Temperature Measurement Error

[Controls_Engineer]

I am using NI 9206 in a Compact DAQ Chassis to measure temperature. K type thermocouples are sued to measure the tempearature. I have created a task in NI MAX to read the temperature from the thermocouples, but the readings are incorrect. At room temperature, the readings are off by 2- 5 deg C but at higher temperatures ( 100 - 2000 deg C) the readings are off by more than 100 deg C.

Can someone help to fix this problem, and help make the correct temperature measurements.

I am using (206 instead of any thermocouple card, because I neede more number of channels. I am onnecting all the negatives to the common grond of the NI 9206.

Thanks.

[K C]

With Thermocouple measurement you also need a cold junction compensation. If you use a module just to measure the voltage it will be wrong !!

[Joseph_Loo]

Are you connecting the thermocopule wires directly from the heat source or go through an intermediate box? If you go through and intermediate box, make sure it is specific for k thermocouples, otherwise it will affect the reading because of disimilar metals.

How are you connecting the thermocouples to gound? This could cause another dissimilar meta match.

I am not sure how NI MAX does the temperature conversion, but does it do 2 or 3 order compensation on the voltage readings?

[Controls_Engineer]

I am directly connecting the thermocouples to the screw terminals on the NI 9206. But I am using long thermocouple extension cables. All the  negative terminals are connect to the common ground by using small pieces of normal 22 gauge single stranded hook up wire.

For cold junction compensation, I was using the "Constant value" option in the DAQmx Create Virtual Channel VI for AI temp TC option and was providing the room temperature as the constant value. Do you think this cold junction compensation is not working and that is what is creating problems? What other options can you suggest for cold junction compensation, as I am not using a card that has in-built cold junction compensation?

Thanks.

[Joseph_Loo]

I hope the wire you are connecting is the same material as the terminal. If yu do not, there is a voltage variation that occurs between the thermocouple wire, which will throw the values off. You can get a brief description or tutorial associated with the web stie. As for a constant, the cold junction compensation, it is a thermistor or some temperature measuring device that is used within the calculation of the temperature computation. My understanding is that when you have large temperature changes, it is not as important. It will definietly play a big role at room temerature.

The extension cable is wire sensitive. You have to make sure that you are using k thermocouple extension cable and you hook it up to the correct wire.

You did not mention if you are using the 2nd or third order correction equation when reading the temperature or MAX takes care of the equation. I am not familiar with that particuliar device and the associated MAX setup.