Thermocouple sensors are the most widely used contact temperature measurement devices in industry. They feature stable performance, a wide temperature range, and long-distance signal transmission. They are also simple in structure, easy to use, and can operate normally even in high-humidity environments such as those found in concrete curing industries. Thermocouples can directly convert heat energy into electrical signals and output DC voltage signals, making display, recording, and transmission easy.
Thermocouple principle
The principle of a thermocouple is based on the thermoelectric effect. As shown in the figure, when conductors A and B, which are made of different materials, form a closed circuit, if the temperatures of the two junctions T and T0 are different, an electromotive force will be generated in the circuit and a current will be formed. This phenomenon is called the thermoelectric effect, and the electromotive force is called thermoelectric electromotive force.
According to their different structural forms, thermocouples are generally divided into three categories: ordinary thermocouples, armored thermocouples, and thin-film thermocouples.
Ordinary thermocouple
Ordinary thermocouples are used in industry to measure the temperature of liquids, gases, and other media. They generally consist of thermocouple wires, an insulating sheath, a protective sheath, and a junction box. In laboratory use, the protective sheath can be omitted to reduce thermal inertia.
Armored thermocouple
Armored thermocouples are thermocouples with a special structure, consisting of thermocouple wires, insulating material, and a metal sheath. They can be made very thin and long, and can also be bent. Depending on the shape of the hot junction, they can be divided into various structural forms, each with different response times, applications, and characteristics. The outstanding advantages of this type of thermocouple are its miniaturization, ease of use, long service life, and low thermal inertia. In addition, these sensors also have advantages such as high strength, pressure resistance, vibration resistance, and shock resistance, and can be manufactured in various lengths, up to 100m, as needed.
Thin film thermocouples
Thin-film thermocouples are made by depositing two thermoelectric materials onto an insulating plate using methods such as vacuum evaporation, followed by the deposition of a layer of silicon dioxide as an insulating and protective layer, forming a thin-film thermocouple. Its thermal junction is extremely thin (0.01-0.1 μm), making it particularly suitable for rapidly measuring the temperature of object surfaces. During installation, it is simply bonded to the surface of the object being measured using adhesive.
In addition to the types mentioned above, thermocouples also include assembled thermocouples, end-face thermocouples, spring-loaded thermocouples, high-temperature thermocouples, platinum-rhodium thermocouples, miniature thermocouples, precious metal thermocouples, fast thermocouples, tungsten-rhenium thermocouples, and so on.
Temperature compensation of the cold junction of thermocouple sensor
Because thermocouple materials are generally expensive (especially when precious metals are used), and the distance between the temperature measuring point and the instrument is often considerable, compensating wires are typically used to extend the cold junction of the thermocouple to the more stable temperature in the control room and connect it to the instrument terminals to save on thermocouple material and reduce costs. It must be pointed out that the function of thermocouple compensating wires is only to extend the thermocouple electrodes, moving the cold junction to the instrument terminals in the control room; they do not eliminate the influence of cold junction temperature changes on temperature measurement and therefore do not provide compensation. Therefore, other correction methods are needed to compensate for the influence of cold junction temperature t0 ≠ 0℃ on temperature measurement. When using thermocouple compensating wires, it is crucial to ensure that the model is compatible, the polarity is correct, and the temperature at the connection point between the compensating wire and the thermocouple does not exceed 100℃.
