Thermal resistance is a primary element that converts temperature changes into resistance changes. It is usually necessary to transmit the resistance signal to a computer control device or other primary instrument through a lead wire. Industrial thermal resistance is installed at the production site, and there is a certain distance between it and the control room, so the lead of the thermal resistance will have a greater impact on the measurement results. At present, there are mainly three types of thermal resistance lead wires. Thermal resistance is a primary element that converts temperature changes into resistance value changes. It is usually necessary to transmit the resistance signal to a computer control device or other primary instruments through lead wires. Industrial thermal resistance is installed at the production site, and there is a certain distance between it and the control room, so the lead of the thermal resistance will have a greater impact on the measurement results. There are currently three main ways to lead the thermal resistance. ○1 Two-wire system: the method of connecting a wire at both ends of the thermal resistance to lead out the resistance signal is called the two-wire system: this lead method is very simple, but there must be lead resistance due to the connection of the wires. The size of r, r is related to the material and length of the wire, so this lead method is only suitable for occasions with low measurement accuracy. ○2 Three-wire system: connect one lead at one end of the thermal resistance and two at the other end The lead method is called the three-wire system. This method is usually used in conjunction with the bridge to better eliminate the influence of lead resistance. It is the most commonly used lead resistance in industrial process control. ○3 Four-wire system: The method of connecting two wires at each end of the thermal resistance is called a four-wire system, in which two leads provide a constant current I for the thermal resistance, convert R into a voltage signal U, and then pass the other two The lead wire leads U to the secondary meter. It can be seen that this lead method can completely eliminate the influence of lead resistance and is mainly used for high-precision temperature detection. The thermal resistance adopts the three-wire connection method. The three-wire system is used to eliminate the measurement error caused by the resistance of the connecting wire. This is because the circuit for measuring thermal resistance is generally an unbalanced bridge. The thermal resistance is a bridge arm resistance of the electric bridge, and its connecting wire (from the thermal resistance to the control room) also becomes a part of the bridge arm resistance. This part of the resistance is unknown and changes with the ambient temperature, causing measurement errors. Using a three-wire system, one wire is connected to the power terminal of the bridge, and the other two wires are connected to the bridge arm where the thermal resistance is located and the adjacent bridge arm, which eliminates the measurement error caused by the wire line resistance
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