Its Important to Know How to Test a Thermocouple With a Multimeter

How to Test a Thermocouple with a Multimeter
A thermocouple is an important safety device which is part of most gas-based appliances. We show you how to test one, so that if it is found to be faulty, it can be replaced immediately.
HomeQuicks Staff
Last Updated: Jun 3, 2018
Tip
Often, a problem with the pilot flame or safety control valve is caused by something other than a malfunctioning thermocouple. Improper maintenance of the appliance can lead to dust and carbon accumulation, which prevents the thermocouple from performing as expected. So before testing/replacing a thermocouple, it is advisable to clean the orifices and air holes on the pilot burner.
A thermocouple is a small electronic component present in most gas appliances, which keeps you safe by preventing the gas from leaking when the flame goes out. Thermocouples are found in water heaters, furnaces, and other gas-based appliances. They form an important safety feature in all these applications, and so it is essential that they are always kept in good working condition.

But a thermocouple, like any other electronic component, is bound to fail once it reaches the end of its working life. At such times, it becomes necessary to have it replaced with a new one. However, before labeling one as faulty, it is better to perform a check on it to ensure that it is actually the source of the problem. In the following lines, we find out how to test a thermocouple. But before that, let's get to know what it is and how it actually works.
What is a Thermocouple?
A thermocouple falls under the category of electronic devices known as transducers. Basically, a transducer converts one physical quantity into another. In case of the thermocouple, the physical quantity temperature is converted to another proportional physical quantity - voltage.

A thermocouple is made up of at least two different conductors (metals), that are joined together in such a manner that they form two distinct junctions. For example, if you take two iron wires and one copper wire, and twist one end each of both the iron wires to either ends of the copper wire, you would effectively get a thermocouple, with two separate iron-copper junctions. One of these junctions is known as the hot junction, and will be connected to the body whose temperature needs to be measured. The other junction is called the cold junction, and is either kept open or is connected to another body whose temperature is known and used as a reference.

When the hot junction is heated, the difference in temperature between it and the cold junction is converted to proportional voltage which is measurable. This forms the basic working principle of a thermocouple. The voltage thus generated is further used to control different voltage-driven circuits in various applications.
Thermocouple in a Gas Furnace
Older generation gas stoves have what is known as a pilot light. A typical gas stove comprises a separate valve-controlled opening through which a small quantity of gas is allowed to pass, which is ignited to make a small flame. This flame is known as the pilot flame. It is responsible for igniting the gas coming out from the main burner.

If the pilot light were to go out, the gas from the main burner would not burn, and instead escape out and fill the kitchen, and maybe even the rest of the house. This could spell disaster if someone or something were to light even a small spark in the vicinity. To prevent such an accidental leakage from occurring, a thermocouple is used.

The hot junction of the thermocouple is placed near the pilot flame. The potential difference that is created by it is used to control an electromagnetic valve, keeping it open and allowing the gas to come out through the main burner. If the pilot flame goes out, the hot junction begins to cool, and the potential difference generated by it begins to drop, which closes off the electromagnetic valve. This effectively stops the natural gas from escaping into the room from the main burner, thus preventing a catastrophe.
How to Test a Thermal Coupler
In the above section, we saw how a thermocouple plays an important role in preventing an accidental leak from gas appliances. When a it goes bad, it usually starts producing less or no voltage when its junction is heated. The good thing about this design is that, a faulty thermocouple will still keep the electromagnetic valve closed and stop the gas from leaking out. The bad part is that, until the thermocouple is replaced, it wouldn't be possible to restart the gas appliance.

But before replacing the thermocouple, one needs to be sure that it is actually faulty. For this, a simple multimeter and a little understanding of basic electronics is sufficient. There are three methods to go about this. In the following lines, we present to you the instructions for each of the three methods of testing a thermocouple with a multimeter.
Instructions for Testing a Thermocouple With a Multimeter
Method 1: Resistance Test
Requirement

1) Digital multimeter capable of reading resistance
2) Crocodile clips

Procedure

Carefully remove the faulty thermocouple from the gas appliance. Connect the crocodile clips to the slots in the multimeter. Now, attach one clip on one end of the thermocouple and attach the other clip on the other end, that is the one that gets screwed into the gas valve. Turn on the multimeter and select the ohms/resistance reading option. The multimeter should display a very small resistance in the order of a few ohms, if the thermocouple is fine. Some multimeters have the continuity-check option wherein, low resistances typically observed in good conductors is indicated by an audible alert. If you are using such a multimeter, then put it on the continuity option. If your thermocouple is good, then you will hear a continuous audio tone.

A high resistance, for example 40 ohms is indicative of a bad thermocouple which must be replaced.
Method 2: Open Circuit Test
Requirement

1) Digital multimeter capable of reading resistance and millivolts
2) Crocodile clips
3) Cigarette lighter

Procedure

In this test, the same setup as above will be used, but instead of measuring resistance, voltage generated by the thermocouple will be measured and checked. For this, the crocodile clips are to be connected as described in the resistance test, and the millivolts option in the multimeter must be selected.

Now, using the cigarette lighter, heat the end of the thermocouple that is kept in contact with the pilot flame (opposite to the one that gets screwed into the gas valve). Typically, the thermocouples that are used in residential gas appliances, such as stoves, heaters, etc., are designed to output a voltage in the range of 25 mV to 30 mV. If the thermocouple under test outputs voltage in this range, then it is fine. If however it outputs voltage that is nearer to 20 mV, then it is advisable to have it replaced.
Method 3: Closed Circuit Test
Requirement

1) Digital multimeter capable of reading resistance and millivolts
2) Crocodile clips
3) Thermocouple adapter

Procedure

As the name itself suggests, this test is performed by placing the thermocouple in its working environment, that is, within the gas appliance. It is a more comprehensive test, as it outlines the thermocouple's performance under load. This is important, because it may happen that a thermocouple outputs normal voltage under no-load condition in the open circuit test, but its voltage may drop under load condition. So in case the thermocouple passes the open circuit test, but your gas appliance still fails to work with it installed, you must perform the closed circuit test as described below.

To perform the closed circuit test, you will need a thermocouple adapter. Many manufacturers provide these adapters for testing purposes, and they are also easily available in stores. This adapter is screwed inside the gas valve. The thermocouple is then screwed into the other end of the adapter.

Attach one of the crocodile clips to the screw that extends from the adapter, and the other one to the exposed end of the thermocouple. Select the millivolt reading option in the multimeter, and turn on the appliance. Ideally, the reading should range between 12 mV to 15 mV. If the thermocouple outputs voltage below 12 mV, there is a significant drop, meaning that it is defective and needs to be replaced.

Thus, using the above mentioned methods, a thermocouple can be easily tested. If it fails these tests, then it is best to get it replaced.