LabView Thermocouple Temperature Readings

[Zawer]

I'm using SCXI-1001 with SCXI 1102 and SCXI 1303 (LabView 8.2) to record temperatures from k-type thermocouples.  My problem is that the temperatures I'm seeing on LabView are off by approximately 5-10 degC AND they drift +/-10 degC throughout the day.  From what I've read, I'm thinking it might be related to CJC.  Right now I just have CJC Source set to "Constant" with a value of "25".  I can make adjustments to this to help with the offset, but the temperatures just drift throughout the day and the readings become incorrect again.  The room temperature does change quite a bit in the area where I have this setup, and the temperature drift seems to be related to that.  Any help or suggestions would be much appreciated. Thank You.

[RavensFan]

Yes, it is related to using a constant cold junction temperature value and the actual temperature of the junctions changing throughout the day.

Set your CJC to use the onboard thermistor rather than constant within your task.

[Zawer]

I have tried changing the CJC source from "Constant" to "Built-In" and the following error message comes up:

"Measurements: CJC Source cannot be set to Built-In for the
specified thermocouple channel.  The physical channel does
not support built-in CJC temperature sensor.

If your hardware contains a CJC temperature sensor on the
physical channel corresponding to the built-in CJC source,
make sure that the hardware configuration (including and accessories
and/or terminal blocks) is correct.  Alternately, specify a different
CJC Source, or use hardware with a built in CJC temperature
sensor."

[EEsRULEtheWORLD · Accepted solution]

Hi Zawer,

Your SCXI-1303 has a high-accuracy thermistor cold-junction temperature sensor, so the "Built-In" setting should be supported.  Try resetting the 1102 in Measurement and Automation Explorer and then restarting LabVIEW.  Is this error still being thrown?

[Zawer]

It works now.  The problem was that the 1102 was not properly configured in Measurement and Automation Explorer.  Thanks for the help.

[Broken_Arrow]

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EEsRULEtheWORLD wrote:

[...]

 Try resetting the 1102 in Measurement and Automation Explorer and then restarting LabVIEW.  Is this error still being thrown?

[...]

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I reset my 1102 pragmatically every time my applications start. I have had a few issues where the temperature will be "stuck" at a strange value a little above ambient upon startup, but since I started resetting all the time - no problems. I think it also helps with initialization, especially when people hit abort rather than doing a proper shutdown the last time the program was used - leaving the 1102 in an unknown state.

The reset certainly doesn't hurt anything, and is a piece of cake with the DAQmx Reset Device vi.

edit: Check out the funny spell check change.

Message Edited by Broken Arrow on 04-16-2010 03:28 PM