To test a thermocouple loop, you’ll need to go back to thermocouple basics, more precisely to the three laws of thermoelectricity. In particular, 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.

thermocouple test diagram
Courtesy of HFO Power Plant

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

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

You’ll find a minimal error margin most of the time.

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