Purpose of the Thermocouple

• A thermocouple is a type of thermometer that works in extremely high temperatures. It is used as a safety feature in pilot light furnaces. These furnaces use a small continuous flame called a pilot to light the gas when the furnace turns on. If the pilot light ever goes off, the thermocouple shuts off the gas. This stops gas from building up, potentially causing asphyxiation or an explosion.
  
  

What a Thermocouple Is

• When one end of a strip of metal is hotter than the other end, it produces a voltage. The greater the temperature difference, the greater the voltage. In a thermocouple, two different types of metal are attached together at the hot end. They are then connected to a circuit at the cold end. Because the types are different, they produce different voltages. The circuit can measure the difference between the voltages to tell how hot the hot end is.
  
  

How the Thermocouple Works

• The thermocouple hot end is placed in the flame of the pilot light, near the hottest part. The flame quickly heats the metal detector, creating a voltage. This voltage flows down the wires of the thermocouple to the detector circuit. As long as the pilot is on, the current continues to flow down the thermocouple and the gas continues to flow. If the pilot goes out, the thermocouple starts to cool and the voltage drops. This causes the circuit to send a signal to a small control motor called a solenoid, which closes the gas valve. The gas valve stays closed until someone manually relights the pilot light.