A good thermocouple should read approximately how many millivolts on a DC meter?

A good thermocouple should read approximately 30 millivolts on a DC meter, which indicates that it is functioning properly in a typical gas furnace application. Thermocouples work by generating a voltage in response to a temperature difference between two junctions made from different metals. The voltage output is proportional to the temperature difference, and for most thermocouples, a reading around 30 millivolts suggests that the thermocouple is effectively sensing the heat generated by the pilot flame or main burner.

This voltage range indicates that the thermocouple is capable of producing sufficient output to be used effectively in safety systems within gas furnaces. If the reading were significantly lower, it might suggest that the thermocouple is not being heated adequately or is malfunctioning, potentially leading to issues with the furnace's operation and safety mechanisms.