According to 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. Three wire system can eliminate the influence of the lead resistance, the measuring accuracy is higher than 2 wire system. As the process of detecting element, its wide application. Three-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. According to temperature range: 500 ℃ or more generally choose thermocouple, usually selected thermocouple below 500 ℃;