What is the method of measuring thermocouple elements nature _ instrument

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 measure the thermal resistance changes of thermocouple, through measuring bridge into a voltage signal is sent to the measured temperature digital display table instructions or record can measure the temperature. Currently widely used thermocouple materials is platinum and copper, platinum resistance is high precision, suitable for neutral and oxidizing medium, good stability, has a certain nonlinear, the higher the temperature the smaller the resistance rate of change; Copper resistance in the temperature range of a linear relationship between internal resistance value and temperature, temperature line number is big, suitable for corrosive medium, more than 150 ℃ susceptible to oxidation. Thermocouple elements under the effect of temperature, the resistance wire resistance change and change. Can be used to measure - 200℃~? Temperature of 800 ℃ range. Its advantage is: minimal deviation. Because of its good electrical output performance, thermocouple can be adjusted for the digital display table, recorder, instrument, scanner and computer to provide accurate input values. Thermocouple elements of two leads are connected at one end, and the other end of the connecting a lead, the lead form is called three wire system. It can eliminate the influence of internal leads, and two wire system, the measuring accuracy is higher than the commonly used in narrow temperature range, lead is too long, or lead wire is easy to change in temperature. Three wire electrodes is often used with a bridge circuit, two wires in the bridge's two bridge arm, another thread on the power of the bridge, to eliminate the effects of lead resistance change. Three wire electrodes can reduce or eliminate the error of measurement caused by lead resistance change. 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) 。 Measurement by thermocouple as the temperature of the element is what will convert why measurement of thermocouple is converted to the temperature change resistance change of an element, usually need to change the electrical resistance signal through wire transfer to the test control unit or other instrument display the corresponding temperature at a time. Commonly used thermocouple wire basically has the following three ways: 1, the two wire: in the thermocouple at both ends of the connection to elicit resistance signal way is called a wire two wire: this kind of wire method is very simple, there must be lead resistance R, but because of the connection wire R size and material of the wire and the length of the factors, so this method is only applicable to lead at a lower measuring accuracy; 2, three-wire system: at one end of the thermocouple roots connect a wire, the other end connected to the two lead way called three wire system, this way, which is equipped with a bridge can be better to eliminate the influence of the lead resistance, is commonly used in industrial process control, the lead resistance. 3, four wire: on both ends of the thermocouple root way each connecting two wire called a four wire system, including two fuses for thermocouples provide constant current I, U R into a voltage signal into, again through the other two root lead to guide U to secondary instrument. Is this way of lead can completely eliminate the resistance of the lead, it is mainly used for very high precision temperature detection.