NI 6356 Thermocouple measurement

[gmanuel]

Hello,

I am using a NI USB 6356 board and am trying to read in 3 thermocouples of type T.

I use this TC:

http://www.farnell.com/datasheets/303421.pdf

and have connected the Brown wire to AI+  and the White wire to AI-, with no connection to the AI GND.

I want to use one as a reference for the other two. However, after connecting the TC to the AI+ and AI-, all the readings start at a different (random) value and start to decrease or increase over time (I don't see a regularity in this). After 15-20 minutes without doing anything to the TC, the temperature has dropped/risen by approximately 15 degC.

I have tried to read in the voltage with LabView SignalExpress 2011  and  Matlab R2012a, and used the preconfigured Thermocouple-type input in LabView SignalExpress. The effect occurs every time.

Do you have any idea what the problem might be? The resolution of the board should be sufficient and self-heating of the TC does not seem realistic since the temperature changes continue even after a long time and the TC itself has a very fast time constant. Is it even possible to read in thermocouples accurately with this board or do I need an additional board/amplifier?

I'm running Win7 64-bit and have DAQmx 9.5.0 installed.

Thank you

Best

Manuel

[gmanuel]

Bump!

Does no one know an answer to this?
Could you at least tell me if I have the wiring correct?

[Julien_De_Freitas]

Hi Manuel,

You need to check your wiring. You are right you need to connect your brown wire to AI+ and your white wire to AI- (because you would like to measure in differential your signal). In that configuration you have to connect 2 resistors as its shown in this paper: Field Wiring and Noise Considerations for Analog Signals.

Please have a look at it:

Field Wiring and Noise Considerations for Analog Signals

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

For your measurement you are interested in the first case in the table 1 (Analog Input Connections) : Input Configuration = Differential (DIFF) and Signal Source Type (Floating Signal Source). Please carefully connect your signal source coming from your thermocouples

In this document (X Series User Manual NI 632x/634x/635x/636x Devices, page 62/297, Figure 4-6), you can find another description how to wire your thermocouple with the resistors.

Please have a look at this document:

X Series User Manual NI 632x/634x/635x/636x Devices

https://www.ni.com/docs/en-US/bundle/pcie-pxie-usb-63xx-features/resource/370784k.pdf

Let me know if it works better.

Regards,