The thermocouple is a sensitive component of the thermoelectric thermometer. It is composed of two different materials of metal conductors. One end of the two conductors is welded together as the measuring end to fully contact the measured medium, and the other end is connected as a reference end to the temperature indicating instrument.
The temperature measurement principle of thermocouples is based on the Seebeck effect, that is, if two homogeneous conductors of different compositions form a closed loop, when there is a temperature gradient at both ends, there is current flowing through the loop, and there is an electromotive force between the two ends. , also known as Seebeck electromotive force. Its value is only related to the nature of the metal material that makes up the thermocouple, the temperature difference between the two ends, and the geometry (length, diameter) of the two hot electrodes with the thermocouple and the temperature along the length of the electrode The distribution is irrelevant. The thermocouple can also be regarded as a transducer, which converts thermal energy into electrical energy, and measures the magnitude of the electromotive force generated in the loop, and uses the function relationship between thermoelectric potential and temperature to achieve the purpose of temperature measurement.
The thermocouple has simple structure, stable performance, wide temperature measurement range, high temperature measurement accuracy and convenient use. It is widely used in temperature measurement and temperature control procedures in heat treatment of mechanical industry, scientific research, etc. According to the materials that make up the thermocouple and The difference in structure, (the thermocouple can be divided into many types). Correct selection and installation of thermocouples is the premise of the rational use of thermocouples, not only can accurately obtain temperature values, ensure product quality, but also save thermocouple materials. Consumption.
The choice of thermocouple should first select the thermocouple's hot electrode and protective sleeve according to the upper limit of the measured temperature and the environment of use. Secondly, select the size of the thermocouple according to the structure and installation characteristics of the object to be tested. Thermocouple according to the structure Different forms can be divided into: general industrial type, armored type, high performance solid type and special type thermocouple. Common common industrial thermocouples have the following types:
(1) Platinum é”—10-platinum thermocouple belongs to noble metal thermocouple. The nominal composition of the positive electrode is platinum-rhodium alloy (SP) containing 10% bismuth, and the negative electrode is pure platinum (SN). Long-term use temperature is 1300 OC, short-term use temperature 1600 cC. Physical, stable chemical properties, strong anti-oxidation performance. It should be used continuously in oxidation and inert environment. It is generally used for high-temperature measurement with high accuracy requirements, but it is expensive, mechanical strength is poor, and thermal electromotive force is small. It is equipped with a display instrument with high sensitivity and the index number is S.
(2) Nickel-chromium-nickel-silicon thermocouple is a low-cost metal thermocouple with a positive electrode of 10% nickel-chromium alloy (KP) and a negative electrode of 3% nickel-silicon alloy (KN). Temperature 1000 9C, short-term use temperature 1200 `C, thermoelectric potential 4 to 5 times larger than S-type thermocouple, excellent linearity, wide temperature range, is the most stable one in the cheap metal thermocouple. It is suitable for oxidation Sexual, inert use in continuous use, the price is cheap, the error is about 7 ° C. But its hot electrode is not easy to do too uniform, easy to oxidize, especially in low temperature, its stability is poor, the index number is K
(3) Nickel-chromium-constantan thermocouple is a low-cost metal thermocouple, the positive electrode is nickel-chromium alloy (EP), and the negative electrode is copper-nickel alloy (EN). Long-term use temperature is 600 0C, short-term use temperature is 800 9C, the biggest feature It is the largest thermal electromotive force rate, that is, the highest sensitivity, but it is easily oxidized, and the graduation number is E.
(4) Copper-constantan thermocouple is a low-cost metal thermocouple with a positive electrode of pure copper (TP) and a negative electrode of constantan (copper-nickel) alloy (TN). A thermocouple commonly used in low temperature conditions, temperature measurement The range is 200-300 OC, the thermal electrode wire has good uniformity, high stability, good sensitivity and low price. The indexing number is Toæº-type thermocouple, which is installed in the metal sleeve with insulating material. The combination is processed into a flexible and compact combination, and then cut according to the required length, and the measuring end and the reference end are processed to prepare an armored thermocouple. The thermocouple can be made very thin and long. In use, it can be bent with the need of measurement. It has wide temperature measurement range, fast response speed, small heat capacity at hot junction, long service life, high mechanical strength, high pressure resistance, wide adaptability and large winding. Easy to install and use, the application is extremely extensive.
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