How Thermocouples Work

A thermocouple operates on a principle discovered by Thomas Johann Seebeck in 1821. He observed that when two dissimilar metals are joined together at two points, and a temperature difference exists between these points, a voltage is generated.

"The voltage generated is proportional to the temperature difference between the measuring junction (hot) and the reference junction (cold)."

This phenomenon is called the Seebeck Effect. It is the basis for all thermocouple temperature measurements. The voltage produced is very small, typically measured in millivolts (mV).

A standard thermocouple circuit consists of two main junctions:

• Hot Junction (Measuring Junction)This is the joined end of the thermocouple that is placed in the environment where temperature needs to be measured.
• Cold Junction (Reference Junction)This is the end connected to the measuring instrument. The instrument must know the temperature of this junction to calculate the accurate temperature at the hot junction.

The voltage generated by a thermocouple is not linear with temperature. This means that a 1°C (1.8°F) change at 100°C (212°F) produces a different voltage change than a 1°C (1.8°F) change at 500°C (932°F).

To handle this, modern thermocouple instruments use complex algorithms or look-up tables (standardized by agencies like NIST) to convert the millivolt signal into a readable temperature value.