Explanation of the Thermocouple Principle

A thermocouple is a sensor used to measure temperature; its principle is based on the thermoelectric effect. The thermoelectric effect refers to the phenomenon whereby, when two different metals or alloys form a closed circuit at their point of contact, an electromotive force is generated if there is a temperature difference across the circuit. Thermocouples utilise this thermoelectric effect to measure temperature.

Specifically, a thermocouple consists of two different types of metallic wire, typically copper and a copper-nickel alloy. These two materials are welded together to form a junction, known as the thermocouple junction. When the junction is at different temperatures, differences arise in the distribution of electron energy levels between the two materials, causing electrons to diffuse between them and resulting in a transfer of charge. The electromotive force generated by this charge transfer is known as the thermoelectric potential, and the temperature difference can be determined by measuring this thermoelectric potential.

The working principle of a thermocouple can be simply described as ‘thermoelectric effect + temperature difference = electromotive force’. Depending on the combination of metallic materials and the temperature range, thermocouples can provide different temperature measurement ranges and levels of accuracy. By comparing the electromotive force of a thermocouple with a known temperature, we can establish a calibration curve or consult a temperature–electromotive force conversion table to accurately measure an unknown temperature.

Thermocouples have a wide range of applications and are particularly suitable for temperature measurement at high and low temperatures, as well as in harsh environments. Owing to their simple structure, fast response time, strong resistance to interference and high reliability, thermocouples are widely used in industrial production, scientific research, energy utilisation and environmental monitoring.