[volt-nuts] Semi-precision high resistance measurement
stevebyan at verizon.net
Thu Jun 20 19:48:11 EDT 2013
Thanks for all the suggestions.
The Fluke 8050A schematic is hard to follow due to the complicated range and function switching, but if I did so correctly, on the 20M ohm and 2M ohm ranges it applies 1.2 volts through a precision 10.00M ohms reference resistance, then through about 2K ohms, then to the resistor under test. It measures the voltage across the resistance reference and then the resistor under test and reports their ratio. The 8050A has a conductance function as well as resistance, but the actual measurement circuit is identical to the resistance function; the dual-slope conversion is simply inverted.
For the 6.837M ohm resistor I measured, the voltage on the resistor under test would be about 0.5 volts, so I can't see thermoelectric effects being an explanation.
For the 683.7K ohm resistor, the voltage would be about 77 mV, so I could see the need to start to worry about thermoelectric effects, but the change in resistance for the smaller resistor is lower, so I think it must be some other effect.
I have a 2.00M ohm 1% wire-wound resistance standard in a nice oak box that I obtained from the recent MIT lab storeroom clean-out. I've yet to clean up the terminals, but measurements of it seem to exhibit similar polarity sensitivity, so I now think the cause must be in my measurement technique or in the meter itself. The 8050A reads about 10 µV DC across the resistance, so that seems like a thermoelectric effect, but one too small to affect the resistance measurement. The kicker is that the 8050A reads about 0.4 volts AC across the resistance, and it is only spec'd at ">60 dB" normal-mode rejection ratio for 60 Hz.
I tried twisting the test probe leads together, but I still get about 0.4 volts AC. I plan to try using coax next, with short leads to the test clips, and see if I can get the AC level down. I should also try moving the meter somewhere further away from the service entrance to my house; my workbench is only about 5 feet away from it.
Steve Byan <stevebyan at me.com>
Littleton, MA 01460
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