Multifunction DAQ

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incorrect thermocouple readings

Problem: Incorrect thermocouple readingsSystem Configuration:DAQ card: PCI-MIO-16E-1SCXI chassis: SCXI-1000SCXI-1102 installed in slot 1 with SCXI-1303 terminal blockSCXI-1180 feedthrough in slot 2 with SCXI-1302 terminal blockInstalled in a HP PC running Windows XP SP3.Thermocouple: J-type connected to AI-ch0 of SCXI-1303 terminal block, red wire to -, white wire to +. TC junction is in the air at room temp.Using LAbVIEW 8.5 and the DAQ Assistant Express VI, DAQ Assistant settings are as follows:Virtual channel: Temperature, physical channel SC1Mod1/ai0, device type SCXI-1102            Signal input range: max 100, min 0            Scaled units: deg. C            TC type: J            CJC source: Built In            Acquisition Mode: Continuous Samples            Samples to Read: 1            Rate (Hz): 1When run, the readings at room temperature are about 182 deg.C. and do not vary when the TC is warmed.  Actual ambient is about 24 deg.C. Any suggestions as to where the problem might lie?



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Hey John


Thanks for posting on the forums!  Sometimes, SCXI issue can be tricky, so I would like you to try the following action items so that we can narrow down the issue.


1) Use a constant CJC value of 24 degC instead of built-in
2) Use a different ai channel

3) Remove the feedthrough panel so that all you have in the SCXI chassis is the 1102/1303


Also, it would be very helpful if you could try and verify that the DAQ card is working  properly.  I suggest putting a known voltage on one of the ai channels of the DAQ card directly.  This means removing the SCXI system from the equation.

If possible, it would also be helpful if you could try using different hardware (just to see if we have any damaged components).  So anything from a different chassis, 1102, 1303 terminal block.  I realize that this might not be possible, but I just want to put that out there as a suggestion. 

A side note: Use Measurement & Automation Explorer to do the troubleshooting steps above.  When reading a simple voltage, you can right-click on the device and choose Test Panels.  You can also create a temperature task here by right-clicking Data Neighborhood and choosing Create NI-DAQmx Task.

Let me know if the above troubleshooting steps shed some light on the problem.  It's ok if you don't have extra hardware, but I'm very interested to see the results of the action items and whether the DAQ card can read a straight voltage properly.  Thanks John, look forward to your next post!

Rod T.
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