How Does a Thermocouple Thermometer Work?

About Thermocouples

A thermocouple is a type of metal thermometer that can measure a tremendously wide range of different temperatures. While thermocouples are often not extremely precise, they can work in environments other thermometers can't. There are specially designed thermocouples that can measure temperatures to nearly absolute zero and others that can work in the hottest ovens.

How They Work

A thermocouple does not measure absolute temperature, but rather the difference in temperature between two points. When one end of a conductor, such as a metal strip, is hotter than the other, it creates a voltage between the two ends. The greater the temperature difference, the greater the current. Different metals react at different rates, and a thermocouple actually makes use of two metals, joined at the sensor end. At the circuitry end, they are attached to a meter that uses the difference in voltages between the metals to calculate the temperature differential.

Reference Temperature

Since a thermocouple only calculates the difference between the sensor end and the circuitry end, the sensor needs to know the exact temperature where the voltage is measured to make it work. Many thermocouples use a sensitive temperature sensor called a "thermistor" to calculate the temperature at the circuitry end. Others keep the circuit at a constant, known temperature through a cooling system. Still other thermocouple systems work well enough without correcting the temperature and don't bother with it. The thermocouple for a pilot light in a gas furnace, for example, only needs to be sensitive enough to detect the difference between the inside of a flame and normal room temperature, so it doesn't have to be very accurate.