We are using NI 9213 module with ungrounded thermocouples. Our thermpocouples do not have any shielding to be connected to COM pins of NI 9213.
I would like to confirm that we can leave COM pins of NI 9213 unconnected. Someone suggested to connect COM pins of NI 9213 to negative terminal of DC power supply like PS-15, but I do not think it is correct.
Thank you in advance.
Are you having any issues with the COM pin not being connected? If your readings are not noisy I would not worry about it. Or try it both ways and see if there is any difference in the data.
Connecting the COM pin to a ground or the negative terminal of a power supply is a way to help get rid of noisy signals.
Start reading on page 4 for some more information.
We do not have a lot of experience with taking temperature data with NI 9213, but have not noticed noise issue that would be detrimental to our measurement purpose. So far COM of NI 9213 has not been connected. We are planning to put cDAQ in protective enclosure along with DC power supply so the wiring will become more permanent. We are looking for the "best practice" how to deal with COM terminal on NI 9213 module. If connection NI 9213 COM to DC power supply COM is a noise reduction measure and it helps measurement in anyway, we would do so.
Here is some more links to take a look at. Putting the DAQ in a protective enclosure might not help if the thermocouple wires are acting like big antennas picking up noise in the area. Another option is to take multiple samples and average them into 1 sample and use that for the data point.
Another helpful hint is if the forum search does not work very well for a topic, try google.