What is the difference between a thermocouple temperature measurement principle and wiring _ instrument

Thermocouple temperature measuring thermocouple temperature measurement principle and the material is based on the metal conductor resistance value increases with the increase of the temperature of the properties for temperature measurement. Thermocouples are mostly made of pure metal materials, the application is a lot of platinum and copper, in addition, now has started using survey, nickel, manganese and rhodium thermocouple materials such as manufacturing. Two lines, three lines, four lines thermocouple from thermocouple temperature measurement principle, the difference between a measured the change in temperature is directly measured by thermocouple resistance change, therefore, thermocouple of all kinds of wire resistance change will affect temperature measurement. Three commonly used to eliminate the influence of the lead resistance wire or four wire system. Thermocouple wiring has respectively, mainly related to the application. General 2 line is used for close range; 3 line used for distance measurement, mainly in order to overcome the effects of line resistance and interference. Another 4 wire thermocouple, is proposed for higher measurement accuracy. Used primarily to applying a current, thermocouple and then measuring the voltage to improve measurement accuracy and sensitivity. Connected to the thermocouple testing equipment ( Temperature controller, PLC input, etc. ) There are four terminals. i+、i- v, v +, - 。 Among them, the I +, I - End is to provide a constant current for thermocouple, v + and v - Voltage change of the thermocouple is used for monitoring, detecting temperature change in turn. Line 4 is 4 line from the thermocouple at both ends, and four terminal connection. 3 line is drawn 3 line, I - this requires testing equipment ,v - The short answer. 2 line makes lead to 2 lines, I - this requires testing equipment ,v - , I +, v + short answer. Two lines, three lines, four lines of thermocouple temperature measurement principle is the same, the only difference. Accurately, current loop and voltage measurement circuit of the question of whether or not to separate wiring. 2 line, current loop and voltage measurement circuit and 2. 1, accuracy is poor. Line 3, the current loop of reference and the reference voltage measurement circuits for a line. Precision is slightly better. 4 lines, separate circuit circuit and voltage measurement circuit, high precision, but the cost line. By thermocouple pick up the bridge of the copper wire resistance will change with the change of environmental temperature, if the wires are only meet on a bridge arm, when the environment temperature changes, connecting wire resistance changes in the value and the change of the resistance of the thermocouple RT value superposition, and produce additional error. So widely used in industry of three-wire system wiring method, take line 2 and 3 respectively to bridge the two bridge arm, when the wire resistance change, can offset part of each other, in order to reduce impact on the value of an instrument said. But the error, there is a limit to the unbalanced bridge, only if the starting point of instrument calibration to get full compensation, and in full scale the additional error is very big. For unbalanced bridge to consider the power wire of the additional temperature error and thermocouple connected to the power supply when a current flows through the wire 1, there will be a voltage drop, when the environment temperature changes, bridge of the upper and lower branch voltage will also change, which brings to the instrument must be additional temperature error. For these reasons, to match the moving coil instrument thermocouple, when the three wire system wiring method, for connecting wire resistance has a regulation, every line of resistance for 5 Ω commonly, if less than 5 Ω, must use manganin resistor complement, to ensure the instrument large additional error is less than 0. 5%. For the use of integrated operational amplifier display control device, the input impedance is high, the change of the external conductor resistance much smaller than the instrument input impedance, so it can ignore the conductor resistance tolerance of the change and will not affect measurement accuracy. So the resistance value, there is no requirement for connecting wires, which can adjust the resistance from outside of work.