Regarding the thermoelectric potential of the thermocouple, the following issues should be noted: 1: The thermoelectric potential of the thermocouple is the difference between the temperature functions of the working end and the cold end of the thermocouple, not the function of the temperature difference between the cold end and the working end of the thermocouple. ; 2: The size of the thermoelectric potential generated by the thermocouple, when the data of the thermocouple is uniform, it has nothing to do with the length and diameter of the thermocouple. 3: When the two thermocouple wire data components of the thermocouple are determined, the thermocouple thermoelectric The size of the potential is only related to the temperature difference of the thermocouple; if the temperature of the cold end of the thermocouple remains constant, the thermoelectric potential of the thermocouple is only a single-valued function of the temperature at the working end. Weld two conductors or semiconductors A and B of different materials to form a closed loop, and two conductors of different components (called thermocouple wires or thermode When the same, an electromotive force occurs in the loop, this phenomenon is called the thermoelectric effect, and this electromotive force is called a thermoelectric potential. When a thermocouple measures temperature, the temperature of its cold end (the measurement end is the hot end, and the end connected to the measurement circuit through the lead wire is called the cold end) is required to remain constant, and its thermoelectric potential is proportional to the measurement temperature. If the temperature of the cold end (ambient) changes during measurement, it will seriously affect the accuracy of the measurement. A certain method is taken at the cold end to compensate for the influence of the temperature change of the cold end, which is called the cold end compensation of the thermocouple. Attachment: Calculation method of thermocouple cold junction compensation: From millivolt to temperature: measure the temperature of the cold junction, convert it to the corresponding millivolt value, add it to the millivolt value of the thermocouple, and convert the temperature. From temperature to millivolt: measure the actual temperature and the cold junction temperature, convert them into millivolts respectively, and subtract the millivolts to obtain the temperature. eg. If the indication surface has cold junction compensation, the actual temperature is 520 degrees; if the indication surface does not have cold junction compensation, first check the K-type thermocouple indexing table and find that the voltage corresponding to 520 degrees is 21.497mv, 25 degrees The corresponding voltage is 1mv; 21.497mv+1mvu003d22.497mv; and then check the K-type thermocouple indexing table, it can be concluded that the corresponding temperature value of 22.497mv is 543.47 degrees, that is, the actual temperature is 543.47 degrees. PS: The graduation number is a standard sequence used to reflect the temperature change of the temperature sensor in the measurement temperature range corresponding to the change of the sensor voltage or resistance value, that is, the temperature value corresponding to the thermal resistance, thermocouple, resistance and potential. Second, the difference between thermocouple and thermal resistance: The first, the nature of the signal, the thermal resistance itself is a resistance, the change of temperature makes the resistance change positive or negative; while the thermocouple is the change of the induced voltage, he Change with temperature changes. .Although they are all touch-type temperature measurement surfaces, their temperature measurement scales are different. Thermocouples are used in higher temperature environments, such as platinum-rhodium 30---platinum-rhodium 6 (B type) The measurement range is 300 degrees~~ 1600 degrees, short-term can measure 1800 degrees. Type S is 20~~1300 (short-term 1600), K type is 50~~1000, short-term 1200). XK type is 50~~600 (800), E type is 40~~800 (900). There are also J type, T type, etc. This type of surface is usually used for higher temperatures above 500 degrees, and the output thermoelectric potential is very high in the low temperature region. When the potential is small, the opposite interference method and secondary table sum requirements are very high, otherwise the measurement is prohibited, and at lower temperatures. The relative error caused by the change of the cold end temperature and the change of the ambient temperature is very prominent, and it is not easy to be fully compensated. At this time, at medium and low temperatures, the temperature measurement range of thermal resistance is usually -200 ~ 500, and even lower temperatures can be measured (for example, carbon resistance can be used to measure a low temperature of about 1K). At present, platinum thermal resistance is normally used. Pt100, (also Pt50, 100 and 50 represent the resistance value of the thermal resistance at 0 degrees. On the top of the old graduation number, BA1, BA2 are used to indicate that the resistance value of BA1 at 0 degrees is 46 ohms, and copper is also used in industry. Resistance, the graduation number is CU50 and CU100, but the temperature measurement scale is small, between 150~150, and in some special occasions, there are indium resistance, manganese resistance, etc.). Second, the temperature range of the two sensors is different. The thermal resistance usually checks the temperature range of 0-150 degrees (of course, it can check the negative temperature), and the thermocouple can check the temperature range of 0-1000 degrees (or even higher). Therefore, the former It is a low temperature inspection, and the latter is a high temperature inspection. Third, from the material point of view, thermal resistance is a metal material, which has a temperature sensitive change of metal material, and thermocouple is a bimetal material, that is, two different metals, due to the change of temperature, in two different metals A potential difference occurs at both ends of the wire. Fourth, on-site judgment in operation Thermocouples have positive and negative poles, as well as positive and negative compensating wires. The first thing to do is to ensure that the connection and equipment are correct. In operation. The most common ones are short circuit, open circuit, poor touch (distinguished by a multimeter) and metamorphosis (distinguished by appearance color). When viewing, to separate the thermocouple from the secondary meter, use a tool to short-circuit the compensation line on the secondary meter, the meter indicates room temperature and then short the thermocouple terminal, and the meter indicates the ambient temperature of the thermocouple location (no, the compensation line has fault), and then use the multimeter mv file to roughly estimate the thermoelectric potential of the thermocouple (if it is normal, please check the technology). The thermal resistance short circuit and open circuit can be judged with a multimeter. During operation, if a short circuit is suspected, just remove a wire end from the resistance end to see the appearance. If it reaches the maximum value, the thermal resistance is short-circuited and returned to zero, and the wire is short-circuited to ensure normal connection and installation. The value appears to be low or unstable, and the possibility of water ingress of the maintenance pipe appears to be the largest.
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