I am having problems with temperature measurements made with FP-TC-120 (and FP-TB-3) and K-type thermocouples. Temperatures start to oscillate from normal ambient temp up to sky.
I am measuring temperatures from conductor loop which I am supplying with AC-current, having some 600 A, appr. 3 V. There are 7-8 temperature measuring points on different places of loop which are measured. I have tried with different FP-TC-120-modules and different K-type thermocouples.
It looks like that the temperatures will go crazy if the voltage difference between measuring points is more than 1 VAC? Is it really so?
One way to "fix" this problem is of course to use 2 modules for measuring and dividing the channels for those, but is there other ones?
Are the T/C's in direct contact with the coil? Is the coil insulated? How are you measuring the 1VAC difference between points?
Can you try just to have one T/C wired up to your system and probe at different points of the coil. Then try 2 connected and placed at different points. If just one works, but 2 yields bad readings then it sounds like ground loops or induced currents are swamping out your mV temperature signals.
At this point it is hard to tell which one may be the culprit- if the culprit is ground loops then you need isolation modules. If it is induced currents from the magnetic field of your coils, that's a tough one.
Let us know what you find.
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