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incorrect thermocouple reading

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I've got a NI PXI-1033 with a NI TB-2709 block and I'm running Labview 9. I'm simply trying to read the ambient temperature with a thermocouple from any of the eight channels. Unfortunately, when I look at the reading in the Measurement and Automation Explorer task menu the value oscillates around with the difference from the peak to valley of about 50 deg C. I've tested the thermocouple on a multimeter and it gives a good reading. What's wrong. Please help.
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Message 1 of 9
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What are you using for your cold junction reference? A multimeter has a built-in cold junction. The TB-2709 is just an analog input module - it has no cold junction, which is required for thermocouple measurements.
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I am trying to just use CJC - constant at a CJC value of 25 (The default setup).
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Message 3 of 9
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It looks like you might have a ground reference issue.  Check whether you have a differential signal or a ground referenced signal and check that the setup in MAX reflects this.  Otherwise, you are measuring a voltage with reference to another voltage that is not attached to anything.  You measure a strange signal, because there is always loose cross-talk and this unattached reference is loosely tied to something, but as charge moves around, this reference shifts.  There are some good knowledgebase articles about single-ended (referenced and unreferenced) and differential measurements on the ni.com site.
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If that thing is just an analog input module, you'll never read a bimetallic sensor voltage reliably. You need a thermocouple amplifier ("thermocouple input module" in NI-speak).

BTW, Isn't CJC indicative of an actual sensor on the cold end and not just a constant value?

Richard






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Message 5 of 9
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Solution
Accepted by topic author twalsh
Once I grounded the chasis to the shielding of the input cable the noise went away.
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Message 6 of 9
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Yes, the Cold Junction Compensation sensor is an active measurement.  Each dissimilar metal interface produces a voltage, including where the thermocouple wires meet the DAQ connector.  However, if you know the temperature and metals of a junction you can factor this out and know the voltage at the thermocouple tip; hence the need for a sensor at (though in reality it is just near, and the environment around it is assumed to be consistent) the DAQ connector. 

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Message 7 of 9
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I understand all that, but how is the OP reading T/C's with plain AI?
Richard






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Message 8 of 9
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What's the problem with plain AI? An oversampled 16 bit AI works fine with a TC.

 

Randy

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