | 50 |  20013.388645 ? 022 | +063.36  +15  | +04.22   +15 |
| 07 | 50 |  20013.374047 ? 019 | -000.73   +0  | +04.18   +15 |
| 08 | 50 |  20013.363585 ? 020 | -000.52   +0  | +04.14   +15 |
| 09 | 30 |  20011.460479 ? 019 | -095.09  -20  | +04.75   -20 |
| 10 | 30 |  20011.463921 ? 019 | +000.17   +0  | +04.75   -20 |
| 11 | 30 |  20011.472355 ? 020 | +000.42   +0  | +04.72   -20 |
| 12 | 60 |  20014.381990 ? 021 | +145.40  +30  | +04.85   +30 |
| 13 | 60 |  20014.352014 ? 021 | -001.50   +0  | +04.80   +30 |
| 14 | 60 |  20014.339421 ? 019 | -000.63   +0  | +04.78   +30 |
| 15 | 30 |  20011.333014 ? 021 | -150.21  -30  | +05.01   -30 |
| 16 | 30 |  20011.346870 ? 020 | +000.69   +0  | +04.98   -30 |
| 17 | 30 |  20011.356837 ? 019 | +000.50   +0  | +04.97   -30 |
| 18 | 30 |  20011.361145 ? 020 | +000.22   +0  | +04.96   -30 |
| 19 | 30 |  20011.368689 ? 019 | +000.38   +0  | +04.95   -30 |
| 20 | 30 |  20011.375579 ? 019 | +000.34   +0  | +04.94   -30 |
| 21 | 30 |  20011.379270 ? 020 | +000.18   +0  | +04.93   -30 |
| 22 | 30 |  20011.384615 ? 020 | +000.27   +0  | +04.92   -30 |
| 23 | 30 |  20011.390463 ? 019 | +000.29   +0  | +04.91   -30 |
| 24 | 30 |  20011.391643 ? 021 | +000.06   +0  | +04.91   -30 |
| 25 | 30 |  20011.399790 ? 020 | +000.41   +0  | +04.90   -30 |
| 26 | 30 |  20011.401468 ? 019 | +000.08   +0  | +04.89   -30 |
| 27 | 30 |  20011.403340 ? 020 | +000.09   +0  | +04.89   -30 |
| 28 | 30 |  20011.406747 ? 019 | +000.17   +0  | +04.88   -30 |
| 29 | 30 |  20011.413715 ? 020 | +000.35   +0  | +04.87   -30 |
| 30 | 30 |  20011.406163 ? 019 | -000.38   +0  | +04.89   -30 |
| 31 | 30 |  20011.410265 ? 020 | +000.20   +0  | +04.88   -30 |
| 32 | 30 |  20011.412707 ? 014 | +000.12   +0  | +04.87   -30 |
| 33 | 30 |  20011.411674 ? 023 | -000.05   +0  | +04.88   -30 |
| 34 | 30 |  20011.412410 ? 021 | +000.04   +0  | +04.87   -30 |
| 35 | 30 |  20011.415800 ? 019 | +000.17   +0  | +04.87   -30 |
| 36 | 30 |  20011.417324 ? 021 | +000.08   +0  | +04.87   -30 |
| 37 | 30 |  20011.418275 ? 020 | +000.05   +0  | +04.87   -30 |
| 38 | 30 |  20011.417811 ? 019 | -000.02   +0  | +04.87   -30 |
| 39 | 30 |  20011.417795 ? 020 | -000.00   +0  | +04.87   -30 |
| 40 | 30 |  20011.415679 ? 019 | -000.11   +0  | +04.87   -30 |
| 41 | 30 |  20011.411685 ? 020 | -000.20   +0  | +04.88   -30 |
| 42 | 30 |  20011.412853 ? 019 | +000.06   +0  | +04.87   -30 |
| 43 | 30 |  20011.406068 ? 019 | -000.34   +0  | +04.89   -30 |
| 44 | 30 |  20011.404360 ? 020 | -000.09   +0  | +04.89   -30 |
| 45 | 30 |  20011.398074 ? 019 | -000.31   +0  | +04.90   -30 |
| 46 | 30 |  20011.392920 ? 020 | -000.26   +0  | +04.91   -30 |
| 47 | 30 |  20011.392633 ? 003 | -000.01   +0  | +04.91   -30 |


This is result from my python script.
The TCR tracing consists of more than 50 steps/measurements points (this run was interrupted at the 47th step).
Every step last 10 minutes circa.
Resistance measurement are made every 30 seconds with K2001, 10NPLC with offset compensation, 4W connection with Pomona leads. So there is 20 measurements circa per each step.
(I back to K2001 as my 3458a is occupied to log the LTZ1000 reference).

1st column is the step number
2nd column is temperature set point
3rd is averaged resistance of n samples
4th is ppm difference and temperature difference referred to previous step
5th is ppm/°K and temperature difference referred to the previous step where temperature set point was different. In other words ppm/°K is always calculated respect to the previous step with lower or higher temperature.
Title: Re: T.C. measurements on precision resistors
Post by: Alex Nikitin on February 04, 2017, 12:24:30 am
I suppose this is the right thread to post my results. Here we have the Ohmite RX-1M (glass encapsulated hermetic) 10G resistor, mounted in an alloy Hammond box with BNC connectors and measured by Keithley 617 electrometer with temperature varied from 21.5C down to 13C. I was expecting much larger tempco, here it is not easily measurable, with deviation around 150ppm p-p and the measurement noise around 50ppm p-p, over 8.5 degrees temperature drop in about an hour time, making the tempco less than 20ppm/C in that range of temperatures. I will leave the resistor warming up overnight and see how it behaves with temperatures higher than the room, up to 45-50C. Vertical scale for the resistance is 0.005% (50ppm)/div, the temperature is measured by a thermocouple fixed to the resistor's enclosure.

Cheers

Alex

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_10G_vT_001.gif)
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 04, 2017, 12:09:00 pm
Hello Alex,

are you shure that you are not below the dew point
at 16 deg C and below when doing the measurement.
Or why is the resistance first rising and then going down?

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Alex Nikitin on February 04, 2017, 12:15:06 pm
Hello Alex,

are you shure that you are not below the dew point
at 16 deg C and below when doing the measurement.
Or why is the resistance first rising and then going down?

With best regards

Andreas

Essentially this change is below my ability to measure it accurately. I don't think this drop is due to a leakage. Here is the full graph going back to +23C as of this morning. This rise could be just a transient effect due to a reasonably fast cooling.

Cheers

Alex

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_10G_vT_002.gif)
Title: Re: T.C. measurements on precision resistors
Post by: CalMachine on February 04, 2017, 02:23:06 pm
I'm looking to get a small benchtop incubator/oven to start doing some TC measurements!  Anyone know of any, fairly decent, cheap units available on the market?  :popcorn:
Title: Re: T.C. measurements on precision resistors
Post by: dr.diesel on February 04, 2017, 02:43:47 pm
I'm looking to get a small benchtop incubator/oven to start doing some TC measurements!  Anyone know of any, fairly decent, cheap units available on the market?  :popcorn:

Not too hard to custom build.  If you end up buying something make sure and get a controller with some kind of IO, you can you script/program/automate the testing.
Title: Re: T.C. measurements on precision resistors
Post by: Andreas on February 04, 2017, 04:31:54 pm
>> For me its not clear: what effect do you mean with "end cycle hysteresis"?

"end cycle hysteresis" is the resistance change measured after one or more thermal cycles.

If you get the following example you can see that at beginning (step 2) resistance was 20011.70 ohm circa.
After the temperature going up and down two times resistance was 20011.39 ohm circa (step 47).
So the resistor has lost 15ppm after thermal cycling.


Ok, thanks for the explanation.

What I see in the diagram is
a) one part with immediate response to temperature
b) a effect in opposite direction with a much longer time constant.

the opposite direction could also be a smaller CTE of
one of the components that build up the resistor.

As you cannot change the readily built resistor
I would ask myself if I have actually such temperature risings in my end application.

In my lab I have measured temperature risings of up to around 2 deg C/hour.

With best regards

Andreas
Title: Re: T.C. measurements on precision resistors
Post by: Alex Nikitin on February 04, 2017, 11:21:04 pm
Hmm, it looks like either I have an exceptional example of 10G RX-1M, or these resistors are really that good. I've ramped up the temperature in the chamber up to 49C (measured on the resistor enclosure) and again, the resistance change is in order of 100ppm for over 25 degrees... . I have another sample of 10G resistor at work and also a couple of 100G of the same type - will measure these eventually. Right now I am measuring the same 10G cooling down, but with a different method - using the I/V mode on the Keithley 617. The difference at 20V supplied is very little comparing with the normal (constant current) method but the cable leakages are less of a problem.

Cheers

Alex

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_10G_vT_003.gif)
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on February 05, 2017, 12:06:58 am
Andreas,

I measured this "overshoot" also with 5° K difference but it was really marginal.

I think with normal natural temperature cycling this overshoot has almost not effect on our LTZ circuit and because of the more slow temperature changes it almost do not reveal.

What I think is that this will also have some impact on long time stability of the resistance. But this will takes many years to prove it.

This effect, the hysteresis due to thermal cycling and maybe drift due to humidity  suggest me that if you want to reduce drift you must keep resistors always at the same temperature. The hole circuit must be in a hoven.

Now I'm checking what is effect of humidity on the resistance of a UPW50. I'm drying a resistors with some "medical" grade desecant. Actually registered -20ppm in two days.

Tomorrow will be third day and I will fire up my MF calibrator to countercheck the resistance readings, but I'm sure the drift is there.

My plan is to leave the resistor to dry for one week, check the "drift" and than countercheck if it creeps back as it will reaccumulate the humidity and in how much days.
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on February 05, 2017, 02:47:19 pm
@Alex Nikitin

That's very odd to me, never expected this to be the case.
From charts it seems you are measuring stability of the Keithley 617 and not of resistor :-)
Title: Re: T.C. measurements on precision resistors
Post by: Alex Nikitin on February 05, 2017, 03:58:28 pm
@Alex Nikitin

That's very odd to me, never expected this to be the case.
From charts it seems you are measuring stability of the Keithley 617 and not of resistor :-)

Yes, the accuracy and stability of the electrometer are limiting factors. The spec on the K617 at 20G range is 1.5%  ::) . Nevertheless, the temperature in my home lab is reasonably stable, so it is mostly the noise and the resolution of the Keithley. Here are two more graphs - a full 22C-10C-50C-25C cycle today and the R(T) curve from that cycle that is reasonably fits a parabola after all. There are obviously some transient effects (the resistor is coupled to the enclosure only by its leads connected to two BNC sockets).

Cheers

Alex

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_10G_vT_004.gif)

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_10G_vT_005.gif)

Title: Re: T.C. measurements on precision resistors
Post by: Alex Nikitin on February 05, 2017, 09:02:51 pm
And here are the graphs for the 100G resistor of the same Ohmite RX-1M series. A very different behaviour, essentially a linear -230ppm/C tempco. Vertical scale 0.1%/div . I may re-measure this resistor in the V/I mode of the Keithley 617 just to confirm this data and check if there is any obvious leakage influence.

Cheers

Alex

P.S. I am considering now if I should build a 100G reference resistor from ten 10G ones. The price however, would be very high - £600 (£500 + VAT) for the resistors only!

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_100G_vT_001.gif)

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_100G_vT_002.gif)
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on February 05, 2017, 09:55:41 pm
Well this last chart from the 100G has much more sense. :-)
Title: Re: T.C. measurements on precision resistors
Post by: Alex Nikitin on February 06, 2017, 11:35:09 pm
The results for the second sample of Ohmite RX-1M 10G resistor. A different tempco shape - almost linear - but still very low, about 12ppm/C.

Cheers

Alex

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_10G_n2_vT_001.gif)

(http://www.ant-audio.co.uk/Test_Gear/K617/RX1M_10G_n2_vT_002.gif)
Title: Re: T.C. measurements on precision resistors
Post by: mimmus78 on February 09, 2017, 04:36:45 pm
I run another experiment to check "overshoot effect" with 40°K temperature difference and a 10K UPW50 resistor.
Experiment than proceed with small steps of 5°K (up) and 2°K step (down).

I still have doubts that this overshoot are coming from my setup so I set a thermistor in contact with one of the two resistor leads near the case of the resistor. I wanted to check that lead temperature is similar to the one of the DUT.
The thermistor is very small NTC that I got from a body thermometer. To keep minimal thermal mass, resistor was soldered with very thin wrap wire. Resistance of this thermistor was measured by the K2001.

I scaled thermistor resistance reading with following equation Y=75-(R/1000) to have it charted on the graph on the same scale of the temperature.

As usual the green line is a digital temperature sensor that is place by side of the DUT resistor.
Yellow dotted line is the scaled NTC resistance.
Blue is the DUT resistance.
The other two dotted lines are 3458a temperature and ambient temperature.
Resistance is measured by the 3458a with 4W, 30 NPLC, offset compensation on and delay of 0.1 s.

RESULTS:

- "overshoot effect" with dt of 40°K is near 6ppm
- "overshoot effect" with dt of 5°K is 1ppm circa and visible in every step even at 2°K
- "overshoot effect" is not visible on the resistance of the NTC
- resistance of the NTC really track very well temperature measured by the digital temperature sensor

I attach three charts from this experiment.

 - first image is the overall experiment
 - second one is the zoomed in part when the DUT reach the fist 40°K temperature increase
 - third is resistance vs temperature
Title: Re: