# Thermocouple compensation

Discussion in 'The Projects Forum' started by Konstabel, Feb 16, 2008.

1. ### Konstabel Thread Starter Active Member

Jan 31, 2008
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Does anybody have a nice schematic for cold junction compensation?

I'm using 3 parallel type E thermocouples.

2. ### hgmjr Moderator

Jan 28, 2005
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214
Maybe the information in the application note AN-28 will provide you with some ideas.

hgmjr

3. ### beenthere Retired Moderator

Apr 20, 2004
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Omega Engineering will be only too happy to sell you one.

4. ### kender Senior Member

Jan 17, 2007
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If you have an A/D in your system, you can measure the temperature of the cold junction with one of the A/D channels and do the actual compensation in the digital domain.

5. ### Konstabel Thread Starter Active Member

Jan 31, 2008
52
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What will the connection digram look like if the compensation is done digitally?

6. ### scubasteve_911 Senior Member

Dec 27, 2007
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Konstabel,

To compensate digitally, two methods are generally used. If you have plenty of memory, then a lookup table is generated. So, you read a particular value, then it corresponds to the actual temperature. Another method is polynomial fitting, which uses much less memory, but requires processing. Basically, you can plot in MATLAB and then use the curve-fitting tool to find the necessary order and coefficients for the polynomial fit. There are other ways to do this, including on paper. Software is the way to go though

Steve

7. ### kender Senior Member

Jan 17, 2007
263
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Here's a block diagram for 3 ground-referenced Type K TCs.

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8. ### Konstabel Thread Starter Active Member

Jan 31, 2008
52
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Thanks for all the replies.

Scuba just to make sure. If doing the compensation digitally, the fluctuation in room temperature and thus the cold junction temp, is not taken into compensation, right?

Thanks for the diagram kender. The 'pizza slices' in your diagram, are the opamps?

9. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Yes. The "pizza slices" represent opamps. You will need to elaborate on their design with additional components since the symbols are just there to show the need for gain yet there are no gain setting components shown.

hgmjr