Otis Singleton
A thermocouple is a safety device that helps control the gas supply in gas-heated furnaces. When it stops working, the furnace's pilot light turns off. To test a non-electric thermocouple, you will need a multimeter that can measure millivolts and ohms/resistance. The multimeter should be set to volts and millivolts, and the thermocouple should be placed in a flame or heated with a lighter. The thermocouple should reach a temperature of 25-35 millivolts, and the multimeter should display a reading between 25 and 35 millivolts. If the reading is below 25 millivolts, the thermocouple needs to be replaced.
Testing a Non-Electric Thermocouple
| Characteristics | Values |
|---|---|
| Basic test | Attempt to turn on the pilot light. If it doesn't stay on, the thermocouple has failed. |
| Multimeter test | Use a multimeter to test the leads on the thermocouple when on and off the pilot setting mode to see if they maintain a charge. |
| Multimeter type | Use a multimeter that can read both ohms/resistance and millivolts. |
| Voltage | A thermocouple should read between 25 and 35 millivolts. |
| Resistance | A thermocouple should exhibit very low resistance. |
| Open circuit test | Place one test lead on the side of the thermocouple and the other on the end that enters the gas valve. Ignite a flame at the opposite end. |
| Closed circuit test | Attach the thermocouple adapter to the gas valve, then attach the thermocouple. Turn the appliance on and place a crocodile clip from the multimeter to the side of the thermocouple. |
| Resistance test | Place one test lead on the side of the thermocouple and the other on the end that screws into the gas valve. |
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What You'll Learn
Use a multimeter to test for continuity
To test a non-electric thermocouple with a multimeter, you must first ensure your multimeter can read both ohms/resistance and millivolts. The thermocouple is a vital safety feature for many gas appliances, so it is important to test it regularly.
To begin the test, disconnect the thermocouple from the gas valve. Then, select the ohms or resistance option on your multimeter. Place one test lead on the side of the thermocouple and the other on the end that screws into the gas valve. If the thermocouple has proper continuity, you should see a small resistance reading on the multimeter. Many multimeters will also indicate continuity with an audio alert. If the reading on the multimeter indicates an "OL" reading, the thermocouple does not have continuity and needs to be replaced. A high resistance reading, such as 40 ohms, also indicates that the thermocouple should be discarded.
Next, turn your multimeter to millivolts to perform an open circuit test. Place one of the test leads on the side of the thermocouple and the other on the end that enters the gas valve. Ignite a flame, or another heat source, at the opposite end of the thermocouple. A properly functioning thermocouple will generate between 15-35 millivolts when heated. If the reading is below 15 millivolts, the thermocouple must be replaced.
You can also perform a closed circuit test using a thermocouple adapter to achieve a true performance reading under load. Attach the thermocouple adapter to the gas valve in the thermocouple's spot. Then attach the thermocouple, screwing the end that would normally enter the gas valve into the adapter. Turn the appliance on and place one test lead on the side of the thermocouple and the other on the screw that extends out of the adapter. The reading should be within 12-15 millivolts. A reading below 12 millivolts indicates that the thermocouple is defective.
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Check the thermocouple's millivolt range
To check the millivolt range of a thermocouple, you will need a multimeter that can read millivolts. A thermocouple will generally produce a reading of around 30 millivolts, but this can vary between 25 and 35 millivolts. If the reading is 15 millivolts or lower, the thermocouple must be replaced.
To test the thermocouple, first disconnect it from the gas valve and connect the multimeter, setting it to millivolts. Then, apply heat to the tip of the thermocouple and check the multimeter reading. The thermocouple should be allowed to heat up for 30 seconds to one minute to obtain an accurate reading. If the thermocouple is functioning properly, the multimeter should display a reading of between 25 and 35 millivolts. If the reading is in millivolts, it should fall within this range. However, if the multimeter only displays volts, look for the meter to move slightly above 0.
It is important to ensure that the tip of the thermocouple is directly in the pilot flame during testing. If the thermocouple is producing a reading above 25 millivolts, but the pilot flame is still not staying lit, there may be an issue with the gas valve.
Additionally, when using a multimeter, it is important to test the device by holding the leads together and pointing them in opposite directions. When held in opposite directions, the meter should stay to the left at infinity. Bringing the leads together should cause the meter to change to 0.
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Perform an open circuit test
To perform an open circuit test on a non-electric thermocouple, you will need a multimeter that can measure millivolts. The test will tell you the amount of voltage the thermocouple is capable of producing without any load, or when it is disconnected.
Firstly, disconnect the thermocouple from the valve by unscrewing the connector nut with a wrench. Next, attach the leads of the multimeter to the thermocouple. Place one test lead on the side of the thermocouple and the other to the end that enters the gas valve or screws into the pilot-on-off knob. Ensure that your multimeter is on the correct range, and ignite a flame at the opposite end of the thermocouple.
A reading of 18 to 30 mVDC should be expected. A reading of 25 to 30 millivolts indicates that the thermocouple is functioning properly. If the reading is under or around 20 millivolts, the thermocouple needs to be replaced. If the reading is between 21 and 25, use your discretion.
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Perform a closed circuit test
To perform a closed circuit test on a thermocouple, you will need a multimeter that can measure millivolts DC (mVDC). This can be done by using the "Range Button" or by looking at the third decimal over on the DC voltage reading. Set the multimeter to measure Ohms or resistance.
The closed circuit reading requires a special adapter that screws into the magnet assembly to allow the meter to connect. The Robertshaw P/N: 10-038 thermocouple Test Adapter is one such adapter. The adapter is attached to the gas valve in the thermocouple's spot.
Once the adapter is in place, place one test lead on the side of the thermocouple and the other to the end that screws into the gas valve. If the thermocouple has proper continuity, a small resistance reading should be visible on the multimeter. Many multimeters also indicate continuity with an audio alert. If the reading on the multimeter indicates an "OL" reading, the thermocouple does not have continuity and is therefore not usable. A high resistance reading, such as 40 ohms, suggests that the resistance will cause problems and the thermocouple should be discarded.
A rule of thumb is that the closed circuit reading should be roughly half of the open circuit reading. With a proper flame, the reading should be about the same as the previous reading. With a lifting main burner flame or excessive drafts, the reading may be reduced.
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Check the thermocouple's resistance
To check the thermocouple's resistance, you will need a multimeter that can measure millivolts DC. The multimeter should have red and black clamps, which are the easiest to use for this test. Turn on the multimeter and set the measurement to Ohms, represented by a horseshoe-shaped symbol. This setting is used to measure electrical resistance. Once the multimeter is set up, hold the black and red clamps or leads apart. When pointed in opposite directions, the meter should stay to the left at infinity. Bring the leads together and the meter should change to 0.
Attach the negative lead to the red wire and the positive lead to the yellow wire. The reading should be about three ohms. If the reading is much higher, then it is likely a thermistor. If the reading is very low, then it is likely an RTD.
It is important to note that resistance cannot confirm if the alloy has the correct thermoelectric specifications. However, it can check for undesirable characteristics such as opens, poor welds, or corrosion of the wires. Always measure the resistance of the thermocouple out of the application to avoid any conflict between the EMF output and the resistance meter.
Additionally, when testing a thermocouple, you can also perform an open circuit test. This is done by disconnecting the thermocouple and attaching the meter leads to the outside of the thermocouple, with one lead attached to the tip. The readings should be above 17 to 18 millivolts.
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Frequently asked questions
You will need a multimeter that can read millivolts and a source of fire, such as a lighter. First, attach the thermocouple adapter to the gas valve in the thermocouple's spot. Then attach the thermocouple as normal, except the end that would screw into the gas valve will screw into the adapter. Place one test lead on the side of the thermocouple and the other on the end that enters the gas valve. The reading should be between 25 and 35 millivolts.
A thermocouple is a device made up of two dissimilar metals joined together at the tip (Hot Junction). When heat is applied to that hot junction, a small millivoltage is created. This voltage is used by the gas valve to keep the pilot valve solenoid internal to the main gas valve open.
Thermocouples are the easiest temperature sensors to identify. They usually come in a 2-wire construction, with one wire coloured red and the other coloured yellow.
