Thermocouples are used to indicate the temperature of what?

• A. The surrounding air
• B. The boiler water
• C. The fuel gas
• D. The metal to which its attached

Answer: (D)

Thermocouples sense the temperature at their junction, and that junction is what the sensor is in contact with. When a thermocouple is attached to a metal surface, its reading reflects the temperature of that metal. It doesn’t directly measure the surrounding air, the boiler water, or the fuel gas. Those media require sensors placed in or exposed to them, but a thermocouple used this way reports the temperature of the metal to which its tip is attached.