For many gas appliances, the thermocouple is a critical safety feature. A thermocouple creates a millivoltage that powers a gas valve using the energy from a fire. If the pilot drops out, the thermocouple will no longer be able to deliver the millivoltage needed to keep the gas valve open, closing the valve to avoid a harmful gas leak.
Tips for Testing the Thermocouple
The thermocouple must be separated from the unit. Ensure compliance with all security protocols. Be sure you can read all ohms/resistance and millivolts from the multimeter to be used.
To check the thermocouple, pick a sample. The three key measures used are the test of open circuit, the test of closed circuit and the test of resistance. While all three are useful, the closed-circuit test will clearly indicate whether the thermocouple is performing under load properly and is often the primary test performed.
Pick your multimeter’s ohms or resistance options. Place one lead on the side of the thermocouple and the other to the end of the gas valve which screws. If the thermocouple has good consistency, the multimeter will show a low resistance reading. The consistency with an audio warning is also demonstrated by many multimeters. If the multimeter reading shows an “OL” reading, the thermocouple has no stability and is therefore not in function. A reading of high resistance, such as 40 ohms, suggests that the resistance will cause problems and should discard the thermocouple.
To run an open circuit check, transform the multimeter to millivolts. Place on the side of the thermocouple one of the multimeter sample leads and the other to the end of the gas valve. Ignite on the opposite end of the thermocouple a torch, or other energy sources with a spark. Thermocouples were built for 30 Millivolts, equipped for domestic gas appliances, such as fireplaces, water heaters, and furnaces. The reading will vary from 25 to 30. Anything under or around the 20 millivolt limit means replacing the thermocouple. If the reading falls between 21 and 25, judgment should be used.
Use the thermocouple adapter to conduct a closed circuit test to achieve a true performance under load. Using crocodile clips tied to the sample ends, this experiment becomes easier to perform. Firstly, the thermocouple connector is mounted to the gas valve at the place of the thermocouple. Instead, add the thermocouple as usual with the exception of screwing into the connector the end that would screw into the gas valve.
Switch the device on and put a multimeter crocodile clip to the thermocouple’s hand. Connect to the screw the other crocodile clip sticking out of the adapter’s hand. The reading is predicted to be in 12 millivolts and 15 millivolts. The thermocouple is faulty if it is less than 12 millivolts.
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