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Potential Drop Measurement

Posted: June 1st, 2015, 11:35 am
by bkagay
I am attempting to perform a direct current potential drop measurement using the 2182 to measure the small change in voltage that occurs across a smooth tensile specimen when a crack or cracks initiate and grow. I am running a current of about 2 amps across the specimen, and I am trying to connect the 2182 to the grips/fixturing.

The problem I am having is that a relatively large voltage drop occurs during the test that has nothing to do with crack growth and I think this may be due to thermal EMFs or noise from AC voltages. The voltage drops from about 7 mV to as low as 3 mV over the course of a few hours. I can't use all copper to copper connections because the grips that I am trying to connect the 2182 are made of 316 stainless steel. I have the model 2107 cable connected to the 2182 and the copper lugs have been removed from the cable. The end wires of the 2107 cable are clamped to a set of tin-coated copper wires with ends that have been covered in silver solder. The other ends of the tin-coated copper wires are silver soldered to the stainless steel grips.

Please let me know if thermal EMFs or noise from AC voltages could be the cause of the drops in voltage or if there may be a different problem and how I might be able to fix it. Thanks.

Re: Potential Drop Measurement

Posted: June 1st, 2015, 12:00 pm
by Dale C
What are you using as a current source?
Can you reverse polarity on this current source?
If it is a Keithley 6220 or 6221, then you can use the delta mode to eliminate thermal offsets.
If it is a Keithley 2400 series or 2450 or 2460 then again, you can use it with the 2182A in a delta mode to eliminate thermal offsets.
I think that is the best method to make this measurement with reliable results.