Low ohm precision resistor standard and testing

[HighVoltage]

For my work, I need to measure lots of low ohm resistors in the range of 0.01 to 0.1 ohm
And for reference I found a couple really nice calibration resistors on ebay for a low price.

This one is a Burster, Made in Germany
Type 1240-0,01
- 10 mOhm
- Tol. : 0.03 %
- Imax: 14 A

I hooked it up to my Keithley 2460 SMU and send 1.00000 A source current through the resistor
The resistance shows at 0.01000 Ohm without any movements of the last digit

So, either I was lucky to have a "perfectly" trimmed resistor or the calibration between the resistor and the instrument are just on spot.

Anyone else here on the forum that has to use low resistance standards?
What are you using and how stable are those resistors?

[plesa]

I'm not using nothing so fancy. Only Vishay VCS332. Values from 1R-R1 mounted on piece of aluminium.With measuring with continuous current I see the temperature drift, one of good features of SMU's - easily you can setup measurement with pulses to eliminate self-heating.
What did you paid for such nice resistance standards?
Hookup the 3458A and measure voltage with it to see how stable it is.

[lukier]

I have Leeds Northrup 4385 box, 0.02%, multiple ranges for various currents.

Like this one: http://www.ebay.de/itm/171945773961

When I get home I can measure the resistances, but none of my meters (K2001,K2015,34401A,3457A) are calibrated and I don't have an SMU to source the current so precisely :/ Maybe a mini-teardown instead?

I bought it, as it was cheap on eBay to measure higher currents than my DMMs allow (2-3A max) and more precisely. For example to calibrate my 6632B in the manual they require Guildline 9230/15 shunt and these Guildline are difficult to get and expensive and I think my L&N is almost as good.

[kxenos]

I think you need one more digit of resolution to judge if it is on spec, no? Is there a typo?

edit: awesome meter and cables there! 

[gamalot]

You may need a low resistance meter. 

[VintageNut]

Instead of meauring ohms, measure voltage on the lowest voltage measure range. You will see 10.0000mV; 5 1/2 digits for 10mV. The ohms function lacks the resolution that you can achieve with voltage measure for low ohms.

The 2450 has the same behavior.

[HighVoltage]

I'm not using nothing so fancy. Only Vishay VCS332. Values from 1R-R1 mounted on piece of aluminium.With measuring with continuous current I see the temperature drift, one of good features of SMU's - easily you can setup measurement with pulses to eliminate self-heating.
What did you paid for such nice resistance standards?
Hookup the 3458A and measure voltage with it to see how stable it is.

It was Euro 125 for this one, a while back on ebay Germany
This was also a test for the 2460 and the resistor of how they would behave under constant 1A current
I am surprised how stable this combination is. 1A seems to be idle for the resistor and SMU

Yes, I will hook up the 3458A soon to this setup. 

[HighVoltage]

I think you need one more digit of resolution to judge if it is on spec, no? Is there a typo?

edit: awesome meter and cables there! 

Actually the meter was set to 5.5 digits
Here are some pictures for 6.5 digits and 10 NPLC setting

[HighVoltage]

You may need a low resistance meter. 

That Hioki meter looks really nice too.
What current is it pushing through the DUT at 1 mOhm?

[HighVoltage]

Maybe a mini-teardown instead?

Yes, please.
Show us some pictures from the inside.

[HighVoltage]

Instead of meauring ohms, measure voltage on the lowest voltage measure range. You will see 10.0000mV; 5 1/2 digits for 10mV. The ohms function lacks the resolution that you can achieve with voltage measure for low ohms.

The 2450 has the same behavior.

Yes, I am getting 1 more digit in the 200mV rage, reading 10.0001 mV for this resistor
At 10 NPLC, only the last digit is changing.

[kxenos]

Cool! Right down in the noise! 

[VintageNut]

If you like low ohms, I have one of these in my office. It is about 4 nano-ohms. It is a challenge to measure.

[gamalot]

You may need a low resistance meter. 

That Hioki meter looks really nice too.
What current is it pushing through the DUT at 1 mOhm?

It is 1A at 10 mOhm range.

[joeqsmith]

For my work, I need to measure lots of low ohm resistors in the range of 0.01 to 0.1 ohm
And for reference I found a couple really nice calibration resistors on ebay for a low price.
...
Anyone else here on the forum that has to use low resistance standards?
What are you using and how stable are those resistors?

Equipment is cal'ed with a report.   I use these as a sanity check:
http://www.ietlabs.com/decaderes/resistance-standard.html

[Vgkid]

Which one Joe?

[lukier]

Yes, please.
Show us some pictures from the inside.

OK, quick teardown then.



It's a very simple device. 4 binding posts for force and sense, L&N made rotary switch and a bunch of wires/bars inside.



I got my unit from eBay very cheap ($45 total), so I'm not surprised it has a long history. High ohm resistors look a bit burned, probably someone tried to push too much current. But nothing is broken and the "event" happened probably long time ago so these resistors aged again.





I've measured them with my HP 3457A (NPLC=100, NRDG=10, OCOMP, AZERO) and Keithley 2001 (NPLC=10, OCOMP, AZERO, LINESYNC etc), nothing calibrated at a lab since I got these meters from eBay:

Range [Ohm]	HP 3457A Mean [mOhm]	HP 3457A StdDev [uOhm]	K2001 [Ohm]
0.01	10.128	43.57	0.010000
0.02	20.103	45.47	0.020008
0.05	50.138	27.66	0.050006
0.10	100.127	44.48	0.100007
0.20	200.148	55.73	0.200056
0.50	500.116	50.82	0.50067
1.00	1000.901	42.52	1.000823
2.00	2000.852	53.89	2.000906

I've also tested this 0.001 Ohm, 1%, 25ppm/C shunt:
http://uk.farnell.com/powertron/fhr-4-2321-0r001-1-q/resistor-metal-foil-0-001ohm-3w/dp/2419239

Range [Ohm]	HP 3457A Mean [mOhm]	HP 3457A StdDev [uOhm]	K2001 [Ohm]
0.001	1.138	50.72	0.001008

I don't have an SMU, but I got curious how the L&N 4385 will behave under load (as multimeters only source ~1mA (3457A) - ~10mA (K2001) for 4W Ohms on this range), so I've selected 1.5A range (0.1 Ohm) and sourced 1A from Agilent 6632B, measured with K2001 as: 1.0007006A, connected that to the shunt box (assumed cold, as 1-10mA from previous measurements probably didn't heat the resistors a lot), sense outputs to K2001 in DC mode, with 1 NPLC and the voltage started with 100.120 mV and after 20 seconds stabilized at 100.105 mV. Therefore, assuming the current from 6632B stayed the same the resistance went from 99.423 mOhm to 99.408 mOhm, a delta of 14.89 uOhm.  Now it is impossible to say anything, K2001 current ranges are not calibrated and not very accurate, 6632B was stated cold and might be unstable and the 14.89 uOhm delta is difficult to measure with my equipment anyway (see the 3457A StdDev).

[joeqsmith]

Which one Joe?

I think they are the SRL series.  Under $2K/ea.  I just use them for a quick sanity check.

[HighVoltage]

OK, quick teardown then.

That seems to be a very nice shunt resistor made by Leeds & Northrup
Thanks for the teardown.

Looks like your K2001 is well calibrated and the HP3457 is a little off.

[HighVoltage]

If you like low ohms, I have one of these in my office. It is about 4 nano-ohms. It is a challenge to measure.

That is interesting.
This is probably an expensive zero ohm standard.
But how complicated can it be to make a zero ohm standard?
Most likely, it has a copper bar connected between the 4 binding post threads?
Would be interesting to see the inside.
Are you allowed to open that one up in your office? Mini teardown may be?

From their website:
http://www.ohm-labs.com/resistance-standards/100-0-standard.html
"Zero ohms in any connection configuration is achieved by using a tetrajunction. A tetrajunction is an ideal connection of four wires, such that current passed through any two creates no potential drop across the other two.

Based on work performed by B. V. Hamon and others at NIST (formerly NBS), a 'copper disc and post' construction was evaluated, and subsequently commercialized in Leeds & Northrup's Hamon Transfer Standards.

Verification of the 100-0 standard can be performed by passing current through various pairs of terminals and noting the potential at the remaining pair.

The 100-0 is factory adjusted to less than 50 nano-ohms in any connection configuration. The tetrajunction will not measurably drift over time.

The binding posts are of gold plated, low thermal emf tellurium copper. A case ground is provided for reduction of electrical interference."

Very interesting, thanks for sharing this zero ohm standard

[HighVoltage]

Here is my second Burster Resistor, it is a 100 mOhm

Type 125.100
- 100 mOhm
- Tol. : 0.02 %
- Imax: 5 A


The graph shows the warmup time of resistor and SMU within 1/2 hour, both started cold.
Starting at: 100.019 mOhm
Ending at: 100.024 mOhm

It is also kind of amazing how fast the SMU is up to specs in only a few minutes.

My cables are DIY with MultiContact gold connectors and LiFY cable.

[lukier]

Looks like your K2001 is well calibrated and the HP3457 is a little off.

Pure luck. I've actually calibrated this K2001 against 3457A after I repaired it, but my transfer accuracy wasn't that great (two 10V references in series on a protoboard, LT5400 1:10 divider, some Vishay 20K and 1M Ohm resistors) so by pure chance it seems OK. Also, L&N shunts are specified to 0.02% and god knows how the resistors drifted over the years.

That's the reason I haven't sent any of my gear to cal lab - everything seems "roughly" OK and agreeing. But at some point I'll probably send at least K2001 for calibration, because it is silly to have 6.5/7.5 digit meters and only have 0.X% accuracy. Another issue is that I often got things for parts/repair for very low prices, therefore calibration cost is significant in proportion

[HighVoltage]

Hookup the 3458A and measure voltage with it to see how stable it is.

OK, here is the first setup with the 3458A measuring the voltage at the sense terminals of the 10 mOhm resistor.
I will hook up the PC and take some measurements over time.
Although my 3458A is not calibrated, it is in very good agreement with the SMU

[VintageNut]

If you like low ohms, I have one of these in my office. It is about 4 nano-ohms. It is a challenge to measure.

That is interesting.
This is probably an expensive zero ohm standard.
But how complicated can it be to make a zero ohm standard?
Most likely, it has a copper bar connected between the 4 binding post threads?
Would be interesting to see the inside.
Are you allowed to open that one up in your office? Mini teardown may be?

From their website:
http://www.ohm-labs.com/resistance-standards/100-0-standard.html
"Zero ohms in any connection configuration is achieved by using a tetrajunction. A tetrajunction is an ideal connection of four wires, such that current passed through any two creates no potential drop across the other two.

Based on work performed by B. V. Hamon and others at NIST (formerly NBS), a 'copper disc and post' construction was evaluated, and subsequently commercialized in Leeds & Northrup's Hamon Transfer Standards.

Verification of the 100-0 standard can be performed by passing current through various pairs of terminals and noting the potential at the remaining pair.

The 100-0 is factory adjusted to less than 50 nano-ohms in any connection configuration. The tetrajunction will not measurably drift over time.

The binding posts are of gold plated, low thermal emf tellurium copper. A case ground is provided for reduction of electrical interference."

Very interesting, thanks for sharing this zero ohm standard

Definitely not a copper bar. I have constructed a copper bar low ohms device and it is difficult to get good results below 1 micro-ohm. You can see from my DIY thread that I achieved almost two orders of magnitude below 1 micro-ohm.

The Ohm Labs device is stated on the front plate to be a balanced tetrajunction. I posted the drawing on my thread to illustrate the geometry of a balanced tetrajunction. It is not a simple linear bar or ingot or rod. It is a balanced geometric shape that is the same very low resistance no matter how you  connect it. A bar or rod could never meet this requirement.

What is done after initial manufacturing is that if there is an imbalance in one orientation, a small amount of material is filed off of one particular face of the device and that orientation is re-measured to see how it behaves. This process is repeated until the device measures the same low number in all three unique orientations. That is why I posted all three unique orientations for my DIY tetrajunction thread.

I paid something like USD$400 for my model 100-0 device. I do not consider that expensive.

Sadly, I cannot break the seal. It is under warranty and is used for my day job.

[plesa]

Although my 3458A is not calibrated, it is in very good agreement with the SMU

After 3 years it will be still good enough to calibrate Keithley SMU
BTW 3458A is only one needed equipment to calibrate SMU. TiN created Python scrips for it.
You are using Pomona 2305 on picture?

What is stability of this standard if you reach 14A ? It can be quite interesting measurement.