cFP-TC120 Inter-channel voltage limit

[Tony_Dean]

Hi,

I am using the TC120 thermocouple modules to record temeratures from a number of test machines. Originally we used sheathed thermocouples but we now wish to use unsheathed thermocouples bonded to the specimens.

The module specification states no channel-to-channel isolation but that the inputs are differential. My question is what voltage can the TC120 tolerate between channels, both in terms of degrading the accuracy and possible damage to the module?

It would be too costly to provide a TC120 module for every test machine as each machine uses only 3 thermocouples but we have 50 machines. The thermocouples are essentially at ground but there may be several volts between the machines.

Thanks

Tony

[K_Bull]

Dear Tony,

Typically you cannot use two channels at different potentials on the same FieldPoint I/O module. As you know, cFP-TC-120 do not offer channel to channel isolation, and they measure signals with a reference to the Common of the terminal base (as defined by the Com terminals). Thus, a signal that is at a potential of 100 V with reference to the isolated backplane could be measured against other signals that are at the same potential -- the circuitry will handle small potential differences (less than 4 V), but not against signals that are at 150 V with reference to the backplane. If you wish to measure two signals that are at different potentials, then you will need multiple modules. (KB)

You will get erratic readings when stray currents run through the module. If two or more of the thermocouples are connected to different potentials, current runs from the thermocouple into the module and out to the other thermocouple. Depending on the range used, if more than 0.5 to 2.0 V is run through the thermocouples, one or more of the readings can become erratic. If these voltages are more than 0.5 to 2.0 V and another thermocouple is grounded, has a common-mode voltage, or is connected to a voltage source, erratic readings will also occur. (KB)

Hope you will find these links helpful: Improve Accuracy, Considerations for Analog Signals.

Regards,

[Tony_Dean]

Thanks for the reply. It's not what I wanted to hear but it's pretty much what I expected. Its a pity that so soon after installation we are finding the limitations of the compact fieldpoint modules.

Thanks again

Tony