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2750 AI8 oscilating measuremets above 975C

Customer is using T2750 AI8-TC with isolated TC's type S and N. Above 975C inputs starts a kind of oscilating. We have proven it is not the module or termination unit itselve. Customer is using a clean instrument earth. Any ideas how to investigate further to get to the root cause are welcome.
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  • Stephen Daly (Software Support Engineer) July 04, 2019 10:18
    Hi Frank,

    A few questions for you:

    - Does the value oscillate when you use a millivolt source instead of the thermocouples?

    - The AI8s have isolated channel pairs - how are the terminal units wired in terms of TC type? Ch1 = S, Ch2 = N, etc.?

    - How did you prove to yourselves that the problem is not to do with the module or the termination?
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  • Stephen thanks for your reply. We have 2 AI8-TC's one on slot 4 the other on slot 5.
    The error is on slot 4. Customer has exchanged the AI-8's between 4 and 5 the error stays on slot 4. Than they exchanged the termination units and again the error stays with slot 4. Than they have moved the termination unit and AI8 from slot 4 to 11 and the error moved to slot 11 aswell. I am suspecting a ground loop or a reduction in isolation of one of the thermocouples at high temperature. I am not sure if type S and N TC's are combined on one input pair.
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