What is a Thermocouple and How Does It Work?

A thermocouple is a type of temperature sensor that converts thermal energy into electrical voltage, which can be used to measure temperature. They are valued for their simplicity, durability, and wide range of operating options, making them suitable for most applications.

Thermocouples consist of two dissimilar metal wires. When one junction of these wires experiences a different temperature than the other, a small voltage is generated. This voltage changes predictably with temperature, allowing thermocouples to provide a reliable way to monitor temperatures across various industries, including high-temperature furnaces and more.

Thermocouple types 

There are many different thermocouple types available for a wide range of applications. Each thermocouple alloy combination determines its temperature range and sensitivity. These combinations are standardised and represented by letter designations.

Why choose thermocouples for temperature measurement?

Thermocouples are preferred for temperature measurement because they offer a wide temperature range, fast response times, high durability, and low cost.

They measure temperatures accurately, even under extreme conditions. Intense heat, constant vibration, corrosive chemicals, and high pressures do not disrupt their readings (provided the appropriate thermocouple is selected). They continuously operate, measure, send data, and support processes that depend on strict thermal control. Conax thermocouples can be used in extremes ranging from -200˚C to 2315˚C. Moreover, they have fast response times and are one of the more affordable options compared to more precise alternatives, such as Resistance Temperature Detectors (RTDs).

How does a thermocouple work?

As mentioned, a thermocouple consists of two different metal wires joined at one end to create a temperature-sensing junction. When this junction experiences a temperature change, it generates a small electrical voltage that corresponds to the temperature difference between the junction and the wire ends.

Thermocouples operate based on the Seebeck effect, first discovered by Thomas Johann Seebeck in the early 19th century. While any two dissimilar metals can produce a voltage, only specific types of thermocouples offer stable, accurate, and predictable temperature readings in various environments.

When one end of the metal wire is heated, the free electrons diffuse toward the colder end. Because different metals react to heat in distinct ways, the electrons in each wire move at different rates. This creates a difference in electrical charge between the hot end and the cold end.

A thermocouple circuit consists of two junctions: the Hot Junction and the Cold Junction.

The hot junction is the joined end placed in the environment or on the surface being measured, while the cold junction is the unjoined end connected to a measuring instrument at a known, constant temperature.

The voltage generated is directly proportional to the temperature difference between the hot junction and the cold junction. By knowing the temperature of the cold junction and measuring the generated voltage, the measuring instrument can calculate the exact temperature at the hot tip.