Thermocouple thermocouple is one of the most commonly used temperature detection devices in industry. The working principle of thermocouple is based on the seeback effect, that is, two different components of the conductor are connected at both ends in a loop. If the two connection ends have different temperatures, then The physical phenomenon of thermal currents in the circuit. Its advantages are: 1 high measurement accuracy. Because the thermocouple is directly in contact with the measured object, it is not affected by the intermediate medium.
2 wide measurement range. Commonly used thermocouples can be measured from -50 to +1600°C. Some special thermocouples can measure at least -269°C (such as gold, iron, nickel-chromium) and up to +2,800°C (such as tungsten-germanium).
3 simple structure, easy to use. Thermocouples are usually composed of two different types of wires, and are not limited by size and the beginning. They have protective sleeves and are very convenient to use.
1. The basic principle of thermocouple temperature measurement Weld two different material conductors or semiconductors A and B to form a closed loop, as shown in -1-1. When there is a temperature difference between the two sticking points 1 and 2 of the conductors A and B, an electromotive force is generated between the two sticking points 1 and 2 so that a current of a size is formed in the loop. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.
2. Types and structures of thermocouples (1) Types of thermocouples Thermocouples can be classified into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to a thermocouple whose national standard stipulates the relationship between thermoelectric potential and temperature, tolerance, and a unified standard indexing table. It has its own display instrument for selection. Non-standardized thermocouples are less than standard thermocouples in terms of range of use or magnitude, and there is generally no uniform index table, which is mainly used for measurement in certain special occasions.
Standardized Thermocouples From January 1, 1988, thermocouples and thermal resistances were all produced according to IEC international standards, and seven standardized thermocouples designated as S, B, E, K, R, J, and T were designed for unified design in China. Thermocouples.
(2) The structure of the thermocouple To ensure reliable and stable operation of the thermocouple, the structural requirements for it are as follows: 1 The welding of the two hot electrodes constituting the thermocouple must be firm; 2 The two thermodes should be well-connected to each other Insulation in order to prevent short circuit; 3 The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable; 4 The protection sleeve should ensure that the hot electrode and the harmful medium are fully isolated.
3. Temperature compensation of thermocouple cold junction Because thermocouple materials are generally more expensive (especially when precious metals are used), the distance from the temperature measuring point to the instrument is very long. In order to save the thermocouple material and reduce the cost, the compensation wire is usually used. The cold junction (free end) of the thermocouple extends into a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the role of thermocouple compensation wire only extends the thermode, so that the cold junction of the thermocouple moves to the instrument terminal of the control room. It does not eliminate the influence of the temperature change of the cold junction on the temperature measurement, and does not compensate. Therefore, other correction methods need to be used to compensate for the effect of temperature at the cold junction temperature t00°C.
When using a thermocouple to compensate for the wire, it must be noted that the model matches, and the polarity cannot be connected wrongly. The temperature of the connection terminal of the compensation wire and the thermocouple cannot exceed 100°C.
Thermal resistance Thermistor is the most commonly used temperature detector in the low-temperature region. Its main features are high measurement accuracy and stable performance. Platinum thermal resistance is the highest measurement accuracy. It is widely used not only in industrial temperature measurement but also as a standard reference instrument.
1. Thermistor temperature measurement principle and material Thermistor temperature measurement is based on the characteristic that the resistance value of the metal conductor increases with the increase of the temperature to measure the temperature. Most of the thermal resistance is made of pure metal materials. At present, platinum and copper are the most widely used materials. In addition, nickel, manganese, and tantalum have been used to manufacture thermal resistors.
2, the type of thermal resistance 1) ordinary type thermal resistance From the principle of thermal resistance temperature measurement, we can see that the measured temperature change is measured directly by the resistance of the thermal resistance changes, therefore, the thermal resistor body lead and other Changes in the resistance of the wire will affect the temperature measurement.
2) Armored thermal resistance Thermistor thermal resistance is a solid body composed of a temperature sensing element (resistor), lead wire, insulating material, and stainless steel bushing. Its outer diameter is generally 28mm, and the minimum is up to mm. Compared with type thermal resistance, it has the following advantages: 1 small size, no internal air gap, thermal inertia, small measurement lag; 2 good mechanical properties, vibration resistance, impact resistance; 3 can bend, easy to install 4 long service life.
3) End face thermal resistance end face The thermal resistance temperature sensing element is wound by specially treated resistance wire, and it is closely attached to the end face of the thermometer. Compared with general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the tested end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.
4) Flameproof type RTD flameproof type RTD is limited to the junction box by the special structure of the junction box, and the explosion of the explosive mixture of its internal shell due to sparks or arcs will not be induced in the production site. Flameproof RTDs can be used for temperature measurement in explosion-hazardous areas within the BlaB3c class.
Pt100: used for thermal resistance, thermocouples, series temperature instruments, temperature transmitters, pressure differential pressure transmitters, hot runner heaters, compensation cables, control cables, etc.
Cu50: used for thermal resistance, thermal resistance thermometer, thermal resistance calibrator, thermal resistance simulator - copper thermal resistance simulator, copper thermal resistance, armored thermal resistance, etc.
The pt100 and cu50 measurement temperatures are different, the PT100 measures a high temperature, and the CU50 measures a low temperature.
Stainless Steel Wire Rope: a flexible rope twisted with multiple or multiple strands of fine steel wire. The steel wire rope is twisted into strands by multiple layers of steel wire, and then takes the core as the center, and is twisted into a helical rope by a certain number of strands. In material handling machinery, it is used for lifting, pulling, tensioning and carrying. The steel wire rope has high strength, light weight, stable operation, not easy to break suddenly and reliable operation. The steel wire rope plays the role of bearing the load, and its performance is mainly determined by the steel wire. The steel wire is a round (or special-shaped) wire made of stainless steel, carbon steel or alloy steel by cold drawing or cold rolling.
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