Digital Multimeters (DMMs) and Precision DC Sources

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Why is the voltage that I am measuring with a Keithley DMM slowly dropping

I have apparatus that I monitor crack growth via potential drop with.  Basically, a constant current (KEPCO ATE @3.00A) is snaked into an oven, connected through a small sample on each side of a notch with spot welds, and then out and returned to the power supply.  The Keithley DMM has wires which enter the oven and are spotwelded neatly on each side of the notch.... as the samples are exposed to conditions that result in crack growth, the potential drop measured by the DMM increases and can be related to the extent of the crack growth.

 

I have had success with these measurements before on the said apparatus and an additional one in my lab - but I have come across a recent problem

 

The potential drop measured across some of the samples is now decreasing very slowly - hard to notice after watching 10 minutes of measurements, but as the test runs overnight in addition to the fact that the growth I'm measuring is very sensitive means this needs to be addressed.

 

I'm pretty sure it is not the power supplies - these particular ones have been used in the past successfully.

 

also, the oddities in the voltage measurements is somewhat intermittent (although more often than not).

 

 

any thoughts?

 

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You posted to the NI DMM board.

 

I'm not sure why you posted to the NI forums at all. You aren't using any NI hardware or software (or any that would cause measurement issues). My best guess is that there is a connection problem. Something expanding, besides the crack, in the heat?

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hmm... you have a point - I use Labview to monitor these instruments and had used this forum for help before.... and therefore thought to come again not realizing the more specific application.

 

This is more just a measurement problem... I think I had some wires touching here of there... and thus am re-wiring the oven.

 

ET

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