The working principle and type of thermocouple is what _ instrument

Commonly used in low temperature zone thermocouple is a temperature testing instrument. Its main characteristic is high accuracy, good stability. A platinum thermocouple measuring accuracy is higher, it is not only widely used in industrial temperature measurement, and is made into standard benchmark instrument. 1, thermocouple temperature measurement principle and material of 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. 2, the type of thermocouple ( 1) Type thermocouple: from the thermocouple temperature measurement principle, the measured the change in temperature is directly measured by thermocouple resistance change, therefore, the wires of a thermocouple body such as a variety of wire resistance change will affect temperature measurement. （ 2) Thermocouple, thermocouple is by thermal components ( The resistor body) , wire, insulation materials, stainless steel casing combination solid body, its diameter is commonly phi - 2 Phi is 8 mm, achieve more xiaoke phi is 0. 25mm。 Compared with conventional type thermocouple, it has the following advantages: (1) small volume, no internal air gap, thermal inertia, the measured value after small; (2) good mechanical properties, resistance to vibration and shock. (3) can bend, easy to install (4) long service life. （ 3) End thermocouple: end thermocouple temperature sensing element resistance wire winding by special processing, close to the face in the thermometer. Compared with general axial thermocouple, it can reflect more correctly and quickly end face of the actual temperature being measured, is suitable for measuring temperature bearing and other parts of the face. （ 4) Flameproof thermocouple: block type thermocouple through the special structure of the junction box, put the shell internal explosive mixed gas by affected by sparks or electric arc explosion limit within the junction box, that wouldn't make production field explosion. Flameproof thermocouple can be used in the bla B3c level zone with explosion danger place temperature measurement. Thermocouple installation considerations for the installation of the thermocouple, 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: 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 pipe and equipment near the corner of the furnish thermocouple or thermocouple. 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: for thermocouple temperature between fluid, usually should be measuring end is inserted into the pipe (at the centre of the Vertical or inclined installation) , such as the current body of pipe diameter is 200 mm, the thermocouple insertion depth should choose 100 mm.