Overview of Thermocouple Temperature Measurement Instruments?

There are many temperature measuring instruments in industrial production, and thermocouple temperature measuring instruments are one of them. It is composed of two conductors or semiconductors with their two ends welded or twisted together to form a closed circuit, which utilizes the temperature difference between the two points to generate electromotive force and work. There are many types of thermocouples, and various classification methods are also different. According to the requirements of industrial standardization, they can be divided into two types: standardized thermocouples and non-standard thermocouples. Thermocouples are one of the specific applications of the thermoelectric effect theory and have a wide range of applications in temperature measurement. They have the advantages of simple structure, stable performance, and easy construction. Thermocouples, when used in conjunction with display instruments, can achieve remote measurement, automatic recording, and automatic temperature control.
1. Types of temperature measuring instruments and thermocouples for temperature measurement
There are many types of temperature measuring instruments, and with the development of science and technology and the needs of modern industrial technology, temperature measurement technology is constantly being improved and enhanced. Due to the increasing range of temperature measurement, different temperature measuring instruments have been manufactured according to different requirements, mainly including LCD thermometers, optical pyrometers, semiconductor thermometers, rotary thermometers, pressure thermometers, glass tube thermometers, pointer thermometers, high-temperature thermometers, thermocouple thermometers, resistance thermometers, gas thermometers, and thermocouple thermometers.
According to the different purposes, installation methods, and structural forms of thermocouples, they can be divided into two categories: ordinary thermocouples and armored thermocouples. The commonly used thermocouples, based on their graduation numbers, include S, R, B, N, K, E, J, T, and so on. Among them, S, R, and B belong to precious metal thermocouples, while N, K, E, J, and T belong to cheap metal thermocouples.

The characteristic of S grade is strong antioxidant performance, and it is suitable for continuous use in oxidizing and inert atmospheres. The long-term use temperature is 1400 ℃, and the short-term use temperature is 1600 ℃. Among all thermocouples, the S scale has the highest accuracy level and is usually used as a standard thermocouple; Compared to the S index, the R index has a thermal electromotive force that is about 15% higher, and its other performance is almost identical; The thermal electromotive force of the B index is extremely small at room temperature, so compensation wires are generally not needed during measurement. Its long-term usage temperature is 1600 ℃ and short-term usage temperature is 1800 ℃. It can be used in oxidizing or neutral atmospheres, as well as for short-term use under vacuum conditions. The characteristics of the N-grade thermocouple are strong high-temperature oxidation resistance at 1300 ℃, good long-term stability of thermoelectric force and reproducibility of short-term thermal cycles, good resistance to nuclear radiation and low temperature, and can partially replace the S-grade thermocouple; The characteristic of K scale is strong antioxidant performance, and it is suitable for continuous use in oxidizing and inert atmospheres. The long-term use temperature is 1000 ℃, and the short-term use temperature is 1200 ℃. The most widely used among all thermocouples; The characteristic of the E-index is that among commonly used thermocouples, its thermoelectric electromotive force is the highest, which means it has the highest sensitivity. It is recommended to use continuously in an oxidizing and inert atmosphere at a temperature of 0-800 ℃; The characteristic of J-grade is that it can be used in both oxidizing atmospheres (with an upper temperature limit of 750 ℃) and reducing atmospheres (with an upper temperature limit of 950 ℃), and is resistant to H2 and CO gas corrosion. It is mostly used in refining and chemical industry; The characteristic of T-scale is that it has the highest accuracy level among all cheap metal thermocouples and is usually used to measure temperatures below 300 ℃.
2. The basic principle of thermocouple temperature measurement
The reason why thermocouples can be used to measure temperature is because they convert temperature signals into thermoelectric signals, which are then converted into the temperature of the measured medium through electrical instruments. The basic principle of thermocouple temperature measurement is that two homogeneous conductors with different compositions form a closed circuit. When there is a temperature gradient at both ends, there will be current passing through the circuit, and there is a thermoelectric electromotive force between the two ends

The reason for the generation of thermoelectric electromotive force is due to the different electron densities of different conductor materials. Even if the same conductor material has different temperatures, its electron density is not the same. When heterogeneous metals A and B form a closed circuit, due to the different temperatures of contacts A and B, the free electron density is higher in the high temperature areas of the same conductor, while the free electron density is lower in the low temperature areas. Therefore, in the closed circuit, the high free electron density needs to diffuse to the low free electron density areas, resulting in a "net" charge flow in the circuit, that is, there is an electromotive force eAB in the circuit, which is the reason for the generation of Seebeck electromotive force. When the material of the hot electrode is fixed, the thermal electromotive force is only related to the temperature of the two contacts. In a closed circuit composed of a pair of heterogeneous metals A and B, if the junction point A is heated, the temperature of the two contacts A and B will be different. If the temperature is different, a current will be generated, causing the ammeter connected to the circuit to deflect. The current generated in the thermocouple circuit is called thermal current. A. B is called a thermoelectric electrode, and contact a is connected together by welding. When measuring temperature, it is placed in the measured temperature field, known as the measurement end or working end. Contact b generally requires a constant temperature, known as the reference end or free end.
The magnitude of thermoelectric potential is related to the difference between t and t0. When the materials of the two thermoelectric electrodes of a thermocouple are known, the distribution theory of thermoelectric potential in the thermocouple circuit determines that the thermoelectric potential difference between the two ends of the thermocouple can be expressed by the following equation:
EAB (t, t0)=EAB (t) - EAB (t0)
In the formula, EAB (t, t0) - thermoelectric potential of thermocouple
EAB (t) - thermoelectric potential at the working end at temperature t;
EAB (t0) - The thermoelectric potential at the cold end at a temperature of t0.
From the above equation, it can be seen that when the temperature of the measured medium at the working end changes, the thermoelectric potential also changes. Therefore, as long as EAB (t, t0) is measured and EAB (t0) is known, EAB (t) can be obtained. The thermoelectric potential can be sent to the display instrument for indication or recording, or to the microcomputer for processing, to obtain the temperature t value at the measuring end.

3. The main advantages and disadvantages of thermocouple temperature measurement
Thermocouples are one of the most commonly used temperature detection components in industry, and their advantages include:
① High measurement accuracy. Due to the direct contact between the thermocouple and the measured object, it is not affected by the intermediate medium.
② Wide measurement range. Commonly used thermocouples can measure continuously from -50 to 1600 ℃, and some special thermocouples
The lowest detectable temperature is -271-2800 ℃, such as gold, iron, nickel, chromium, and tungsten rhenium.
③ Simple construction and convenient use. Thermocouples are usually composed of two different types of metal wires, and are not limited by size or beginning. They have a protective sleeve on the outside, making them very convenient to use.
However, thermocouples also have shortcomings, such as the use of a constant reference temperature, otherwise the measurement results will be distorted; In high temperature or long-term use, degradation occurs due to the influence of the tested medium or atmosphere (such as oxidation, reduction, etc.), reducing the service life.
4. Current research status and development trend of thermocouple temperature measurement in China
Thermocouples are mainly used to measure higher temperatures. Regarding the development status of high-temperature thermocouples in China, high-temperature thermocouples generally refer to those above 1300 degrees, including platinum rhodium 10 platinum thermocouples, platinum rhodium 13 platinum thermocouples, platinum rhodium 30 platinum rhodium 6 thermocouples, tungsten rhenium 5 tungsten rhenium 25 thermocouples, platinum rhodium 40 platinum rhodium 20 thermocouples, iridium rhodium thermocouples, silicon carbide graphite thermocouples, and so on. At present, contact type temperature measurement thermocouples are the most reliable temperature measurement method. Apart from using optical pyrometers and infrared thermometers in vacuum, medium frequency, and high-frequency areas that are not suitable for thermocouple temperature measurement, there are basically no other materials or technologies that can replace it.
From the current situation, the main development trend is to enhance corrosion resistance and oxidation resistance through technological means to improve service life.