Author Topic: Looking for infos/suggestions about accurate Low- and High-Ohm measurement  (Read 25542 times)

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Offline quarks

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it is a long time ago, but from my memory the Fluke 720 method was about as accurate as my TEK DMM 4050 (which turned out to be very accurate). But in my tested setup the 4050 was the Null Detector and I do not know if a Fluke 845 would lead to even better accuracy.

About Lead compensation, I have only played around with this setup and therefore have not really solid data. But as described in the 721 Manual "It is particularly useful in those situations demanding the utmost in accuracy".

Although with the Fluke 720/721 it was very interesting to see how they where used for unknown resitance measurements in the old days , I would (and did) go for an easier DMM solution.
 

Offline acbern

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thanks for answering this old thread.
if you say you would rather go for an easier DMM solution, I do not really see which one would be more precise than a 720A.
when working from a 10k standard accurate to better than 0.5ppm, the error propagation calc shows derived values accuracy to be 5ppm and better from 10ohms to 1Mohm (using voltage divider method and a 3458a).
however at 1 ohm and 10Mohm, the inaccuracy increases to arround 50ppm, one reason being low voltage range inaccuracies. so for these values, a 720a seems to be the only solution to get something better.
do you have any suggestions (other than a measurements internatinal bridge). Also, I am using these resistors to vaildate the accuracy of a 3458a artefact cal, so using my 3458a is not a solution
 

Offline quarks

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Overall I found the Fluke 8508A to be the best way to go for. Accuracy combined with convenience seems to be very hard to beat with other gear.
 

Offline acbern

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sure, the 8508 is a very good instrument, however its transfer accuracy is worse by about a factor of 2 than the 3458A, hence it does not solve this problem better unfortunatelly, it makes it worse.
 

Offline quarks

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if a 8508A is still not good enough, maybe you should look at ESI 242D Resistance Measuring System (very old but a great piece of art) or have a look at Guildline Model 6625A. According to Guildline "the 6625A System is the industry standard replacement
system for the historical ESI 242".
« Last Edit: August 06, 2014, 09:32:40 am by quarks »
 

Offline acbern

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yes, thanks, if money was no issue the 6622A xr would be a fine piece of gear. my guess is this is arround 50k though. that does not make sense.
the 242d was actually what I was trying to emulate by using the 720a and the 721 lead compensator, and given the fact that the 720a is self-cal (the 242d resistor decade is more complicated to calibrate, also it has quite a high temp drift of several ppm/K, and I see fluctuations of 1-2 K during a measurement cycle), I would think it should not be worse in resistance comparison. at least on the paper the 720a solution looks actually better.
the nullmeter can be adjusted to better than 1uV I would think, and at 10v or 100v total (for the 10megs measurement) with 0,1ppm referenced to input should give some reasonable results. i have not done the math yet, will do soon, and am waiting for the 721 to arrive, so doing tests to see how the noise is doing will be important too. I thought that you might have had some results here overall.
 


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