Thermocouple

What is a Thermocouple?

A thermocouple, is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage. This voltage can be interpreted to measure temperature. Thermocouples are widely used as temperature or flame sensors.

How to test?

If you find that the flame, monitored by a thermocouple isn't staying lit, or is going out after a short period of time. This is possibly due to a faulty thermocouple. Depending on the type of thermocouple and gas control used, will depend on how the thermocouple is tested.

Standard Gas Millivolt Thermocouple

Disconnect the thermocouple from the gas valve
using a multi-meter with clamp leads
place the red (+) clamp on the outer (Copper) of the thermocouple, anywhere along its length
connect the black (-) clamp to the end of the thermocouple, which has been removed from the gas valve
put your multi-meter on to mV DC (Millivolt Direct Current), this may show as the ⎓ symbol. You may need to change the range on your multi-meter for mV.
with this all connected, ignite the burner or pilot of the appliance, keeping any buttons pressed in to keep the flame lit on to the thermocouple.
with the thermocouple still in the flame, check the mV displayed on your multi-meter.

Bad Thermocouple	Good Thermocouple

For a millivolt gas valve, such as a Robertshaw, you should expect a voltage of over 20mV for the gas valve to work. For other controls, such as gas taps the voltage generated can go as low as about 15mV and would still work, however, it would be recommended to replace the part as soon as possible, before it stops working completely.