DAQ assistant won't accept thermistor channel for thermocouple CJC

[sethlyman]

I'm using DAQ Assistant to measure temperature with a thermocouple.  My data aquisition device is a USB-6215.  I already have a thermistor hooked in, and it's channel in the DAQ Asisstant is called "thermistor."  I want to use the thermistor temperature as the cold junction compensation for the thermocouple, but I can't get the DAQ Assistant to let me.  I select "Channel" from the "CJC Source" box, and then I type "thermistor" as the channel name, and it won't accept it.  It says

"Error 200488 occurred at DAQ Assistant.
Possible reason(s):
Specified virtual channel cannot be added to the task because it does not exist.
You cannot specify a physical channel. Instead, create a virtual channel using DAQ Assistant or DAQmx Create Virtual Channel and then add the virtual channel to the task."

 I also tried "Dev1/ai7"as the channel name, but that won't work either. How can I get it to recognize my thermistor channel as the CJC?

Thanks for your help.

-Seth

[EEsRULEtheWORLD]

Hi, Seth.

Great question.  This error is usually a result of the local virtual channel that is used as a CJC channel appearing after the thermocouple channel in the channel list.  For example, I was able to generate the same error when I put my CJC channel (which I named "Thermistor") after my thermocouple channel (which I named "Thermocouple"), as shown:

 

When I put the "Thermocouple" virtual channel after the "Thermistor" virtual channel, the error was resolved, as shown:

Rule of thumb:  You always want to put the CJC virtual channel (in this case, "Thermistor") before the Thermocouple virtual channel in the list of channels.

If you have any more difficulties successfully configuring your CJC channel, do not hesitate to reply to this post.  I hope you're having an incredible day!

[sethlyman]

Okay.  I got DAQ assistant to accept the "thermistor" virtual channel as the CJC.  But it still isn't working correctly.  First, when I use "thermistor" as the CJC then the output in Labview for the Thermocouple (called "Tekrn334temp" in the attached schematic) goes to zero, and the thermistor output changes to the expected output of the thermocouple.  In other words, if CJC is set to constant the thermistor output is about 26 degrees C and the thermocouple output is about 15 degrees C, but if CJC in the thermocouple is set to the thermistor channel, the thermistor output goes to about 15 degrees C and the thermocouple output goes to zero. 

Secondly, if I select the "Calibration" tab in the "Thermocouple Setup" window of the DAQ Assistant and then push "Calibrate" it still gives me that same error message as before, which doesn't make sense, since it allows me to use "thermistor" as long as I don't push "Calibrate."

Also, if I make another thermistor virtual channel called "thermistorCJC" using the same physical channel as "thermistor" (ai7), so that "thermistorCJC" is identical to "thermistor", then if I use "thermistorCJC"as the CJC for the thermocouple, then the thermistor output stays in it's normal range (i.e. about 26 degrees C), but the thermocouple output is still zero.

I should also note that the thermistor I am using is atypical.  It is a Vex 3 wire thermistor, with external 4.85 V exitation and 1K ohms resistor.

Does this all make sense?  Can you help me with this problem?

Note that the attached Labview schematic  doesn't show the thermistor output from the DAQ Assistant because it is off the page.

Thanks.

-Seth

[EEsRULEtheWORLD]

Hi, Seth.

What pins do you have your thermocouple and thermistor connected?  When using AI7 for the thermistor, the positive terminal is connected to pin 31 and the negative connected to pin 32.  When using AI9 for the thermocouple, the positive terminal is connected to pin 18 and the negative connected to pin 26.  Verify that these are correct.

In the meantime, I'll escalate this issue to some of our DAQ experts to see if there is some small detail that I'm missing.  I hope you're having a great day!

[sethlyman]

Thanks for getting back to me.  You are right about the pins for the thermistor, but the thermocouple pins are 18 and 28.  Pin 28 is the universal ground.  For some reason it is measuring the thermocouple as single ended, not differential.  I tried to measure the thermocouple just as a voltage so I could process the signal after the fact in Labview, and for some reason I couldn't get the DAQ assistant to let me make a voltage virtual channel that was differential, either. 

Any more information you might have would be helpful.  Thanks..

-Seth

[EEsRULEtheWORLD]

Hi, Seth.

I apologize for the mistype.  You're right; it's pin 28.

Are you putting both the thermistor and thermocouple in an ice bath?  Have you tried just using the thermocouple and thermistor in your system and eliminating the other measurements in your task?

Could you attach your code, so I could take a closer look at how you're configuring your DAQ Assistants?

I hope you're having a great day!

[sethlyman]

Here is my .vi file.  It is in a zip drive with a .vi for computing arithmetic mean, which is necessary to run the program.  I tried moving the thermocouple to a different physical channel but it worked just the same.

-Seth

[aNIta B]

Hi Seth,

      I took a look at your code, but one thing that I'm still curious about (which I think Sara had also mentioned) is what are your results like if you strip everything except for that DAQ assistant (or test from a Test Panel in Measurement and Automation Explorer)?  What temperature are you expecting to read from your thermistor and thermocouple?  And finally, have you tried setting this up with the DAQmx API, rather than with the assistant?  

[sethlyman]

I tried eliminating pretty much everything from the labview program except for the DAQ assistant, but the problem was still the same.  I also tried it in Measurement and Automation Explorer, but with the same result.  I tried using the DAQmx "read" function, also, but the result was the same.

-Seth

[EEsRULEtheWORLD]

Hi, Seth.

I was successfully able to run your VI using my DAQ card without any errors.  If it is also failing in MAX, then this is not a software issue. 

I want to make sure that your device is functioning correctly.  Could you could connect a 5V battery (or some known voltage source) to your input channels and verify that your analog input channels are reading the correct voltage using Test Panels in MAX?