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Thermocouple connected to one channel and AI GND wrong reading while connected to two channels is OK

Hi everybody.

 

When I connect the thermocouple to two channels, lets say AI 6 and AI 14, the reading is alright, but when I connect the thermocouple to AI 14 and AI GND, the reading is shifted 12 Celsius degrees. They react always to change of temperature, if I touch them the temperature increases as expected. Although in the thermocouple connected to AI GND the value is around 12 degrees higher.

 

This situation is repeated with different channel combinations, for example: the reading using AI5 and AI13 is alright but the reading using AI13 and AI GND is shifted again the same value. If I connect only one thermocouple or several thermocouples this situation happens. To me, it looks like the AI GND is not a good reference for the values of the different channels. Why?

 

I tried to selfcalibrate the instrument PXI-6220 and the measurement improved, but the shift remained exactly the same. The measurement improved because the temperature is now 3 degrees closer to the real one but the shift did not change at all between both thermocouples or connecting one thermocouple alone.

 

I am using the DAQ assistant express in Labview 7.1, MAX is version 5.0.0f1. I have a chassis PXI-1033 with a PXI-6133 Simultaneous series and a PXI-6220 M series that is connected to the Terminal Block SCC-68. I am using a constant temperature for the cold junction of 18 Celsius degrees. The DAQ assistant express is defined as thermocouple type K with constant temperature in the cold junction. I use the default connection for the physical channel given by the DAQ-assistant.

 

I think that either the GND or the level of the AI channels is shifted. but I am not sure if this is the problem. I would appreciate any help. Thanks.

 

Gonza

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Hey Gonzo

To correctly use cold junction compensation (CJC) for you thermocouple you need to use a differential configuration. this is where you were wiring AI6 and AI14 for example. CJC can not work correctly when you connect to ground hence the strange reading.

Field Wiring and Noise Considerations

Connecting Thermocouples to a Data Acquisition (DAQ) Device

I have linked a couple of pages that should help explain how to correctly configure your system

hope this helps.

 

Matt

Matthew Trott
Applications Engineer
National Instruments UK
www.ni.com/ask
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