A good thermocouple should read approximately how many millivolts on a D.C. meter?

A good thermocouple typically produces an output voltage in the range of 30 millivolts on a D.C. meter when measuring temperature differences. Thermocouples function by generating a small voltage that corresponds to the temperature difference between the two junctions of dissimilar metals. This voltage output is generally very low, typically in millivolts, and for many types of thermocouples, around 30 millivolts is a practical and expected value when measuring a significant temperature difference.

While the other options represent lower and higher outputs, the 30 millivolts mark is recognized as a standard reading that indicates the thermocouple is functioning correctly and accurately reflecting the thermal conditions it is measuring. Values significantly lower or higher could indicate issues such as calibration problems, incorrect installation, or a malfunctioning thermocouple.