What type of sensor proves the presence of a pilot or main burner flame?

A flame rod sensor is designed specifically to detect the presence of a pilot or main burner flame. This type of sensor typically consists of a metal rod that is positioned within the flame. When the flame is present, it ionizes the air around the rod, allowing an electrical current to flow between the rod and ground. This current serves as an indication that the flame is lit, enabling the control systems to allow gas flow to the burner and maintain safe operation.

The other options do not perform this specific function. For example, a bimetallic sensor relies on the expansion and contraction of two different metals to detect temperature changes but does not respond to the presence of a flame. A mercury switch operates based on mercury's movement within a sealed glass tube, typically used for detecting tilt or liquid levels rather than flame presence. Lastly, while thermocouples can indicate temperature changes and are used for safety interlocks in some systems, they do not specifically sense the presence of a flame in the same way a flame rod does.