What type of pilot sensor is typically used on a standing pilot furnace?

The type of pilot sensor typically used on a standing pilot furnace is a thermocouple. A thermocouple is a reliable device that generates a small voltage when there is a temperature difference between its two junctions. In the context of a standing pilot furnace, the thermocouple is placed in the flame of the pilot light. When the pilot light is burning properly, the heat from the flame keeps the thermocouple hot, producing a voltage that signals to the gas valve that it is safe to keep the main gas supply open. If the pilot light goes out, the thermocouple cools down, the voltage drops, and this prompts the gas valve to close, thus preventing gas from leaking into the furnace and creating a potential hazard.

Other types of sensors listed would not perform the same role effectively in this context. For instance, a thermistor is typically used for temperature sensing in various electronic applications but does not provide the same safety mechanism as a thermocouple in a gas furnace. Similarly, photoelectric sensors are used for detecting light levels and would not be applicable to sensing flame conditions. Bimetallic sensors, while useful for temperature regulation, do not offer the specific safety features required for monitoring pilot flames in gas-fired appliances. Therefore