PCI-4351 temperature reading problem

[Craig_APG]

I'm using a PCI-4351 with K-type thermocouples and a TBX-68.  The readings are correct (verified using a standalone reader) until any of the thermocouple readings exceeds about 315 C.  Then all of the connected thermocouples read wild values, both negative and possible.  All it takes is for one of the TCs to exceed 315 C to make all of the connected TCs exhibit this behavior.

What could be causing this?

[RT4CY]

Hello Craig_APG,

Thanks for posting on the forums!  A couple things to check are the following.

1) Ensure that you are not getting noise from the system.  One possible source is from whatever you are using to increase the temperature.  Review this article for other possible causes of noise.

2) Measure the voltage in Measurement & Automation Explorer (MAX) using Test Panels.  Increase the temperature past 315 degC and lef me know what voltages you see.

3) Try replicating the problem using only one input channel

4) Take measurements abovew 315 degC, and then lower the temperature.  Let me know if the reading stabilizes when the temperature decreases.

I also have some questions for you.

1) Have you seen this behavior before?

2) What are you measuring exactly (what is your temperature source)?

3) Do you have access to another PCI-4351 or TBX-68 you coud try?

4) Do you have access to other thermocouples you could try? 

5) Could you reply with the allowed temperature range of the particular thermocouples you are using?

Thanks Craig_APG, I look forward to your reply!

[Craig_APG]

Thank you for the reply.

I found a problem that was causing the strange behavior.  I had a heat tape that had shorted in a way that it was in electrical contact with the thermocouples in my system.  I don't know why they worked below 315 C, but the problem was definitely caused by that short.

After removing the faulty heat tape from the system, the temperatures are now reading correct values.

[RT4CY]

Hey Craig_APG,

I'm happy to hear you were able to figure out the problem!  Also, thanks for posting the solution.  This thread will likely help others in the community that may be experiencing similar issues.  Have a good one!