How to measure 100mR? - Page 1
Forum section: electronics / general

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 21, 2017, 03:38:43 pm
content: Hi,I was thinking about how to measure my 0,1R Shunts.One way could be a transfer with a SR1010 1Ohm/step to Transfer 0,1R to 10R with 1ppm and the measuring 10R with a 3458A within 20ppm. Unfortunately, I don't have access to a SR1010-1. Direct measurement of 0,1R with the 3458A will only give 515ppm. (All specs refer to 1year calibration)The next best way I can think of is to use two 3458A to measure 100mA within 40ppm and 10mV within 39ppm -> 79ppm. (I would switch the current to compensate for offset voltages)Better ideas without some resistance bridges or something like that?

author: TiN
author_rank: Super Contributor
timestamp: February 21, 2017, 04:31:24 pm
content: I had some challenge not so long time ago and resolved it using delta-measurement, using calibrator 100mA or 1A as constant current source with reversal, and reading voltage drop using nanovoltmeter.Thread with some resultsHere.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 21, 2017, 04:42:43 pm
content: Thanks, TiN!Unfortunately my nanovoltmeter (Keithley 182) is worse in uncertainty specification at 10mV than the 3458A.And for me the most challenging problem is measuring the current with small uncertainties.

author: quarks
author_rank: Frequent Contributor
timestamp: February 21, 2017, 04:55:23 pm
content: have a look athttps://www.eevblog.com/forum/metrology/low-ohm-precision-resistor-standard-and-testing/msg1063840/#msg1063840andhttps://www.eevblog.com/forum/chat/help-wanted-low-and-high-ohm-measuremet/msg169139/#msg169139

author: ap
author_rank: Frequent Contributor
timestamp: February 21, 2017, 05:01:43 pm
content: Had the same issue, started out with 1ohms though (Leeds&Northrup 4210, that was derived from an externally calibrated 10K standard in a ladder calibration, similar as below):-Using the 3458A, apply 100mA (or more) to the series connection of the 0.1 and 1ohm, and measure the voltage drops relative to each other. This way the transfer accuracy applies, which is much better (you can do that also at higher resistances, then apply a constant voltage instead of the constant current; sources need to be stable short term). If you do the math, the accuracy is pretty good (starting with 10k/1ppm, I ended up somewhere around 10 to 20ppm for the 0.1ohm iirc). Watch out for self heating/overloads...-The SR1010 as you said, just came in, but have not started doing the cal with it.

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 21, 2017, 05:54:22 pm
content: @quarks: Thanks for the interesting links! The SMU doesn't deliever real low uncertainty I think, but before any discussion I should read the threads@ap: Thanks. That should be the way to go. I throwed a little table together, only considering the 3458A transfer specs. This seems to be very good

author: ap
author_rank: Frequent Contributor
timestamp: February 21, 2017, 06:04:53 pm
content: Yes, but that is NOT all your uncertainties. You have short term drifts of the source, EMF (even if just 100nV), and you need to do a GUM error propagation consideration (weighing factor of uncertainties, sum square of partial differentials, not going into details here). So the overall will be a little worse.Practically, you could switch between front and rear for the two different voltages, that makes things faster. Consider potentiel zero-offsets between front/rear (NULL?)!

author: e61_phil
author_rank: Frequent Contributor
timestamp: February 21, 2017, 06:17:02 pm
content: I'm aware of the fact that the transfer accuracy isn't the whole story.I did some measurements on my "new" VCS332 1R shunts a few days ago. At home I have no 3458A. Therefore, I used my 3456A together with a 3488A muxer and my Knick JS3010 as a current source. With this setup I compared the 1R in my Fluke 5450A to the VCS332 at 100mA. Control of the current source and the muxer was done in python.I took 200 of this measurements and the standard deviation of this 200 measurements was 1,25ppm. I think most of the "noise" is due to the fact that only 100nV is needed to get 1ppm of error. Therefore, I assume the standard deviation with 0,1R at 100mA will increase to more than 10ppm.Perhaps, the Keithley 182 is better for measurements in the range of 10mV (300mV Range for transfer).The Knick JS3010 seems to be very stable. 100mA is within 1ppm over several minutes.