Testing a Thermocouple With a Multimeter

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.

How to Test a Thermocouple

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

Note: Consider a Thermocouple installed in a gas appliance line.

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.

Also Read: Peltier Effect Theory