# Thermocouple compensation

#### Konstabel

Joined Jan 31, 2008
52
Does anybody have a nice schematic for cold junction compensation?

I'm using 3 parallel type E thermocouples.

#### hgmjr

Joined Jan 28, 2005
9,029
Maybe the information in the application note AN-28 will provide you with some ideas.

hgmjr

#### beenthere

Joined Apr 20, 2004
15,819
Omega Engineering will be only too happy to sell you one.

#### kender

Joined Jan 17, 2007
264
Maybe the information in the application note AN-28 will provide you with some ideas.
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.

#### Konstabel

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

#### scubasteve_911

Joined Dec 27, 2007
1,203
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

#### kender

Joined Jan 17, 2007
264
What will the connection digram look like if the compensation is done digitally?
Here's a block diagram for 3 ground-referenced Type K TCs.

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#### Konstabel

Joined Jan 31, 2008
52
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?

#### hgmjr

Joined Jan 28, 2005
9,029
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