Analysis Of The Principle Of Thermocouple

As one of the most widely used temperature sensors in industrial temperature measurement, thermocouples, together with platinum thermal resistance, account for about 60% of the total temperature sensor. Thermocouples are usually used in conjunction with display instruments to directly measure various productions. The surface temperature of liquids, vapors and gaseous media and solids in the range of -40 to 1800. Thermocouple working principle: Thermocouple working principle: Two different components of the conductor (called thermocouple wire or hot electrode) are joined at both ends into a loop. When the junction temperature is different, an electromotive force is generated in the loop. This phenomenon is called the thermoelectric effect, and this electromotive force is called the thermoelectric potential. Thermocouples use this principle for temperature measurement. The one 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 compensating end); the cold end and the display The instrument or the supporting instrument is connected, and the display meter indicates the thermoelectric potential generated by the thermocouple. A thermocouple is actually an energy converter that converts thermal energy into electrical energy and measures the temperature with the generated thermoelectric potential. For the thermoelectric potential of a thermocouple, the following problems should be noted: (1) The thermoelectric potential of the thermocouple is The difference between the temperature function of the thermocouple, not the temperature difference between the thermocouples; (2) The magnitude of the thermoelectric potential generated by the thermocouple, when the material of the thermocouple is uniform, independent of the length and diameter of the thermocouple , only related to the composition of the thermocouple material and the temperature difference between the two ends; (3) When the composition of the two thermocouple wires of the thermocouple is determined, the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; if the thermocouple The temperature at the cold end is kept constant, and the thermoelectric potential of the thermocouple is only a single-valued function of the temperature at the working end. The basic structure of thermocouple: the basic structure of thermocouple: thermocouple for industrial temperature measurement, its basic structure includes thermocouple wire, insulation tube, protection tube and junction box. I. Commonly used thermocouple wire and its properties 1. Platinum rhodium 10-platinum thermocouple (index S, also known as monoplatinum thermocouple) The positive electrode of the thermocouple is a platinum-rhenium alloy containing 10% bismuth. The negative electrode is pure platinum; its characteristics are: (1) stable thermoelectric performance, strong oxidation resistance, continuous use in an oxidizing atmosphere, long-term use temperature up to 1300, super-1400, even in air, pure Platinum wire will also recrystallize, causing coarse and broken grains; (2) high precision, it is the highest accuracy level among all thermocouples, usually used as standard or measuring higher temperature; (3) use Wide range, uniformity and interchangeability; (4) The main disadvantages are: differential thermoelectric potential is small, so the sensitivity is low; the price is relatively expensive, the mechanical strength is low, and it is not suitable for the reducing atmosphere or the condition of metal vapor. Use under. 2, platinum rhodium 13-platinum thermocouple (index number R, also known as single platinum rhodium thermocouple) The positive electrode of the thermocouple is 13% platinum-rhodium alloy, the negative electrode is pure platinum, compared with the S-type, Its potential rate is about 15%, and other properties are almost the same. This kind of thermocouple is used most in the Japanese industry as a high temperature thermocouple, but it is used less in China;