Thermocouple and the thermocouple, are we often can see the temperature of the instrument at ordinary times, both in industry application is very extensive. When measuring temperature, however, need to know if the measured object under 200 ℃ temperature range selection of thermocouple; If more than 200 ℃ temperature range of the object being measured in the selection of thermocouple. Of thermocouple and thermocouple installed at the same time, should pay attention to temperature measuring accurate, safe and reliable and convenient maintenance, and shall not affect the equipment operation and production operations. To meet the above requirements, in the choice of thermocouple and thermocouple installation site and insertion depth should pay attention to the following points 1, in order to make the measurement end of the thermocouple and the thermocouple, and had good heat exchange between the measured medium, should choose the observation points, try to avoid the valve, elbow and pipe and equipment near the corner of the furnish thermocouple or thermocouple, thermocouple with a protective casing and thermocouple with heat transfer and heat loss. At the same time in order to reduce the measurement error, thermocouple and thermocouple should have enough insert depth: 1, for the measurement of pipeline fluid temperature thermocouple, generally should be measuring end is inserted into the pipe ( Vertical or inclined installation) 。 As the current body of pipe diameter is 200 mm, the thermocouple or thermocouple insertion depth should choose 100 mm; 2, for high temperature and high pressure and high velocity of temperature measurement, Such as main steam temperature) , in order to reduce cases of fluid resistance and prevent cases break under the action of a fluid, can take protecting tube shallow inserted way or use hot thermocouple. Shallow inserted type thermocouple protection tube, its depth into the steam pipe should be no less than 75 mm; Heat sets the standard of thermocouple insertion depth is 100 mm; 3, if need to measure is the temperature of flue gas, although the flue is 4 m in diameter, thermocouple or thermocouple insertion depth of 1 m. 4, when measuring the original insertion depth of more than 1 m, shall be installed vertically, as far as possible or add racks and protect casing. Thermocouple elements in the nature of the thermocouple is a kind of commonly used low temperature zone temperature meter, and they are widely used in the thermal power units to understand the working principle of thermal resistance and apply it to the actual production, to ensure accurate and reliable in the thermal measurement is of great significance, chongqing instrument co. , LTD. Through the element in this paper, the thermocouple temperature measurement principle, combining with the actual application, analysis of common failures. Thermocouple temperature measurement is based on the heating effect of the resistance, the resistance of the characteristics of the resistance varies with temperature. Therefore, as long as measure the thermal resistance changes of thermocouple, through measuring bridge into a voltage signal is sent to the measured temperature display instrument instructions or record can measure the temperature. Currently widely used thermocouple materials is platinum and copper, platinum resistance is high precision, suitable for neutral and oxidizing medium, good stability, has a certain nonlinear, the higher the temperature the smaller the resistance rate of change; Copper resistance in the temperature range of a linear relationship between internal resistance value and temperature, temperature line number is big, suitable for corrosive medium, more than 150 ℃ susceptible to oxidation. Thermocouple elements under the effect of temperature, the resistance wire resistance change and change. Can be used to measure - 200 ℃ ~ 800 ℃ ﹢ range of temperature. Its advantage is: minimal deviation. Because of its good electrical output performance, thermocouple can be adjusted for the display device, recorder, instrument, scanner and computer provide accurate input values. Thermocouple elements of two leads are connected at one end, and the other end of the connecting a lead, the lead form is called three wire system. It can eliminate the influence of internal leads, and two wire system, the measuring accuracy is higher than the commonly used in narrow temperature range, lead is too long, or lead wire is easy to change in temperature. Three wire electrodes is often used with a bridge circuit, two wires in the bridge's two bridge arm, another thread on the power of the bridge, to eliminate the effects of lead resistance change. Three wire electrodes can reduce or eliminate the error of measurement caused by lead resistance change. From the perspective of the change of resistance with temperature, most of the metal conductor has this property, but not all can be used for temperature measuring thermocouple, thermocouple as general requirements of metal materials, as far as possible big and stable temperature coefficient, resistivity ( Reduce the size of the sensor) under the same sensitivity , using temperature range has stable chemical physics performance, good replication of material and resistance change with temperature have the function relationship between ( A good linear relationship) 。