To test a thermocouple loop, you’ll need to go back to thermocouple basics, more precisely to the three laws of thermoelectricity. In case you don’t remember, we have the law of homogenous materials, the law of intermediate materials, and the law of intermediate temperatures.

How to do a 4-20 mA thermocouple loop test

Here we need the law of intermediate temperatures. This law states that the sum of the electromotive force (EMF) from a thermocouple with its ends at temperatures T1 and T2 and another thermocouple with its junctions at temperatures T2 and T3 equals the EMF from a thermocouple between T1 and T3.

Confused yet? This picture should make testing a thermocouple easier.

3rd law thermocouple basics - How to test a thermocouple
Courtesy of HFO Power Plant

Now that you have the thermocouple basics needed to test a thermocouple, let’s move on to a more specific case. Say we want to test a loop by injecting a millivoltage in its compensating cable back to a substation and a converter card with cold junction compensation. Let’s make the field 30 degrees Celsius and the substation 10 degrees Celsius.

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Type K Thermocouple Ø1.5 mm with standard plug Thermoelementsensor mit Standardstecker und 1.5 mm Durchmesser
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K-S625-15

You’ll have to sum up the millivolts (mV) for each temperature from the sensor to the input card. You’ll also need the temp where you have the card.

To get the mV for each temperature, you’ll use a thermocouple reference table. If you can’t find your mV reading on the table, then you just do a little interpolation.

Don’t know how to read thermocouple charts? We have an article about that right here!

Thermocouple basics
Calculation for real temperature into input card – Courtesy of SENAI

You’ll find a minimal error margin most of the time. Take a look at this handy-dandy pic to give you a better idea of what to do!

Want to know more about Thermocouple loops, get in touch with us, please ask our engineers!

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