Two wire system: The method of connecting a wire to each end of a thermoelectric resistor to lead out a resistance signal is called a two wire system. This lead method is very simple, but due to the presence of lead resistance r in the connecting wire, the size of r is related to the material and length of the wire. Therefore, this lead method is only suitable for situations with low measurement accuracy
Three wire system: The method of connecting one lead at the root of the thermoelectric resistor and two leads at the other end is called a three wire system. This method is usually used in conjunction with an electric bridge to effectively eliminate the influence of lead resistance, and is the most commonly used method in industrial process control.
Four wire system: The method of connecting two wires at each end of the root of the thermal resistor is called a four wire system, where two leads provide a constant current I for the thermal resistor, converting R into a voltage signal U, and then leading U to the secondary instrument through the other two leads. 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 a three wire connection method. The use of a three wire system is to eliminate measurement errors caused by the resistance of connecting wires. This is because the circuit for measuring thermal resistance is usually an unbalanced bridge. As a bridge arm resistor of an electric bridge, a thermistor's connecting wire (from the thermistor to the central control room) also becomes a part of the bridge arm resistor. This part of the resistor is unknown and varies with the ambient temperature, causing measurement errors. Using a three wire system, one wire is connected to the power supply end of the bridge, and the other two are respectively connected to the bridge arm where the thermal resistor is located and its adjacent bridge arm, thus eliminating measurement errors caused by wire line resistance.