Thermocouples with USB-6210 DAQ, readings show big variations from true temp.

[DSharp]

I have several type T and type K thermocouples connected to a USB-6210 DAQ.  In VI logger, I have set up a thermocouple task, selecting CJC source as constant.  Is it correct that software takes care of the CJC, as the connection diagram shows just the thermocouple connected directly into the device?  With the CJC value of 18 (room temp), the values start off OK, but over ten minutes they drift to about 40 degrees, and seem to increase and decrease randomly.  Do I need additional hardware, and/or what is going wrong?  Thank you.

[lmtis]

Can you post your VI?

[Joseph_Loo]

Check the number of bits of resolution available for your measurments. My guess you have 16 bits which translates to about 20 degrees of resoultion on your temperature measurements.

[dansch]

Hi DSharp,

You are correct that the software will account for the CJC value which you set as constant when computing the returned temperature value from the voltage reading. One thing I would suggest trying would be to see if touching the thermocouple or otherwise providing a path to ground for the thermocouple makes the signal return back to a value within the valid range. When measuring a floating source such as a thermocouple in differential mode, it is necessary to provide a path from the instrumentation amplifier inputs to the instrumentation amplifier ground, which can be accomplished through the use of bias resistors being placed between the inputs and AIGND. For more information on bias resistors and properly connecting signals, please refer to the Field Wiring and Noise Considerations for Analog Signals tutorial. There is also more information about the problem you are seeing in "Appendix B: Troubleshooting" of the NI USB-621x User Manual (page B-1). On this page, you will see a topic titled "I am using my device in differential analog input ground-reference mode and I have connected a differential input signal, but my readings are random and drift rapidly. What is wrong?" which also discusses making proper connections. Hope this helps,

[DSharp]

Thanks everyone for your help.  I tried using LabVIEW SignalExpress and it seems to work, though the parameters were pretty much the same as I'd put in the logger task.