Low ohm precision resistor standard and testing

[HighVoltage]

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

That might be a challenge, how do I get a stable 14A and how to measure it?

I had the 10 mOhm resistor hooked up to the SMU for 18 hours
Here are the results:

[plesa]

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

That might be a challenge, how do I get a stable 14A and how to measure it?

You have 2460, so it should deliver 7A or it is only in pulse mode?

[HighVoltage]

Here I did a 10A test with an Agilent high current PSU,
The 34470A is used to measure the voltage at the Resistopr
The 34461A is used to measure the current (Limited to 10A)

OK, this following setup failed, if looked from a metrology point of view.
The current and voltage is fluctuating too much.
Banana plugs are not good to be used for 10A current transport in to the resistor, because the binding posts get hot.

Results are:
Average Voltage: 100.17695 mV
Average Current: 10.004859 A
Average Resistance: = 10.0128 mOhm

It seems I have to look in to the metrology way of measuring high DC currents exactly with high precision.
Not easy and probably be very expensive.

OK, back to the Keithley 2460 and up to 7A

[plesa]

If you needs to make test at higher current than 7A (Keithley limit) you can still can use your Agilent/TDK PSU.
But you will needs to use one of your fancy precision resistors for monitoring current and set up feedback to voltage or current regulation terminal ( on rear DIP switch you you needs to set it to voltage controlled voltage or current source).
Input signal level is  between 0-5V or 0-10V, so some OpAmp gain needed.
This will make your setup far more stable.

[HighVoltage]

The 2460 SMU does go to 7A continuous current, although it does sound like an airplane when you do.

So, I started with 5A for about 15 min
But for this I had to build a new set of cables, starting at 4A the binding post of the resistor got warm.
Now I am using good quality lugs.

The readings are very stable at 5A
Actually standard deviation is 65 nOhm
Peak to Peak is about 500 nOhm

Interestingly, the voltage of the 34470A increased by about 1.3 uV
The resistor seems to not warm up at all, (not measured) and stays at room temperature.

[HighVoltage]

Ok, here is the last set of measurements at 7A.
At least this SMU has reached its limit

Also interesting, the fan stays at full power for a long time, after the current has been reduced from 7A to 100 mA and the instrument is very loud.

This cable I assembled seems to work very well for this test.

[zlymex]

It seems I have to look in to the metrology way of measuring high DC currents exactly with high precision.
Not easy and probably be very expensive.

One way to do this is to use a LEM IT-600S to scale down the current by 1500 times. The 1500:1 ratio is guaranteed and the linearity is ppm level. Even if used as a 15A:0.1A(10 turn primary and 10 Ohm secondary resistor), the parameter is also not degrade very much.

[Conrad Hoffman]

This thread triggered a memory of an old L&N low resistance box I keep stashed on the lower shelf of a scope cart. It needs a good cleaning, but I brushed off the dust. It doesn't seem to have a model number, but it's 300 ohms max in 0.001 ohm increments. I can't believe the cal sticker is original, as I doubt they made that style in '92.

[HighVoltage]

This thread triggered a memory of an old L&N low resistance box I keep stashed on the lower shelf of a scope cart. It needs a good cleaning, but I brushed off the dust. It doesn't seem to have a model number, but it's 300 ohms max in 0.001 ohm increments. I can't believe the cal sticker is original, as I doubt they made that style in '92.

That Leeds & Northrop resistance box looks fantastic.
Just amazing, what they built in those days.
Do you know, when it was built?
I would not be surprised, if it was right on to specification as well.

Reading up on it, Leeds & Northrup did built a Thomas 4210 with one Ohm and they built it to 1 ppm specification.
I bought one of these 4210 a few weeks ago  but it has not arrived.

[manganin]

I can't believe the cal sticker is original, as I doubt they made that style in '92.

Those aluminium stickers were used by the L&N lab at least until late 80's. And for one-off calibrations probably later than that.

If you post the serial number I can tell you the approximate year of manufacture.

[VintageNut]

Ok, here is the last set of measurements at 7A.
At least this SMU has reached its limit

Also interesting, the fan stays at full power for a long time, after the current has been reduced from 7A to 100 mA and the instrument is very loud.

This cable I assembled seems to work very well for this test.

Hello High Voltage

Are you performing an offset compensated measurement? If not, the multiple metal-to-metal contacts in the current path will induce random microvolts each causing significant error in your measurement.

There are multiple ways to accomplish the technique. In the end, you want two different currents forced, calculate the slope of the line, (v-v)/(i-i) and that is the true resistance of your device. The Y-axis intercept of the y=m*x+b equation is the lumped offset.

In a DMM, the highest current possible and lowest current possible are used to create the points to calculate the slope and thus perform the offset compensated ohms measurement.

For a SMU, you can use +7A and -7A which gives a much larger range and should be less error than a DMM all else being equal.

The old-school current reversal technique involves actually reversing the plugs at the current source. A more elegant solution is a polarity reversal switch between the current source and resistor.

The different methods have different sum-of-errors calculations.

For my 10 nano-ohm DIY tetrajunction, I used a KE2450 to force current. This allowed the use of TRIAX-to-alligator cables and placing the resistor and unshielded alligator clips into a shielded box which also prevented air flow over the device and alligator clips.

[Conrad Hoffman]

My only reference resistors (that I trust) are a 100 ohm L&N that I had measured some years back, and a Guildline 10 ohm that's lost its oil, like the one in the tear down thread. As far as I can tell, the L&N box is still within spec, but I don't have a good fractional ohm resistor I trust for comparison. NIST seems to be good at scaling things over large ranges, but I get nervous beyond 10X.

The L&N box has two metal plates that I thought were owner tags, but looking closer I think they're the L&N tags. The top one is two lines, "L.T." followed by "4171". Below that is a similar tag "Sp. Inst." The only other marking is "2377" stamped on the side of one of the taper keys, and a pencil notation inside the case I'm not sure about. I can also see the tape residue next to the cal sticker from a previous one, so it likely went back to L&N on a regular basis. I bought it at a hamfest decades ago when such things were being cleaned out of local businesses and universities. I seem to remember there were several of them, and they had trouble selling them at all. It's big and heavy. When I get a chance, I'll post some pictures of the insides. The 0.001 step section is basically a manganin bar with taps that's been filed slightly between taps to trim it. The rest is fairly conventional. I may also have a photo of a nice L&N knife-style reversing switch.

Another question- can an accurate ratio transformer be used in an AC measurement to compare resistors? Or are the big wound reference resistors so bad, even at 1 kHz or so, that it's not a good method?

[VintageNut]

Hello Conrad

I am curious how you connect to your L&N resistor box since the box has two connections. Do you use 4-wire ohms? What instrument do you use to measure the resistor box?

[Conrad Hoffman]

The giant binding posts are actually double nuts on a stud, so sort of a 4-wire connection, at least at that point. I put the device in series with a known resistor (my L&N 100 ohm) and use a 6.5 digit meter to measure the drop across that and the DUT. My resistor was certified about ten years ago, so it's as good as an L&N resistor is after ten years of drift!  I have a fairly large collection of them, plus a friend with more, so by intercomparison I know I'm not too far off, but no way would I guess better than 100 ppm or so without a fresh cert. An interesting aside, the resistor is quite old (chrome body) and when I first tried to have it certified, they rejected it as defective- too far out. Then we realized it was built before the ohm was adjusted (can't remember the year, '60s?) and it was actually almost dead on what it should have been.

Here's the insides, a bar for .001, loops for .01, coils for .1 and then wound and waxed coils for the rest. Also a not-great photo of the L&N switch that I believe was used for bridge reversals.

[VintageNut]

That is a very interesting box. I could see that used as a low volts, low offset, divider.

The solid copper bus bars are on every decade.

Looks like two correction/adjustment resistors?

[Conrad Hoffman]

The big resistors to the left are 100 ohms each. The box total is 300 ohms, the 100 ohm total decades plus the two 100 ohm coils that can be switched in and out. The odd value and "Sp. Inst." plate make me think this was a special for somebody, maybe based on a standard 100 ohm box. Alas, I have almost zero documentation on L&N, nor is there much on the web.

Proving anything resistance-wise requires a decent standard. I have tons of 0.1% parts, quite a few 0.01% parts, but only the one 4030 (or similar) L&N resistor that was measured to 6 places a decade ago. I think they charged me at least $100 back then, so I'm reluctant to do it again. Same problem with voltage- I have three standards that can be inter-compared that were calibrated a decade ago. They still compare closely, so I assume my volt hasn't drifted much. I just need somebody to ask for a traceable measurement and pay for a new round of calibration!

[manganin]

The L&N box has two metal plates that I thought were owner tags, but looking closer I think they're the L&N tags. The top one is two lines, "L.T." followed by "4171". Below that is a similar tag "Sp. Inst."

The secondary L.T. serial numbering system was for selected units better than the normal specifications. Used in L&N own labs and factories and possibly in some selected partner labs.

The number was engraved afterwards above the original serial number. It was usually not given at the time of manufacture, but based on years or sometimes decades of calibration history. For example I have a Rosa style standard resistor which has smaller L.T. serial number than a similar resistor manufactured almost 20 years earlier.

The Special Instrument tag could be the key for the missing main serial number. Maybe it was never built for sale.

[quarks]

Reading up on it, Leeds & Northrup did built a Thomas 4210 with one Ohm and they built it to 1 ppm specification.
I bought one of these 4210 a few weeks ago  but it has not arrived.

the LN 4210 is the very best 1 Ohm resistor I know of

more standards and details can be seen in zlymex post
https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg891744/#msg891744

[The Soulman]

An interesting aside, the resistor is quite old (chrome body) and when I first tried to have it certified, they rejected it as defective- too far out. Then we realized it was built before the ohm was adjusted (can't remember the year, '60s?) and it was actually almost dead on what it should have be.

It was on the 1st of January 1948 the standards were increased with 495 ppm.

I didn't even know it was changed at all, found it here:

https://www.nist.gov/sites/default/files/documents/calibrations/87mscohm.pdf

Thanks!

[Conrad Hoffman]

Good find- I saw that pub a decade ago but lost track of it. My resistor was certified 12/30/04, 12 years ago! Time flies. Back then it measured 100.05047 ohms, which seemed to upset the cal lab more than I would have thought. I should probably change the oil again, as it can become acidic over time and cause the value to drift.

I have various other standards, including a Fluke 5450A, but the 100 ohm L&N is the only thing with any calibration history, though I fully realize they're not the greatest standard.

I've experimented a bit with winding my own standards from both manganin and constantan. I've also got a spool of "800" alloy that's probably better than either of them, but you can't solder to it- has to be spot welded. Still, it all comes down to having a trusted standard to compare to.

IMO, it's best to have ancient standards with a known history of calibration so you can get some idea of long term drift! Or, being unable to afford much else, I keep telling myself that. 

[CalMachine]

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.

I would love to try

[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.

I would love to try

Where are you located? What instruments do you have that will force 1A DC? Do you have a nanovoltmeter?

[CalMachine]

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.

I would love to try

Where are you located? What instruments do you have that will force 1A DC? Do you have a nanovoltmeter?

I am located in Ft. Wayne, IN.

As far as equipment, I have 2 Datron 4808s available, 2 Fluke 5520As, currently 1 Fluke 5700, as far as meters go, Keithley 181 or perhaps one of our 2002's or Agilent 3458As.  I also have a Fluke 752A Reference divider and Fluke 845AB Null detector if I want to get stupid with it

[e61_phil]

As far as equipment, I have 2 Datron 4808s available, 2 Fluke 5520As, currently 1 Fluke 5700, as far as meters go, Keithley 181 or perhaps one of our 2002's or Agilent 3458As.  I also have a Fluke 752A Reference divider and Fluke 845AB Null detector if I want to get stupid with it

4nR * 1A = 4nV

Only the Keithley 2002 in your list is capable to resolve 1nV. Isn't it?

[CalMachine]

As far as equipment, I have 2 Datron 4808s available, 2 Fluke 5520As, currently 1 Fluke 5700, as far as meters go, Keithley 181 or perhaps one of our 2002's or Agilent 3458As.  I also have a Fluke 752A Reference divider and Fluke 845AB Null detector if I want to get stupid with it

4nR * 1A = 4nV

Only the Keithley 2002 in your list is capable to resolve 1nV. Isn't it?

They keithley 181 is our nano voltmeter with resolution down to 1 nV.