Faulty Thermocouple Measurement Due to Common Mode Voltage

Issue Details

I would like to measure the temperature of a metal surface, which is under external voltage. For the temperature measurement I use a thermocouple connected to a C Series Temperature Input Module module in a CompactRIO Chassis. Over a specific voltage level connected to the measured surface, the result of the temperature measurement changes suddenly exactly to 0°C.

I also get an error message from LabVIEW with the Error Code -65583 and the following message:

CompactRIO: (Hex 0xFFFEFFD1) A common-mode range error was detected.

What may cause this faulty measurement?

Solution

Most probably the thermocouple is common mode voltage exceeded the allowed range, according to the device's specifications refer to the Related Link section to further details.

For example, the NI-9213 common-mode range is the maximum voltage between any channel and COM, which is +/-1.2 V at minimum. If COM is not connected, then the common-mode voltage range is the maximum voltage between any two channels. The NI-9213 measures the common-mode voltage level of each channel and returns a warning in the software if the signal is outside the common-mode voltage range.

As a solution to the problem, apply better electric isolation between the thermocouple and the measured surface under voltage to decrease the leakage currents and to prevent that the common mode voltage of the thermocouple inputs of the module exceeds the allowed limits.