The working principle of thermocouples

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What is a thermocouple? This begins with the principle of temperature measurement using thermocouples. A thermocouple is a temperature-sensing element and a primary instrument; it measures temperature directly and converts the temperature signal into a thermoelectromotive force signal, which is then converted, through electrical instruments (secondary instruments), into the temperature of the medium being measured. The basic principle of thermocouple temperature measurement is that a closed circuit is formed by two conductors made of different materials. When there is a temperature gradient between the two ends, an electric current flows through the circuit; at this point, an electromotive force – namely thermoelectromotive force – exists between the two ends. This is what is known as the Seebeck effect. A homogeneous conductor with two different compositions serves as the thermoelectrode; the end with the higher temperature is the working end, while the end with the lower temperature is the free end, which is usually at a constant temperature. Based on the functional relationship between thermoelectromotive force and temperature, a thermocouple calibration table is created; this table is obtained under the condition that the temperature at the free end is 0°C, and different thermocouples have distinct calibration tables. When a third metal material is introduced into the thermocouple circuit, as long as the temperatures at the two contacts of this material are the same, the thermoelectric potential generated by the thermocouple remains unchanged; in other words, the presence of the third metal in the circuit has no effect on it. Therefore, when using a thermocouple for temperature measurement, a measuring instrument can be connected; by detecting the thermoelectromotive force, the temperature of the medium being measured can be determined. B: Principle of operation of thermocouples: Two conductors made of different materials (referred to as thermocouple wires or thermal electrodes) are connected at both ends to form a circuit. When the temperatures at these connection points differ, an electromotive force is generated within the circuit. This phenomenon is known as the thermoelectric effect, and such an electromotive force is called a thermoelectromotive force. Thermocouples use this principle to measure temperature; the end that is directly used to measure the temperature of the medium is called the working end (also known as the measuring end), while the other end is called the cold end (also known as the compensation end) ； The cold end is connected to a display instrument or associated meters, which indicate the thermoelectric potential generated by the thermocouple. A thermocouple is essentially an energy converter that converts thermal energy into electrical energy; temperature is measured using the thermoelectric potential generated. Regarding the thermoelectric potential of a thermocouple, the following points should be taken into account: 1: The thermoelectric potential of a thermocouple is a function of the difference in temperatures at its two ends, rather than a function of the temperature difference between those two ends ； 2: The magnitude of the thermoelectric potential generated by a thermocouple, when the material of the thermocouple is uniform, depends neither on its length nor diameter; it depends only on the composition of the thermocouple material and the temperature difference between its two ends ； 3: Once the material compositions of the two wires in a thermocouple are determined, the magnitude of the thermoelectric potential of that thermocouple depends only on the temperature difference across it ； If the temperature at the cold end of the thermocouple remains constant, then the thermoelectromotive force of the thermocouple is a function of the temperature at the working end only. Commonly used thermocouple materials include: Thermocouple type, Thermoelectric electrode material – Positive pole, Negative pole. S: Platinum-rhodium 10, Pure platinum; R: Platinum-rhodium 13, Pure platinum; B: Platinum-rhodium 30, Platinum-rhodium 6; K: Nickel-chromium, Nickel-silicon; T: Pure copper, Copper-nickel; J: Iron, Copper-nickel; N: Nickel-chromium-silicon, Nickel-silicon; E: Nickel-chromium, Copper-nickel