What reading should a properly functioning thermocouple produce on a DC meter?

A properly functioning thermocouple should produce a reading of approximately 30 millivolts on a DC meter. This voltage output is typical for thermocouples because they generate a small voltage in response to temperature differences between their junctions. This small voltage is directly related to the temperature they sense; different types of thermocouples will have different output characteristics, but around 30 millivolts is a common benchmark under standard conditions for many thermocouples used in gas furnace applications.

The voltage output is critical for the operation of safety mechanisms in gas furnaces, where the thermocouple helps ensure that the gas valve remains open only when there is a sufficient heat source (such as a pilot flame). If the thermocouple generates too low a voltage, it may not signal a proper heat source, resulting in the gas valve closing and no combustion occurring, which is a safety feature to prevent gas leaks. Understanding these voltage readings is essential for diagnosing issues with thermocouples in gas furnace applications.