Thermocouples software linearization

[raul200020]

Hello,

I'm using a K type thermocouple which outputs milli-voltage. This voltage is amplified by using a DC voltage amplifier.

I measured the output of the amplifier using a voltmeter. The temperature was measured with a simple thermometer.

For a temperature of about 26deg.C I got 1.4V. For about 100deg.C I got 5.2V.  The amplified voltage values were sent to the USB9215A daq for analog voltage input.

I have a Labview application which works fine when reading the voltage from the daq but I have trouble with linearization.

Since we do not have a dedicated module for thermocouples, how can I implement the software linearization? Are there any vi's which linearize K type thermocouples?

I also have other available daq devices (NI6251,NIUSB6218, USB9233). 

Thank you. 

[aeastet]

You should have available on the panel thermocouple vi's see below. I am not sure if it matters what version of LabVIEW you are running.

[raul200020]

Thank you for your answer.

I found the linearization VI but I still have to read and understand the parameters I have to give  as inputs.

I'm still having problems understanding the CJC Voltage, CJC Sensor and Type of Excitation parameters. 

Do I need an extra sensor? I only have the thermocouple (2 wires) which goes in the amplifier. The amplifier output goes in the DAQ device. 

Have a nice day. 

[aeastet]

If you are using a National Instruments hardware for data collection then it may have CJC built in and all you would have to do is turn it on. If you are not then you might need a seprate channel for CJC. I would expect that you will need CJC to get accurate results.

What amplifier are you using? How accurate do you need to be?

[Wayne.C]

raul200020,

CJC stands for Cold Junction Compensation and you don't need to worry about it because of your external amplifier.  If you open up the block diagram of the thermocouple linearization vi, you will see that it uses a sub vi named Volts to Temperature.vi.  Use a copy of this in your vi to do the conversion.  Remember to scale the voltage into this fuction to compensate for the gain in your amplifier.

[raul200020]

Hello and thank you for your help.

I’ve tried to use the V->T.vi but the results are not as expected.

I placed my K type thermocouple in hot water and using a thermometer I measured

the temperature of the liquid. Using a voltmeter I measured the output of the

amplifier to which the thermocouple was connected. The following values are the

approximate correspondents: Temperature (Celsius) -> Voltage (Volts)

43 degrees -> 2.2 V

57 degrees -> 3 V

78 degrees -> 4 V

100 degrees -> 5.2 V

I used the indicated vi (as presented in the attached image) and I have a problem with

the results. I set the voltage to 3 Volts and the program multiplies this value in order to get microvolts. Then I see that the program chooses the option number 2 for temperature

calculation. As I understand this corresponds to the 500-1372 degrees range. Which is not my situation.

Can you give me any other suggestions? What am I doing wrong?

[Wayne.C · Accepted solution]

raul200020,

You need to account for the gain of the external amplifier you are usings.  Try this.

Message Edited by Wayne.C on 04-08-2010 09:04 AM

[raul200020]

Thank you for your answer.

If i take into account the amplifier gain, the program works just fine.

Thank you again and have a nice day.

[Wayne.C]

raul200020,

Thanks for the update.

[Henrik_Volkers]

raul200020,

To make correct TC measurements you need to measure the CJC (cold junction connection, hence the temp on the the other end of the TC 😉 )  temperature and do some compensation calculation. 

That might be done in an explicit TC amplifier, however a basic (&correct) understanding of the physics of a TC help to prevent errors. 

A good source for TC knowledge:
Manual on the use of thermocouples in temperature measurement,
ASTM PCN: 28-012093-40,
ISBN 0-8031-1466-4

(Page1): 'Regardless of how many facts are presented herein and regardless of the percentage retained,

                all will be for naught unless one simple important fact is kept firmly in mind.

                The thermocouple reports only what it "feels." This may or may not the temperature of interest'