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<p>Thermocouple (thermocouple) is a commonly used temperature measuring element in temperature measuring instruments. It directly measures temperature, converts the temperature signal into a thermoelectromotive force signal, and converts it into the temperature of the measured medium through an electrical instrument (secondary instrument). The appearance of various thermocouples is often very different due to needs, but their basic structure is roughly the same. They are usually composed of main parts such as thermoelectrodes, insulating sleeve protection tubes and junction boxes, usually with display instruments, recording instruments and electronic adjustments. Used in conjunction with the device.</p><p><br></p><p><br></p><p>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 (Seebeck effect). 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.</p><p><br></p><p>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 the temperature of the measured medium can be known after the thermoelectromotive force is measured. When the thermocouple measures temperature, the temperature of its cold end (the measuring end is the hot end, and the end connected to the measurement circuit through the lead is called the cold end) is required to keep the temperature unchanged, and the thermoelectric potential is proportional to the measured temperature. If the (environment) temperature of the cold junction changes during measurement, it will seriously affect the accuracy of the measurement. Taking certain measures at the cold junction to compensate for the influence caused by the temperature change of the cold junction is called the cold junction compensation of the thermocouple. Dedicated compensation wire for connection with measuring instrument.</p><p><br></p><p>Calculation method for cold junction compensation of thermocouple:</p><p><br></p><p>From millivolt to temperature: measure the temperature of the cold junction, convert it to the corresponding millivolt value, and add the millivolt value of the thermocouple to calculate the temperature;</p><p><br></p><p>From temperature to millivolt: Measure the actual temperature and the cold junction temperature, convert them to millivolts, and subtract the millivolts to obtain the temperature</p><p><br></p><p><br></p><p>It is a kind of temperature sensing element, a kind of primary instrument, thermocouple directly measures temperature. A closed loop composed of two conductors with different composition materials. Due to different materials, different electron densities produce electron diffusion, and a potential is generated after stable equilibrium. When there is a gradient temperature at both ends, current will be generated in the loop, generating thermoelectromotive force, the greater the temperature difference, the greater the current. The temperature value can be known after measuring the thermoelectromotive force. Thermocouple is actually a kind of energy converter, which converts heat energy into electrical energy.</p><p><br></p><p>The technical advantages of thermocouples: the thermocouple has a wide temperature measurement range and relatively stable performance; high measurement accuracy, the thermocouple is in direct contact with the measured object, and is not affected by the intermediate medium; the thermal response time is fast, and the thermocouple responds flexibly to temperature changes; The measuring range is large, the thermocouple can measure the temperature continuously from -40~+1600℃; the thermocouple has reliable performance and good mechanical strength. Long service life and convenient installation.</p><p><br></p><p>The galvanic couple must be composed of two conductor (or semiconductor) materials with different properties but meet certain requirements to form a loop. There must be a temperature difference between the measuring terminal and the reference terminal of the thermocouple.</p><p><br></p><p>The conductors or semiconductors A and B of two different materials are welded together to form a closed loop. 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, so a large current is formed in the loop. This phenomenon is called the thermoelectric effect. Thermocouples work using this effect.</p><p><br></p><p><br></p><p>Two conductors with different components (called thermocouple wires or thermoelectrodes) are connected at both ends to form a loop,</p><p><br></p><p>When the temperature of the two junctions is different, an electromotive force will be generated in the loop. This phenomenon is called the thermoelectric effect, and this electromotive force is called the thermoelectric force. Thermocouples use this principle to measure temperature. Among them, the end 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 instrument or supporting instrument is connected, the display instrument will point out the thermoelectric potential generated by the thermocouple.</p><p><br></p><p>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:</p><p><br></p><p>1. The thermoelectric potential of a thermocouple is the temperature difference between the two ends of the thermocouple, not the temperature difference between the cold and working ends of the thermocouple;</p><p><br></p><p>2. The size of 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;</p><p><br></p><p>3. After the two thermocouple wires of the thermocouple are determined, the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; if the temperature of the cold junction of the thermocouple remains constant, the thermoelectric potential of the thermocouple is only Single-valued function of working end temperature. Solder two conductors or semiconductors A and B of different materials 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.</p>
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