Taking thermocouple measurements with BNC-2120

[thomasjm]

Hi,

I am trying to take thermocouple measurements with a PCI-6110 card by plugging a type K thermocouple into a BNC-2120 connector block. When I use the built-in LabView example called "Cont Acq Thermocouple Samples-Int Clk.vi," I'm getting both bad accuracy and bad resolution. The resolution seems to be about 4 degrees, and the data is centered around -5 degrees Celsius when the thermocouple is at room temperature. However, if I take 100 samples (for example) I get widely varying values, as high as 0 degrees and as low as minus 10 degrees. However, I know that I'm measuring something because when I hold the thermocouple tip in my fingers the average value increases.

I am confused about this because the thermocouple voltage should be on the order of 5mV, and when the 12-bit PCI-6110 is set to its minimum voltage range (-200mV to 200mV) it should have a resolution that is more than adequate when the voltage is converted to a temperature.

The trouble is, the voltage range seems to be set in the "DAQmx Create Channel (AI-Temperature-Thermocouple).vi" virtual instrument, and it is set in units of temperature, not voltage. I would suppose that putting in a temperature range of 10 degrees to 50 degrees would cause the Create Channel block should figure out an appropriate voltage range based on the thermocouple type specified...but this doesn't seem to be happening, and the data doesn't look good.

Can anyone help me understand this? Any assistance would be greatly appreciated...

[jschwartz]

Hi Thomas,

DAQmx does change the range based on temperature, so if you input a temperature max and min, DAQmx will convert this to voltage and set the range appropriately. Do you have your thermocouple connected to the thermocouple input of the 2120?  If not, where and how do you have it wired?  One thing to try would be to switch the positive and negative lead of the thermocouple.  If these are switched, you could be getting crazy values.  What kind of values are you using for CJC?  Are you using internal or constant?  Try using a constant CJC source with the value set to about what you expect your room temperature to be. 

How to Determine the Thermocouple Type and the Correct Polarity of Signal Connections