Thermocouples

Sometimes referred to as flame-failure devices, thermocouples consist of two dissimilar metals welded together at one end (hot junction).

When the hot junction is warmed, an electromotive force, or voltage, is produced, usually measured in mV.

This mV output is used to hold open an electromagnet assembly located somewhere within the gas control.

If the flame is accidentally extinguished (i.e draft, spillage, etc.) the thermocouple cools, and the produced mV output drops.

When the mV output drops, the electromagnet de-energises, and closes the gas valve, thus preventing any further flow of gas.