How Thermocouples Work

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How Thermocouples Work

 

A common way engineers measure temperature is with a thermocouple. Unlike thermometers which use pressurized liquid, thermocouples create an electrical signal which can be converted to a temperature reading. Two dissimilar metals are connected together at the measurement point and the same two at the reference point. Different metals, when heated, absorb energy at different rates when in contact. The contact creates an electrical gradient between them and starts an electrical current between the measurement point and a reference point. The reference point is generally at room temperature, for the most accurate readings it should be at a known temperature.

 

Any two dissimilar metals when subjected to thermal gradient will generate a voltage, called the thermoelectric or Seebeck effect, named after Thomas Johann Seebeck who discovered that any two metals in contact will create a voltage potential. Seebeck was a Baltic-German physicist who in 1821 happened upon this discovery after using a compass near two heated metals and the needle deflected in response to the metals. Although the term thermoelectric was not coined by Seebeck he was first to have recorded the effect.

 

Thermocouple systems are more expensive than traditional thermometers, but cheaper than other electrical temperature measurement systems. The advantage in using thermocouples is they have a higher temperature range, readings are quick, measurements are fairly precise, and they are relatively inexpensive. In comparison, other systems such as the resistance temperature detector, or RTD has a lower temperature range, it’s slower to stabilize readings but readings are stable and repeatable. Cost, durability, and variability make thermocouples a common thermal measurement device for many applications.