Thermocouple Temperature Shift

[TCNoob]

Hello there,

I am having an issue with my thermocouples type k. I have configured them using a "DAQ Assistant" by selecting the appropriate analog input, defining them as thermocouple type k, setting the cjc to "in built" and the "auto zero mode" to "every sample".

At the beginning the measured temperatures look quite promissing (about 21.8° in a lab where other Temperature Sensors show about 22°), but with time, or better said in the evening, these values shift to much higher values (about 24.x°). What can cause such an issue?

One guess is that the electronics (and the surrounding electronics), respectively the heat which they are emitting, is disturbing the cjc/reference measurement? Can such a behavior appear becaus of the age of the NI-Modules?

At the moment three modules (see list below) are mounted on the "mainboard", directly one next to the other. The board itself is mounted in an Electrical Control Panel.

As an absolute electronics and LabView beginner I do not really know what to try next and therefore would be happy for any suggestions!

The used Block Diagramm is attached. If you need further information or if anything was not explained good enough please let me know!

Regards,

TCNoob

Used Hardware:
-NI cDAQ-9188

Mounted on the above board:

-NI9211 (3 Thermocouples Type k: T-PreCooler, T-AfterCooler, T-ProdVessel)

-NI9207 (used for other signals, such as T-R and T-Set and Pressures in the attached Screenshot)

-NI9481 (used for a heater... not important for this problem)

[aeastet]

I'm not sure if you saw this but I would start here:

Using the Autozero ChannelThe NI USB-9211/9211A has an internal autozero channel for measuring the offset error. If the ambient temperature of the NI USB-9211/9211A is less than 15 °C or more than 35 °C, use this channel to read the offset error. The NI USB-9211 always has this channel enabled. Use NI-DAQmx to configure the behavior of the autozero channel for the NI USB-9211A. Refer to the LabVIEW Help for 8.0 or later, accessible from Start»All Programs»NationalInstruments»LabVIEW 8.5»LabVIEW Help, and search for Auto Zero Mode for more information

[TCNoob]

Thank you for your answer.
I have now tried several arrangements regarding the "auto zero" option.. which unfortunately made no difference. I guess that the "auto zero" option  will not be my solution, since ambient temperature (in the Electronic Control Panel) is about 26 degree.

I have further tried to use different (also shielded) connection cables, which did not improve my measurement either.
Is there a possibility to "convert" the cjc values to directly temperatures? 
I have come so far that i can read the voltage of the "_cjtemp" channel, but I am not sure if I am doing it correctly. Can anyone confirm or disapprove my approach?

Regards,

TCNoob