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I have a controller with two k-type thermocouple inputs. I'd like to have one of those inputs be the difference between the two TCs. Since TCs are voltage devices, it looks like an opamp would be perfect here. Maybe three: one for each TC, then one to compare the two signals and send the difference to the TC input. I've just never cared about microvolt levels before. Would this work?

The temperature ranges I will be measuring will be from 20C to 200C, they will never be negative.

I also need to have one TC still connected to the controller, so the circuit diagram below won't work for me, I don't think.

The controller is Japanese from 1997 and has no manual. It has K-type inputs.

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  • \$\begingroup\$ TC amplifiers are well defined for grounding, and reference metallic junction. Read the App Notes. Don't guess. \$\endgroup\$ Sep 4, 2017 at 23:03
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    \$\begingroup\$ What range of temperatures will you be measuring (absolute)? Same question for difference? What accuracy for the difference do you want to achieve? Please post a link to the manual for you controller. \$\endgroup\$ Sep 4, 2017 at 23:47

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[I have posted a comment with practical questions. In the meantime, here's a preliminary theoretical answer.]

Thermocouples measure temperature difference - inherently. The temperature difference can be obtained without an additional OpAmp.

enter image description here

An important nuance is to turn off ice point compensation in the controller.

Further reading:
Differential-thermocouple temperature measurement system
Differential Temperature Measurement with Thermocouples

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  • \$\begingroup\$ Note that your circuit will, in general, only work for the situation where T1 is at a greater temperature than T2 - standard thermocouple conditioners only work on positive inputs. \$\endgroup\$ Sep 5, 2017 at 4:05

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