I'm using 4 MAX31855 ICs to measure 4 K-type thermocouples and realized that 2 of my thermocouples have grounded junctions; I simultaneously also realized the MAX31855 is stated to not support grounded thermocouples (returns a "nan" error). The 2 grounded TCs in my setup (of the 4 total) continuously give me errors, no matter which channel they're connected to - so clearly the issue is the grounded junction. Using non-grounded TCs for my specific application isn't a feasible solution since I need my device to be compatible with existing systems which use welded/grounded TCs. What options are there to read the grounded TCs in this kind of scenario? So far I've come up with the following:
- Modifying the circuit: I looked into what I could do to change my IC circuit and/or the TC setup, but haven't been able to find anything yet that works as a solution. The grounded TCs are welded to earth-grounded components which are grounded to prevent operators from getting shocks when servicing the units. Un-grounding these components is not a good option, and so far tricks to make the IC work with the grounded TCs haven't provided any success. Isolating my TC-measuring device isn't very feasible since I need to permanently run off the target system's internal power supply and also connect via USB to a laptop - it may be possible to isolate but likely not practical.
- Replacing the grounded TCs with Un-grounded TCs: This doesn't work as a solution because my TC-reading device needs to be able to work with existing system setups that already use welded grounded TCs which can't be swapped out without incurring large expenses.
- Alternative ICs: While looking for ICs that can read grounded TCs my search results are much more meager than I hoped. I'm still combing through, but any specifically recommended ICs would be great. Writing new code for a different IC wouldn't be ideal but it's a potential solution.
Below are examples of TC grounding configurations. I believe the thermocouple weld in my grounded TCs is like the middle configuration in this figure, though it's possible it may also be like the top option. My multimeter beeps during continuity check between ground and each side of the TC, implying a grounded junction weld, but I don't trust this as an "official" test since the resistance of each TC wire is rather low to begin with, so I suspect it may be possible it could be the top configuration in the figure: