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Precise Offset Compensation Ohm Measurements and Validation of DMMs

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TimFox:
I believe that I have seen a similar problem with 1 Megohm wire-wound resistors in IET (originally GR) SR1 resistor standards.
I have two 1 M units that give unstable short-term readings on 6.5 digit DMMs, while a normal RN65 1 M metal film gives stable short-term readings.

TiN:
Hi Frank,

That's for thorough investigation, this is interesting. Good trap for young players, to check and triple check everything, not just believe "magic features" to improve performance.

K2001 have max 200k range if OCOMP enabled, while 2002 is 2M max autorange. I can check timings/capture some data to add for your measurements.
There is not much needed to upset 100K resistance reading as even 10GOhm in parallel with 100K gives 10ppm error.

What was your current leakage setup? 32V PSU in either polarity to case and floating terminal of resistor?

Also to fully complete this experiment would be cool to have guarded measurements test.
I'd expect those should resolve issues with OCOMP even without delay.

Dr. Frank:

--- Quote from: TiN on January 03, 2016, 03:40:38 pm ---Hi Frank,

That's for thorough investigation, this is interesting. Good trap for young players, to check and triple check everything, not just believe "magic features" to improve performance.

--- End quote ---

Hi TiN,
I'm sorry, but I'm already an old-player..
Everybody will fall into that trap, as this is not explained in the documentation.


--- Quote from: TiN on January 03, 2016, 03:40:38 pm ---K2001 have max 200k range if OCOMP enabled, while 2002 is 2M max autorange. I can check timings/capture some data to add for your measurements.
There is not much needed to upset 100K resistance reading as even 10GOhm in parallel with 100K gives 10ppm error.

--- End quote ---

Well, please do so. I expect the same effects. Keithley instruments prefer shorter NPLC numbers, obviously. So NPLC 1 will even worsen the measurement.
Isolation resistance is another subject, which is not covered here, and which will provide a constant decrease of the reference resistance, but which will not be affected or mitigated by the delay.


--- Quote from: TiN on January 03, 2016, 03:40:38 pm ---What was your current leakage setup? 32V PSU in either polarity to case and floating terminal of resistor?

--- End quote ---

This setup was for demonstration purposes only..not for quantitative results.

Yes, I applied +/-32V between the case and one jack.
The resistor was applied, so you would probably measure the sum of all three jacks..

I also measured one jack w/o a resistor connected, like on the picture of the interior of the box.
Anyhow, I found loading time & current of the same order of magnitude.
You also have to scale with the 1V or 5V which are normally applied


--- Quote from: TiN on January 03, 2016, 03:40:38 pm ---
Also to fully complete this experiment would be cool to have guarded measurements test.
I'd expect those should resolve issues with OCOMP even without delay.

--- End quote ---

I did that, but the passive guard of the 3458A did not change anything.

An actively driven Guard only may help, I assume.
That can be found on some Keithley electrometers only, using Triax cables.

Vgkid:

--- Quote from: TimFox on January 03, 2016, 03:21:55 pm ---I believe that I have seen a similar problem with 1 Megohm wire-wound resistors in IET (originally GR) SR1 resistor standards.
I have two 1 M units that give unstable short-term readings on 6.5 digit DMMs, while a normal RN65 1 M metal film gives stable short-term readings.

--- End quote ---
The ESI SR1 resistors should be flat mica card wound, not much inductance.

TimFox:
Yes, they are mica-card-wound resistors.  Perhaps the problem is due only to the shunt capacitance in the aluminum box?

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