How to Test a Thermocouple: A Comprehensive Guide

Introduction

Most natural gas-based appliances, such as water heaters, stoves, and furnaces, use a thermocouple as a safety shut-off device to prevent harmful gas leakage if the pilot light goes out. According to our research, even though standing pilot lights have become somewhat obsolete in recent years due to the advent of direct ignition technology, there are still many older, fully serviceable appliances in use today that rely on a pilot light and thermocouple. Testing a thermocouple with a multimeter is a task that most DIY enthusiasts can perform with ease.

Understanding Thermocouples

Before starting the testing procedures, it's essential to understand what a thermocouple is and how it works. A thermocouple is a type of electrical device known as a transducer, which is found in various forms, from small toasters to massive industrial kilns. It converts the temperature it senses into a measurable DC voltage. The device contains at least two distinctly different metals that form two separate junctions: the live (hot) junction, connected to the part where temperature needs to be monitored, and the base (cold) junction, which is either kept open or connected to a known temperature source as a point of reference. The difference in temperature between the hot and cold junctions is then converted into a measurable voltage.

Basic Testing Procedures

Knowing how to test a thermocouple with a multimeter is crucial in diagnosing why a pilot light goes out or won't ignite at all. Before starting the tests, thoroughly clean the part to eliminate any dust or carbon buildup that might have contributed to its failure. To perform the basic open circuit checks, you'll need a multimeter (either digital or analog) and a pair of double-ended alligator clips. Follow these steps:

1. Set your digital multimeter to the millivolt (mV) scale on the DC voltage range.
2. Attach the alligator clip wires to the meter lead ends, then clip one test wire to the gas valve (source) and the other to the exposed copper wire leading to the thermocouple.
3. Light the pilot light by turning the on/off switch to "pilot" and pushing down on the pilot valve opening button.
4. Hold everything in place for up to a minute to obtain the proper reading. The meter should show about 30 mV ±5mV.

Advanced Testing

If the basic test results are within the proper range, the thermocouple is likely not the source of the problem. However, before calling in a professional, you can perform a more involved closed circuit test using the same multimeter and clips as before. These tests can provide a more thorough assessment of the thermocouple's functionality and may reveal underlying issues such as draft/flue problems, fuel pressure, or gas valve failure. You'll need a special screw-in adapter, available through most HVAC dealers, to perform these tests.

1. With the pilot light off and the adapter in place on the thermocouple, check the millivolts drawn by the electromagnetic coil inside the thermocouple. The reading should be approximately 15 mV.
2. Make a second measurement with the pilot light on, introducing a certain "load." This test determines if the cold junction portion of the unit is being altered by other heat factors. If the flame is proper, the reading should be about the same as before (15 mV).
3. The final test is to see how the magnet reacts to pilot light blow-out. Maximum drop-out time is around 3 minutes, with 1.5 minutes being the norm. If the part is good, the voltage will drop to 1-2 millivolts, and there will be an audible "click" when the magnet valve shuts.

Using the Thermocouple as a Test-Bed

Although commercial thermocouples are relatively inexpensive, replacing them without understanding the root cause of the failure is a mistake. Testing a thermocouple can lead to a better understanding of why your appliance failed in the first place. Blindly replacing the part without knowing the real reasons for failure doesn't fix the original problem.

One of the best ways to get to the root of thermocouple malfunction and determine your appliance's efficiency is to examine the pilot flame and its relation to the tip of the thermocouple. When the flame is adjusted for optimal performance, it should envelop the upper 1/2 to 3/8" of the unit, and the tip should glow a "dull-red" color. A flame brighter than a soft, blue color will tend to heat the tip excessively to a "cherry-red" color, eventually causing the tip to weld together and the part to fail. Pay particular attention to the cold junction portion of the thermocouple, as it is highly susceptible to higher temperatures resulting from an improperly adjusted flame.

Safety Precautions

Safety concerns linked to thermocouple testing mostly relate to the nature and volatility of natural gas. Perform these tests with adequate ventilation in place, preferably with fresh air flow-through. Take extra care when reconnecting the thermocouple fittings to prevent cross-threading. To ensure there are no gas leaks associated with an improperly tightened fitting, use soapy water to check for bubbles after turning the gas back on.

Conclusion

Even though thermocouples are fairly cheap to replace, it's important to know how to test them to expose any other problem areas beyond the part itself. According to our findings, performing a simple function test can give you a good idea of whether your appliance needs an inexpensive repair or a more costly fix. With this knowledge, you can decide on the best course of action and be better prepared for the eventual repair or replacement that follows. Being able to troubleshoot appliances can be a rewarding and empowering experience for any DIY enthusiast.