How Does a Thermocouple Work?

By: Contributors  | 
Two water heaters on a wall.
Water heaters regulate their temperature range thanks to thermocouple probes.
Scott Bryant

Inside your gas-operated water heater is a tiny gas flame called a pilot light, whose purpose is to ignite the burner whenever the temperature falls below the thermostat setting. Right next to the pilot light is a small metallic tube known as a thermocouple. But how does a thermocouple work, exactly?

Primarily, this is a safety device. Its purpose is to monitor the pilot light and send a signal that shuts the gas burner valve should the pilot light ever go out — and keep it shut until the thermocouple senses that the pilot light is lit. If the thermocouple stops working, the pilot light will go out, preventing the burner from working [source: Family Handyman].


In this article, we'll not only explain how thermocouples measure temperature, but also discuss different thermocouple types and applications. Finally, we'll show you how to test a thermocouple on a gas valve.

How a Thermocouple Works

A thermocouple is a simple yet ingenious device used to measure temperature. It consists of two dissimilar metal wires, joined at one end. When this junction is heated or cooled, it generates a small voltage, known as the Seebeck effect. The amount of voltage produced is directly related to the differential temperature between the other ends of the two wires. By measuring this voltage, the temperature at the junction can be accurately determined.

The beauty of a thermocouple lies in its simplicity and responsiveness, making it an essential tool in a wide range of applications, from industrial furnaces to household appliances. Additionally, thermocouples are known for their durability and ability to measure extreme temperatures, which is not easily achievable with other types of temperature sensors.


The Hot and Cold Junction Compensation, Explained

When these two dissimilar metal wires are fused together at one end, they form the 'hot junction.' This is the point where temperature measurements are taken. When this junction is exposed to a temperature - be it hot or cold - a thermoelectric effect occurs, where a voltage is generated due to the difference in the electron density between the two dissimilar metals.

The other ends of these wires, which are not joined together, form the 'cold junction' or 'reference junction.' Contrary to its name, the cold junction does not necessarily need to be at a lower temperature than the hot junction; it simply refers to the end where the wires are separate and typically connected to a temperature measurement device.


The role of the cold junction is crucial in a thermocouple's operation. It serves as a reference temperature, allowing for the accurate calculation of the temperature at the hot junction based on the voltage generated between the two junctions. This thermocouple voltage is directly related to the temperature difference between the hot and cold junctions. Therefore, knowing the ambient temperature at the cold junction (often maintained or measured at a known, stable temperature) is essential for accurately determining the temperature at the hot junction.

Applications of Thermocouples

Thermocouples are widely used due to their simplicity, durability, and wide temperature range. Some common applications include:

Industrial Applications

Thermocouples are extensively utilized in industrial settings, where their robustness and ability to withstand extreme temperature ranges are of great value. They play a critical role in monitoring and controlling temperatures in furnaces, kilns, and reactors. Their reliability in these high-temperature environments ensures safety and efficiency in various industrial processes.


Household Appliance Usage

In the realm of household appliances, a thermocouple probe is commonly found in devices such as ovens and water heaters. They serve as a key component for temperature regulation, ensuring these appliances operate safely and effectively. Their ability to promptly respond to temperature changes makes them an ideal choice for maintaining desired temperature levels in these everyday appliances.

Scientific and Research Applications

In scientific research, thermocouple probes are indispensable tools due to their precision in temperature measurement. They are frequently employed in experiments that require accurate readings across the temperature gradient. Their versatility allows them to be used in a variety of research settings, from basic laboratory experiments to advanced scientific studies.

Automotive Industry Applications

The automotive industry also benefits from the use of thermocouples, particularly in measuring exhaust gas temperatures and monitoring engine performance. These measurements are crucial for optimizing engine efficiency and reducing emissions. Thermocouples in vehicles contribute to better engine management and enhanced vehicle performance.


Advantages of Thermocouples

Thermocouples offer a significant advantage due to their ability to measure a wide range of temperature difference. These thermocouple types can be suitable for many different applications, from extreme cold to intense heat. Additionally, their durability is a key factor, especially in industrial and automotive contexts where they are exposed to harsh conditions and high temperatures. This resilience, coupled with their simplicity, makes thermocouples a popular choice in various fields.


Limitations of Thermocouples

Despite their numerous advantages, thermocouples do have some limitations. One of the primary drawbacks is their accuracy, which can be less than that of other types of temperature sensors. They also produce a non-linear output, meaning that the relationship between temperature and voltage is not straightforward. This non-linearity necessitates the need for calibration and complex electronic systems for precise temperature measurement, which can be a drawback in certain applications.


Case In Point: Your Everyday Water Heater

All of this talk about exposed junction thermocouples and thermocouple extension wires can confuse this conversation around a simple measuring instrument. So, let's discuss how it aids a water heater.

As previous discussed, the thermocouple consists of a probe made from dissimilar metals, known as the thermocouple wire, which is specifically designed to measure temperature changes within the unit. This probe extends into the pilot light's flame and is sensitive to the ambient temperature in its environment.


When the water heater is active, the flame heats the thermocouple probe, causing the dissimilar metals to produce a small electrical current. This current is a direct response to the measuring of temperature, keeping the gas valve open and thereby maintaining a continuous gas flow to support the flame. This process is crucial for the consistent heating of water.

If the pilot light extinguishes, the ambient temperature around the thermocouple wire drops. This cooling effect reduces the electrical current generated by the thermocouple. As a result, the gas valve automatically closes, halting the gas supply. This feature is a vital safety mechanism, protecting against the risks of gas leaks, which could lead to hazardous conditions such as explosions or fires.


How to Test a Thermocouple on a Gas Valve

Here's how to test a suspected thermocouple in a gas operated water heater:

  1. Light the pilot by pressing and holding the pilot light button on the gas control valve. If you can't light the pilot, the problem most likely lies with the gas control valve or the gas flow itself, not the thermocouple.
  2. Continue pressing the pilot button for 30 to 60 seconds after lighting the pilot light.
  3. Release the button. If the pilot light goes out, the trouble is most likely a defective thermocouple.

The inclusion of a thermocouple in a water heater exemplifies a perfect blend of functional efficiency and critical safety. By effectively measuring temperature changes and responding accordingly, the thermocouple ensures that the water heater operates safely and effectively, automatically adapting to the ambient temperature and preventing potential hazards.


This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.