Thermocouple readings not accurate at 0 deg C

[cjlandry]

We are using an NI-PCI-6032E Multifunction DAQ and an SCC-68 connector block.  We have configured 6 thermocouples on various AI inputs setup in differential mode using the built in CJC.  The CJC is wired to its own AI input per a tech note from NI.   At room temperatures all 6 thermocouples measure within 2 to 3 degrees of each other as well as the CJC. However, when I put each thermocouple in a glass of cold water the temperature reading in the VI is 7 to 9 deg C.  I compare this with a calibrated digital thermometer we have and it is reading -1 to 0.5 deg C.  What gives? 

I have been reading a lot of tech notes on the NI website and tried many different scenarios... such as creating a virtual channel using the CJC scale.  Nothing changes.

For testing purposes, I created another VI using a two thermocouples that are setup differently.  One thermocouple is read in using a signal conditioner from Omega Engineering (Super MCJ provides 1mv/deg C) and the other is setup like my 6 thermocouples above (using DAQ Assistan, Type K, built in CJC)  Any insight into this would help.

Thanks,

CJ

[BROhan]

Hi CJ,

I would suggest making sure that your input range for the temperature measurements is set to 0-100mV on your PCI-6032E. If the range is on the default -10 to 10 V, you will not get the maximum accuracy. If this is already done, then you may want to calibrate the device within the DAQ Assistant.

[cjlandry]

Thanks Rohan.  Performing the calibration helped a lot.  I heard that you can send in your PCI-6032E to NI and have them calibrate it. I'm going to inquire about this.

Thanks again BROhan!!

CJ

[BROhan]

Glad to hear it, CJ! You can check out Calibration Services for some more calibration information.