How Does a Thermocouple Work?

By G.K. Bayne

Where are They Used?

Thermocouples are widely used in manufacturing and industry whenever precise temperature control is required. They are rarely incorporated in the residential home, although newer HVAC units are utilizing them in a few of the more efficient models of combustion burners for the home market. Because they are so precise in reading the temperature differences down to 0.1 of a degree, the thermocouple can save energy by not having the heating or cooling activate with such a wide margin of inaccuracy.

How do Thermocouples Work?

Most thermocouples are all manufactured in the same way and operate on the same principle. They use dissimilar metals that when heated or cooled will generate a small voltage or current. The voltage is called a discreet signal. This voltage is so small most volt ohmmeters are not equipped to read the value. The voltage is in the mille volts or .001 volts. The same for current generated thermocouples they register in the mille amps or .001 amps.

Are There Ranges in Voltage?

Some models of the voltage thermocouples are fed a voltage in the range of 0 to 5 volts and feed back a higher voltage, but with a discreet change in the signal, generally again in the .001-volt range. Current generating thermocouples have a range typically, in the 4 to 20-mille-amp signal. Where the 4-mille amp is the low end and the 20-mille amp is the high side of the signal. You might also this written as 4ma to 20ma.

How are Thermocouples Made?

The thermocouples are constructed with very thin wires most in the diameter of .010 inches. They are then welded at the ends to form a circuit or conduction path. These small wires are encased in a metal tube and electrically insulated from the outer tube. Generally they are immersed in a heat conducting liquid or paste so the environment in which they are placed can be read, temperature taken and a small voltage or current generated from the difference of the two wires.

How is the Signal Interpreted?

The signal or voltage from the thermocouple is then fed into a controller which interprets the signal. Generally it amplifies the signal so it can be used to control the device that is either heating or cooling.

What Makes Thermocouple Wiring?

Many companies use proprietary substances in the thermocouple wires. So much so that if you attempt to take one apart to see the material used they will generally be ruined. Also the conducting substance can be toxic to your skin and or health. It is never recommended to take a thermocouple apart.