What is the mode of thermocouple lead _ instrument

Thermocouple lead way has 3 kinds: namely the 2 wire and 3 wire system, four wire system. 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. 3 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 application is very wide. Four 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. Thermocouple temperature measurement principle is based on a conductor or semiconductor resistance changes with the temperature change and this feature to measure temperature and parameters related to the temperature. Thermocouple are mostly made of pure metal materials, the application is a lot of platinum and copper, now have begun to use the materials such as nickel, manganese and rhodium thermocouple. Thermocouple is usually need to get the resistance signal through wire transfer to the computer control device or other secondary instrument. Ordinary thermocouples 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. 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 and resistance to vibration and impact resistant; 3, can bend, easy to install; 4, long service life. Capillary thermocouple ends 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. Flameproof thermocouple junction box flameproof thermocouple with special structure, its 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 temperature measurement principle and the thermocouple temperature measurement principle, the thermocouple temperature measurement is based on the heating effect of the resistance, the resistance of the characteristics of the resistance varies with temperature. Therefore, as long as the measured temperature thermocouple resistance changes, can measure the temperature. At present there are mainly metal thermocouple and thermistor two types of semiconductor. Metal thermocouple resistance value and temperature can use the following general approximate relation said, namely the Rt = Rt0 [ 1 +α( t- t0） ] Type of Rt for temperature t resistance; Rt0 for temperature t0 ( Usually t0 = 0 ℃) When the corresponding resistance; Alpha for temperature coefficient. Relationship between resistance and temperature of semiconductor thermistor for Rt = AeB in Rt for temperature for t/t type of resistance; A and B depends on the structure of the semiconductor material constant. In comparison, the thermistor temperature coefficient is bigger, room temperature resistance higher ( Usually in the thousands of the above) But poor interchangeability, serious nonlinear and only - temperature measurement range Around 50 ~ 300 ℃, a large number of used in home appliances and automotive temperature detection and control. Metal thermocouple generally applies to - Within the range of 200 ~ 500 ℃ temperature measurement, its characteristic is measuring accuracy, good stability, reliable performance, is widely application in process control. Industry commonly used metal thermocouple from the perspective of the change of resistance with temperature, most of the metal conductor has this property, but not all can be used for temperature measuring thermocouple, thermocouple as general requirements of metal materials, as far as possible big and stable temperature coefficient, resistivity ( Reduce the size of the sensor) under the same sensitivity , using temperature range has stable chemical physics performance, good replication of material and resistance change with temperature have the function relationship between ( A good linear relationship) 。