How to correctly ground/isolate a grounded enclosure type thermocouple?

Question

My K-type thermocouple (grounded enclosure type) connected to MAX31855 adapter connected to Arduino Mega gives me faults every few readings. It's really, really bad. I've tried swapping thermocouples, adapters, and Arduino's in various permutations and reached the conclusion that this happens whenever the thermocouple sheath/shielding touches the metal frame I've got the rest of my contraption set up on. Also gives faults when I touch it. Works perfectly fine when floating or completely isolated from everything else.

I've tried connecting myself to ground, and I've tried connecting the frame to ground, but nothing works. What should I do? I thought this was the whole purpose of the shielding. :(

When I check continuity on the probe, I get 3 ohms between probe tip and one lead and 7 ohms between probe tip and other lead. This is exactly the same for another thermocouple I've tried. No surprise here, as it's a grounded type enclosure thermocouple.

Answer 1

Disclaimer: As Chris Knudsen has rightfully indicated, this answer is only applicable to shielded TCs without a junction-shield bond (i.e. the "ungrounded" type). However, the OP asks about the "grounded" type with a junction-shield bond, to which the discussion/suggestion below are not applicable.

---

The shield is only effective if is at a potential that the thermocouple wire pair is, too. At least, it should have a constant voltage to the wire pair. Else, the capacitance between the shield and wire pair will couple in common-mode noise currents. These are not perfectly rejected by the thermocouple readout instrumentation amplifier and a part of it will mimic a fake differential signal. AS the TC voltage is so small, a small differential noise voltage will make a large impact on the reading.

If your setup is powered from an isolated switching converter, that usually means that its common-mode voltage with respect to Earth is something like 50-100 VAC at mains frequency.

If you don't touch the shield anywhere and just co-float it with the rest of the setup, it crudely-speaking "disappears" and you obtain approximately the behavior of an unshielded wire pair.

However, when you touch the shield or when you attach it to PE, you force the shield onto a different potential than the wire pair. In this case, the aforementioned large noise can appear.

If you are looking for a robust and shielded layout of your setup, I suggest you attach the shield to your system power supply negative rail. Just to be clear: I am talking about the negative supply of - in particular - the MAX adapter, i.e. its "Ground", which might be also the "Ground" of the arduino.