From your description, I would suspect that your thermocouples are in an environment that is highly electrically noisy, such as being near a motor. The FP-TC-120 module does not have channel to channel isolation, and can only reject several volts of common mode voltage between each channel. Since the readings become erratic when one thermocouple is longer than the others, this is a very good indication that external common mode noise is being induced on the thermocouple leads. As long as the leads are the same length, the same voltage gets induced on all the thermocouples and the module is able to reject the voltage. In order to remedy your situation, I would offer the following suggestions:
1) Make sure the FP-TC-120 common is free to float, to allow it to
better reject common mode noise. This is done by making sure that you have not wired the C or Com terminals of the FP-TC-120 to the C or Com terminals of any other FieldPoint module.
2) Use shielded thermocouple wires. Connect the shield of the thermocouple to the C terminal of the FP-TC-120.
3) Route the thermocouples away from the electrical noise source.
4) Do not ground the ends of the thermocouples. If they need to be touching a metal object, use an insulator and thermal conduction grease.
5) Route the thermocouple cables through grounded metal conduits.
Regards,
Aaron
LabVIEW Champion, CLA, CPI
