Wide variation on thermocouple readings

[Brad G]

I have K-type thermocouples (two long wires twisted and soldered together at the end) connected to an SCC-68 board (no modules). This board connects to a PXI-6221 multifunction DAQ with CJC compensation.

I've setup a global virtual channel for the CJC, connected it according to the diagram in the user manual, etc. etc. - that all seems to work fine. When I connect my thermocouples, however, their readings are very sporadic; they might jump from 69 deg F up to 89 deg F or more.

Any suggestions as to how I can get a more even reading? Honestly, I don't care if it jumps around a few degrees, but 10-20 degree variations seem a bit much.

I've attached a copy of my VI (LabVIEW 8.2) that I'm using to view the temperatures (it's part of a larger control system, so ignore everything except the two temperature zones on the left :P). 

[Brad G]

Quick update..

I've gone through the Calibration Wizard in the DAQ Assistant and the values it shows (e.g. the Uncalibrated column) are actually quite steady - they only vary by about 1 degree Fahrenheit.

I tried using the same sampling rate (1kHz) and number of samples (1k) in DAQ Assistant and yet the values it shows still vary wildly. I'm very confused at this... why would the values shown in the Calibration wizard be fine yet the values shown in the DAQ Assistant task are way off??

[Henrik_Volkers · Accepted solution]

Do you measure in diffential mode?

How does the signal look like ? Hum , spikes, white noise ?

How about using a filter? Fist try: sample with 1kS/s and use the mean of 300 samples (300ms work for 50Hz and 60Hz power line noise)  

Message Edited by Henrik Volkers on 01-28-2010 09:10 AM

[Brad G]

Thanks for the tip; using a higher sampling rate and taking the mean gave a much steadier/accurate reading!