Temperature Verification with PCI-6071E

[ccdjl]

I am trying to perform a temperature verification on my PCI-6071E DAQ board.  When the temperature channel terminations are shorted or a thermocouple is attached, the room temperature values are stable within a degree.  When I plug in a constant voltage using a Yokogawa 2554 DC Voltage Current Standard, to simulate various temperatures between 20 and 100 degrees C, my converted temperature values are noisy and jump around by 5 to 6 degrees.  I have tried changing sampling rates (currently 120 samples per second) and changing the input ranges (currently +/- 100 mV).  Nothing seems to help.  I would rather not have to use a water bath to verify the temperature readings.  Previously I had a ATI-MIO-64-E3 board and this constant voltage method worked just fine with exactly the same settings.  Any suggestions would be appreciated.

Cindy L.

[NI-Bongo]

Hi Cindy,

Are you operating the voltage source in the wall-powered or battery powered mode?  If it is not floating, you may need to consider how your grounding is being handled.  Do you take the thermocouple measurements in differential or single-ended mode?  This document discusses the different terminal configurations and grounding considerations when connecting analog signals:

Field Wiring and Noise Considerations for Analog Signals
http://zone.ni.com/devzone/cda/tut/p/id/3344

Have you verified with a scope that the voltage source does output what you expect?  This might be something to check if it's convenient.

[ccdjl]

John,

Thanks for the reply.  Our thermocouple measurements are made in differential mode.  This morning I changed voltage sources (similar model) to one which had a working battery.  Changing to battery mode made a huge difference in the signal noise.  I am also using a multimeter (which is AC powered) to confirm the voltage signal.  On the advice of our instrumentation staff, I also tried measuring the thermocouple signal with the multimeter removed.  Although the average temperature readings did not change, the standard deviation of the readings was greatly reduced.  The reading are now acceptable for our verfication purposes.

I will also follow up with reading the document you suggested.

Cindy