What type of flame does a thermocouple detect?

A thermocouple is a sensor that detects temperature by generating a voltage based on the difference between the temperature of its two junctions. In the context of gas appliances, it is primarily used to monitor the temperature of a pilot flame. The pilot flame is a small, continuously burning flame that ignites the main burner when needed. The thermocouple ensures that the pilot flame is lit and functioning properly; if it goes out, the thermocouple will cool down, causing the voltage to drop and triggering a safety mechanism to shut off the gas supply.

While the main burner flame is important for the operation of gas appliances, it is typically monitored through other mechanisms rather than directly by a thermocouple. Therefore, the role of a thermocouple is specifically associated with detecting the presence and stability of the pilot flame to ensure safe operation of gas systems.