[volt-nuts] Semi-precision high resistance measurement

Steve Byan stevebyan at verizon.net
Mon Jun 17 13:50:36 EDT 2013

I'm restoring my old HP 410B VTVM, and I'm interested in seeing how much the resistors have drifted since it was built, particularly the precision resistors in the input voltage divider. I don't have volts-nuts caliber equipment (well, there is a busted HP 3456A on the shelf waiting to be repaired someday), just a Fluke 8050A and a Fluke 27/FM.

I didn't expect to have much trouble making consistent measurements as I don't think the 8050A has the resolution to see temperature coefficient changes or thermocouple effects. But I'm seeing some odd results on the higher resistance values. First, I seem to see some contact resistance effects: I don't get consistent measurements just using the probes as the count varies a little with contact pressure and probe placement. The contact resistance would have to vary by thousands of ohms for it to affect the meter; I can't believe that could be the explanation. However, I'm able to get consistent measurements by slipping alligator clips on to the probe tips and clipping on to the range switch terminals. Maybe the old solder is so oxidized that the contact resistance can really vary that much?

Second, the Fluke 27/FM measurements track those of the 8050A better than the spec'd limits, but I see some odd behavior in the last digit of the 8050A. The last digit of the resistance value varies with the direction of the current through the resistor, and in one direction, it bobbles up and down about three counts. In the other direction, the reading is stable. The bobble doesn't seem to be sensitive to placements of the test leads.

For example, the 6.837M Ω 1% resistor (R6) measures 7.037M Ω one way, and between about 6.994M and 6.996M Ω when I reverse the 8050A test leads. That's a difference of nearly 0.6%

The 2.163M Ω 1% resistor (R5) measures 2.220M Ω one way, and between about 2.215M and 2.217M Ω when I reverse the leads, for a difference of about 0.2%.

The 683.7K Ω 1% resistor (R4) measures 697.9K Ω one way, and between about 697.5K and 697.7K Ω when I reserve the leads, for a difference of about 0.05%.

I did try switching off some potential nearby RFI sources - fluorescent lights, switching power supply, laptop computer - and saw no difference in behavior, although I didn't do an exhaustive search for RFI.

Finally, I did some quicky measurements with the Fluke 27/FM about two months ago, and the current measurements seem to be a bit off (I don't have the old recorded measurements handy as I write this, but I think they are outside the accuracy limits spec'd for the 27/FM). This is a non-climate-controlled New England basement, so the temperature is probably up about 5 degrees C and the humidity has shot up recently. But again, I wouldn't think my instruments are good enough to notice these environmental effects on the components themselves.

Any ideas as to what's going on? How can I improve my measurement procedure to get repeatable results? Do I really need better climate control even at the 3 1/2 or 4 1/2 digit level of precision? What's with the polarity sensitivity of the 8050A resistance measurements? Suggestions and advice would be gratefully accepted.

Best regards,

Steve Byan <stevebyan at me.com>
Littleton, MA 01460

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