Purpose of the Thermocouple

• A thermocouple is basically a metal thermometer used to measure extreme temperatures. In an old-style gas furnace with a pilot light, the thermocouple is a crucial safety device. The pilot light stays burning all the time, using a small trickle of gas. If the pilot light goes out and the furnace turns on, gas will build up. This can cause an explosion or asphyxiate the people in the building. The thermocouple is designed to stop this from happening.
  
  

How a Thermocouple Works

• If one end of a piece of wire is hotter than the other end, a current will flow between the two ends. The bigger the temperature difference, the more electricity flows across the wire. Each metal will produce a different amount of voltage. In a thermocouple, two different metals are welded together at the hot end and attached to a circuit which detects voltage at the cold end. By measuring the voltage coming from the thermocouple, the circuit can detect whether the hot end is significantly hotter than the surrounding air temperature.
  
  

Putting it All Together

• The tip of the hot end is placed right in the middle of the pilot light flame--usually in the hottest part. When the pilot light is on, the heat from the flame creates a voltage in the thermocouple, which is detected by the circuit. If the pilot light goes out, the hot end starts to cool very quickly, and the voltage drops. The circuit immediately detects this drop in voltage and triggers a small control motor called a solenoid. The solenoid closes the gas valve, effectively shutting off the flow of gas to the furnace. The gas valve will stay closed until someone lights the pilot light again, making the furnace safe to use.