Testing a Thermocouple With a Multimeter

What is a Thermocouple?

A thermocouple belongs to the transducer category of electrical devices. A transducer is a device that changes one physical quantity into another. The physical quantity temperature is translated to another proportionate physical quantity – voltage – in the case of the thermocouple.

A thermocouple is made up of at least two separate conductors (metals) that are connected in such a way that two unique junctions are formed. Take two iron wires and one copper wire, for example, and twist one end of each of the iron wires to either end of the copper wire to make a thermocouple with two independent iron-copper junctions. One of these intersections

The cold junction, on the other hand, is either left open or linked to another body whose temperature may be measured and used as a reference.

When the hot junction is heated, the temperature differential between it and the cold junction is converted to a detectable proportional voltage. A thermocouple’s basic functioning concept is as follows. The voltage produced is then used to regulate various voltage-driven circuits in a variety of applications.

How to Test a Thermocouple

However, before changing the thermocouple, make sure it’s the one that’s broken. A simple multimeter and a rudimentary understanding of electronics are all that is required. There are three approaches to this. The instructions for each of the three techniques of testing a thermocouple with a multimeter are presented in the following lines.

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

Remove the faulty thermocouple from the gas stove with care. Connect the crocodile clips to the multimeter’s slots. Attach one clip to one end of the thermocouple and the other to the other end of the thermocouple, which will be screwed into the gas valve. Select the ohms/resistance reading option on the multimeter. A very low resistance should be seen on the multimeter, i.e. Low resistances, such as those found in good conductors, are indicated by an auditory alert on some multimeters with the continuity-check option. If you’re using a multimeter like this, make sure it’s set to continuity. A steady audible tone will be heard if your thermocouple is working properly.

A high resistance, such as 40 ohms, indicates a faulty thermocouple that needs to be replaced.

Method 2: Open Circuit Test

Requirement

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

Procedure

The same setup as before will be utilized in this test, but instead of resistance, the voltage created by the thermocouple will be monitored and evaluated. To do so, connect the crocodile clips as specified in the resistance test, and pick the millivolts option on the multimeter. Heat the end of the thermocouple that is in contact with the pilot flame using the cigarette lighter (opposite to the one that gets screwed into the gas valve). The thermocouples used in home gas appliances, such as stoves and heaters, are typically intended to output a voltage between 25 and 30 millivolts. If the thermocouple under test produces voltage in this range, it’s a good sign.

Method 3: Closed Circuit Test

Requirement

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

Procedure

The thermocouple is placed in its operating environment, which is within the gas appliance, for this test. It’s a more thorough test because it examines the thermocouple’s performance when it’s under load. This is significant because a thermocouple may output normal voltage in an open circuit test when there is no load, but its value may drop when there is a load.

If the thermocouple passes the open circuit test but your gas appliance still won’t operate with it fitted, you’ll need to do the closed circuit test, which is outlined below. A thermocouple adapter is required for the closed-circuit test. These adapters are readily available in stores and are provided by many manufacturers for testing purposes.

One crocodile clip should be attached to the adapter’s screw, while the other should be attached to the thermocouple’s exposed end. Turn on the appliance and select the millivolt reading option in the multimeter. The measurement should be between 12 and 15 millivolts. There is a considerable decline in the thermocouple’s output voltage below 12 mV, indicating that it is damaged and has to be replaced.

Also Read: What is DCS? (Distributed Control System)