Multimeters can test thermocouple millivolts.
A thermocouple plays a vital role in protecting a building's occupants from dangerous situations that can arise from gas appliances such as furnaces. A thermocouple's function within a furnace is to allow the gas valve to stay open when it senses that the pilot flame is hot enough for the burner to sustain burning the incoming gas. It does this by sending millivolts to the gas valve only when the pilot flame is hot enough, which then enables the gas valve to stay open. When the flame is not hot enough to sustain burning because the furnace is turned off or not working properly, it closes the gas valve and keeps it closed. The millivoltage produced by the thermocouple can be measured with some common electrical tools.
Instructions
Measuring Millvolts on the Thermocouple
1. Clean the thermocouple with 000 steel wool to remove any loose sediment baked on by the flame. Any dirt will create obstructions that reduce the surface area used to transfer heat to millivolts. Be careful not to damage the thermocouple by making marks or scratches, because this will lower the effectiveness of the thermocouple.
2. Set the multimeter to read ohms or resistance to perform a pretest to see whether any other tests are needed. Place one test lead on the side of the thermocouple and the other on the end that would connect to the gas valve. A small resistance reading should be displayed, and you should hear an audio tone if continuity is achieved. High number readings suggest too much resistance and the thermocouple may fail to function properly now or in the near future. If no reading is visible, or if the multimeter displays OL(Open Loop), the thermocouple needs to be replaced and there is no reason to continue testing for millivolts.
3. Begin an open circuit test with the multimeter by first selecting the millivolts option. With one test lead on the side and the other at the cold junction end where it enters the gas valve, place a flame from a lighter at the opposite end. The reading on an open circuit test should read between 25 and 30. Less than 25 millivolts means that the gas valve will not receive, or will not consistently receive, enough millivolts to keep the solenoid in the gas valve powered.
4. Test the thermocouple under load with a closed circuit test. With the thermocouple adapter attached to the gas valve where the thermocouple would normally go, attach the thermocouple to the adapter and to where it secures at the flame. Use the crocodile clips by clipping one test lead to the side of the thermocouple and the other test lead to the screw that comes out of the side of the thermocouple adapter. Turn on the appliance and record the reading on the multimeter. The reading on a closed circuit test should read between 12 and 15 millivolts.
5. Discard and replace the thermocouple if it fails any of these tests.
Tags: circuit test, test lead, flame enough, lead side, side thermocouple, test lead side, thermocouple adapter