In metals, the carrier for the free electrons, when temperature increases, although the number of free electrons basically remain unchanged, When the temperature change range is not very big) , but each free electron kinetic energy will increase, and therefore under certain electric fields, to make these veins of electronic directional movement will encounter greater resistance, lead to metal resistance increases with the rise of temperature. Thermocouple is using resistance increases with temperature rise and this feature to measure temperature. Thermistor of thermocouple sensor is a new type of semiconductor temperature measuring element. Semiconductor in conductive carrier, due to the number of carriers in semiconductor far far less than the number of free electrons in the metal, so it's resistivity. With temperature rise, more the valence electrons in the semiconductor heating stimulates the transition to a higher level and produce new electronic - hole pair, and attend to the electric carrier number increases, the resistivity of the semiconductor is reduced ( Increase conductivity) 。 Because the carrier number according to the index law increases with temperature rise, so the resistivity of the semiconductor is along with the temperature rise according to the index law. Thermistor is using the semiconductor carrier number changes over temperature made of the characteristics of a kind of temperature sensitive components. When the temperature change 1 ℃, some semi-conductor thermistor resistance changes will reach ( 3 ~ 6) %。 Under certain conditions, according to the measurement of thermal resistance change of the change in temperature pressure sensor. Thermocouple sensor installation requirements for the installation of the 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, thermocouple with protecting tube with heat transfer and thermal loss, 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 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 is need to measure the temperature of flue gas, although the flue is 4 m in diameter, 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 weighing sensor.