Selection of thermocouple method according to the temperature range 500 ℃ or more generally choose thermocouple, usually selected thermocouple below 500 ℃; According to the accuracy of measurement choice: thermocouple with high accuracy requirement choice, to choose thermocouple accuracy is not high. According to the measurement range selection measured by thermocouple temperature generally refers to the 'point', generally refers to the space average temperature measured by thermocouple; 2 wire thermocouple wiring simple, but to bring in additional error lead resistance. So shall not apply to the manufacturing accuracy of grade a thermocouple, and when use and lead wires have shoulds not be too long. Wire system can eliminate the influence of the lead resistance, measurement accuracy is higher than 2 wire system. As the process of detecting element, its application is very wide. Wire system not only can eliminate the influence of the lead resistance, and the connection wire resistance phase at the same time, also can eliminate the influence of the resistance. When in high precision measurement, must use the four wire system. The characteristics of the thermocouple thermocouple thermal inertia small, response rapidly, such as pipe diameter - maintenance 2 mm plain platinum resistance, the time constant of 25 s; The metal casing diameter - 2 mm kai thermocouple, the time often only about 5. Components and protection for the elastic compression at the bottom of the tube structure, the thermal response time shortened and further improve the vibration resistance. Thermocouple with flexible performance, kai with thermocouple in addition to the head, can make the direction of bending, so it is suitable for the structure is relatively complex, small temperature measurement equipment. Thermocouple long using life, kai in resistance of thermocouple by iron power twist of insulation materials cover and metal casing maintenance, thermocouple wire is not easy to jung corrosion by harmful medium, so it has normal thermocouple, thermocouple life with good resistance to vibration and shock resistance.