Thermocouple sensor working principle of thermocouple sensor is to use the characteristics of the resistance of the conductor changes with temperature, related parameters of temperature and temperature testing device. Thermocouple temperature measurement is based on the metal conductor resistance value increases with the increase of the temperature of the properties for temperature measurement. Thermocouples are mostly made of pure metal materials, the application is a lot of platinum and copper, in addition, now began to use the materials such as nickel, manganese and rhodium thermocouple. They have the temperature coefficient of resistance, good linear, stable performance, use temperature scope width, easy processing, etc. And with the development of science and technology of the thermocouple sensor temperature measurement range and with extension, the low temperature has been successfully used in 1 ~ 3 k temperature measurement, the high temperature side also appeared a variety of thermocouple sensor used in 1000 ~ 1300 ℃. The classification of the thermocouple sensor NTC thermocouple sensor: this kind of sensor for negative temperature coefficient, namely, the sensor resistance decreases with the increase of temperature; PTC thermocouple sensor: this kind of sensor for positive temperature coefficient, namely, the sensor resistance increases with the rise of temperature. Installation of thermocouple, should pay attention to accurate temperature measurement, safety examination and maintenance is convenient, and does not affect the equipment operation and production operations. To meet the above requirements, in the choice of the installation site of the thermocouple and insertion depth should pay attention to the following points: 1, in order to make the measurement end of the thermocouple, and had good heat exchange between the measured medium, should be a reasonable choice of observation, try to avoid the valve, elbow and thermocouple pipeline and equipment near the corner of installations. 2, with a thermocouple with heat transfer and heat loss protection casing, in order to reduce the measurement error, thermocouple and thermocouple should have enough: (insert depth 1) For measurement of pipeline fluid medium temperature thermocouple, generally should be measuring end is inserted into the pipe (at the centre of the Vertical or inclined installation) 。 As the current body of pipe diameter is 200 mm, the 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) 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. ( 4) If is need to measure the temperature of flue gas, although the flue is 4 m in diameter, thermocouple insertion depth of 1 m.