A flame sensing relay strategy that takes advantage of the ionization of flame is called

• A. Thermocouple
• B. Flame rod
• C. Ion sensor
• D. Cad cell

Answer: (B)

The idea being tested is sensing flame presence through the ionization produced in a flame. A flame rod is a small electrode placed in the flame; when the flame exists, ions in the combustion products conduct current between the rod and the circuit. The control system uses this ionization current as a proof that the flame is present; if ionization drops (no flame), the circuit detects the loss of current and can shut off the gas for safety. The other options rely on different mechanisms: a thermocouple detects flame heat to generate a small voltage, a cad cell responds to flame light, and an ion-sensor term is less specific to flame ionization. Thus, the flame rod is the device that takes advantage of flame ionization.