What is a Thermocouple?

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In HVAC we work with thermocouples for temperature measurement all the time and we may not understand the difference between a thermocouple and other types of temperature measurement sensors.

A thermocouple is just two different types of metal connected together at a “junction” that generate a small voltage with a change in temperature. Older furnaces would often use a thermocouple as a sort of flame sensor that would be immersed in the pilot flame and would generate a small control voltage that would “prove” that the pilot flame was lit. You will still often see this sort of millivolt control system on water heaters, fireplaces and some pool heaters.

We often use a “K-type” thermocouple for line and air temperature measurements. This thermocouple uses one wire made of a chromium/nickel alloy called chromel and another made of an aluminum/nickel alloy called alumel. The voltage across this junction changes as the temperature changes and the thermometer is calibrated to track this voltage change and display a temperature readout.

A thermocouple can often be confused with a thermistor which is a resistor that changes in resistance with a temperature change but they do not function in the same way at all.

Thermocouple sensors must be calibrated to the thermometer or meter they are plugged into using and ice bath to be accurate and they tend to be slightly less accurate than thermistors.

— Bryan

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