1, the difference between the thermocouple and thermocouple signal properties: thermocouple itself is resistance, thermocouple thermometer is according to the principle of wire resistance change with temperature of work; Thermocouple: had two different materials of the conductor or semiconductor welding together, constitute a closed loop, when the conductor when there is a temperature difference between the two contact occurs between the electromotive force, thus forming a certain size of current in circuit, this kind of phenomenon called the thermoelectric effect. Thermocouple is in using this effect to work. 2, two kinds of sensors to detect temperature range: thermal resistance 0 - general detection 150 degrees temperature range, higher measurement range can be up to 600 degrees, and thermocouple can detect 0 - Temperature range of 1000 c ( Even higher) 。 3, the material cost: thermocouple is a kind of metal material, with a temperature sensitive metal materials, thermocouple is resistance signal, the output is the resistance value ( Ω) , nothing is negative. Commonly used pt100, cu50. 。 And thermal coupling is double metal materials, both two kinds of different metallic materials, due to the changes of temperature, in two different electric potential difference at both ends of the wire. Can be divided into k, s, e, r, t, n equal degrees, can be used separately in different measurement medium, the temperature measurement range. Thermocouple is a voltage signal, the output is millivolt values ( mv) , have a positive negative. There are also more expensive than resistance, cheaper than resistance, compensation conductor, but on comprehensive cost of the thermocouple is high. 4 and the relationship with PLC, PLC corresponding thermocouple and thermocouple input module is also different, this sentence is no problem, but the general PLC are directly connected to the 4 ~ 20 ma signal, and the thermocouple and thermocouple are generally transmitter to access with a PLC. If access to DCS don't have to use the transmitter. Thermocouple is a sign of RTD, thermocouple is tc signal! ( PLC has thermocouple module and thermocouple module, can be directly input resistance and thermocouple signals. 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) 。