Thermocouple is a kind of temperature sensing element. It directly measures the temperature, and converts the temperature signal into a thermoelectromotive force signal, which is converted into the temperature of the measured medium through an electrical instrument (secondary instrument). The basic principle of temperature measurement is that two conductors of different components form a closed loop. When there is a temperature gradient at both ends, a current will flow through the loop. At this time, there is an electromotive force-thermoelectromotive force between the two ends. This is the so-called Seebeck effect.　When the third metal material is connected in the thermocouple circuit, as long as the temperature of the two junctions of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the third metal in the circuit. Therefore, when the thermocouple measures the temperature, the measuring instrument can be connected, and after the thermoelectromotive force is measured, the temperature of the measured medium can be known. Two homogeneous conductors with different compositions are thermoelectrodes, the end with a higher temperature is the working end, the end with a lower temperature is the free end, and the free end is usually at a certain constant temperature. According to the functional relationship between thermoelectromotive force and temperature, a thermocouple index table is made; the index table is obtained when the free end temperature is at 0℃, and different thermocouples have different index tables. The working principle of thermocouple temperature sensor Two conductors of different components (called thermocouple wire or thermoelectrode) are connected to a combined circuit at both ends. When the temperature of the junction is different, an electromotive force will be generated in the circuit. This phenomenon is called It is the thermoelectric effect, and this electromotive force is called thermoelectric force. Thermocouples use this principle for temperature measurement. Among them, the end that is directly used to measure the temperature of the medium is called the working end (also called the measuring end), and the other end is called the cold end (also called the compensation end); the cold end and the display When the meter or supporting meter is connected, the display meter will point out the thermoelectric potential generated by the thermocouple. A thermocouple is actually a kind of energy converter, which converts heat energy into electrical energy, and uses the generated thermoelectric potential to measure the temperature. For the thermoelectric potential of the thermocouple, the following issues should be paid attention to: 1. The thermoelectric potential generated by the thermocouple When the material of the thermocouple is uniform, it has nothing to do with the length and diameter of the thermocouple, but is only related to the composition of the thermocouple material and the temperature difference between the two ends; 2. The thermoelectric potential of the thermocouple is a function of the temperature of the two ends of the thermocouple. The difference is not a function of the temperature difference between the cold junction and the working end of the thermocouple; 3. When the material composition of the two thermocouple wires of the thermocouple is determined, the thermoelectric potential of the thermocouple is only the temperature difference of the thermocouple Related; If the temperature of the cold junction of the thermocouple remains constant, the thermoelectric potential of the thermocouple is only a single value function of the temperature of the working junction. Thermocouple measurement accuracy and temperature measurement range selection Two conductors or semiconductors A and B of different materials are welded together to form a closed loop, as shown in the figure. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, thus forming a large current in the loop. This phenomenon is called the thermoelectric effect. Thermocouples use this effect to work. The selection of a thermocouple should be based on comprehensive considerations such as the use temperature range, the required accuracy, the use atmosphere, the performance of the measurement object, the response time and the economic benefits. 1. The choice of atmosphere: S type, B type, suitable for strong oxidation and weak reduction atmosphere, J type and T type thermocouple are suitable for weak oxidation and reduction atmosphere, if you use a protection tube with better airtightness , The requirements for the atmosphere are not too strict. 2. Selection of measurement accuracy and temperature measurement range. When the temperature is between 1300~1800℃, when the accuracy is relatively high, the B-type thermocouple is generally used; the accuracy is not high, and the atmosphere allows the use of tungsten rhenium thermocouple, higher than 1800℃ Tungsten-rhenium thermocouple is generally used; the working temperature is 1000~1300℃, and the accuracy is relatively high. S-type thermocouple and N-type thermocouple are available; K-type thermocouple and N-type thermocouple are generally used below 1000℃, below 400℃ E-type thermocouple is generally used; T-type thermocouple is generally used for measurement at 250°C and negative temperature. T-type thermocouple is stable and high in accuracy at low temperatures. 3. Selection of durability and thermal responsiveness. Thermocouples with larger wire diameters have better durability, but have a slower response. For thermocouples with large heat capacity, the response is slow. When measuring temperatures with large gradients, under temperature control , The temperature control is poor. It requires a fast response time and a certain degree of durability, so it is more appropriate to choose an armored couple. 4. The nature and state of the measurement object and the choice of thermocouple The temperature measurement of vibrating objects, vibrating objects, and high-pressure vessels requires high mechanical strength. Chemically polluted atmospheres require protective tubes. In the case of electrical interference, higher insulation is required. 5. Precautions ◆The insertion depth of the thermocouple is very small: it should not be less than 8-10 times the outer diameter of its protective tube (except for special products) ◆Thermocouple [1] Nominal pressure: generally refers to the protection tube at working temperature Can withstand static external pressure and rupture. ◆Insulation resistance: when the surrounding air temperature is 15-35℃, relative humidity
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