measuring temperature using thermocouple and SCB-68 box

[Imaginer]

I used thermocouple and SCB-68 box to measure temperature. But the readings were fluctuating around the expected value. So I removed the thermocouple and short-circuited the differential input channels. As expected, the voltage reading was not zero and also fluctuating. It seems that there was something wrong with the SCB-68 box. What might lead to this and how to solve this problem? Thanks.

[JustinBritten]

The problem could be either the SCB-68 or the DAQ card you are using to make the measurement. You should first ensure that the DIP switches (S1 - S5) of your SCB-68 are in the correct position for your application. Please refer to the SCB-68 User Manual for more information on this subject.

SCB-68 68-Pin Shielded Connector Block User Manual
http://digital.ni.com/manuals.nsf/websearch/74C86ADEF0E4813F86256C84007CB3AB?OpenDocument&node=1180_US

What do you mean by fluctuating? Full swings from 10V to 10V? Hitting one rail? Close to 0V but looks like noise in mV or uV? If your fluctuation is very slight, for example off by a few millivolts, than you could be picking up noise or your board could need to be calibrated. You can read more about accuracy and
calibration from the following web page:

Calibration Solutions from National Instruments
http://www.ni.com/support/calibrat/

If you have an E-Series Card you may be interested in running our Online E-Series Diagnostic Utility. This utility takes about 3 minutes to run, will perform a calibration, and will generate a report of anything that may be wrong with your board.

Online E Series Diagnostic Utility
http://www.ni.com/support/selftest/

Regards,
Justin Britten

Applications Engineer
National Instruments