What is the scope of the PT100 wiring diagram for thermal resistors and how to connect the three types of leads?

PT100 is a platinum thermistor, and its resistance value is proportional to changes in temperature. The relationship between the resistance of PT100 and temperature change is: when the temperature of PT100 is 0 ℃, its resistance is 100 ohms, and at 100 ℃, its resistance is about 138.5 ohms. Its industrial principle: When PT100 is at 0 degrees Celsius, its resistance is 100 ohms, and its resistance will increase uniformly with the temperature.

Thermistor is a component that converts temperature changes into changes in resistance values, typically requiring the transmission of resistance signals through leads to computer control devices or other instruments. Industrial thermal resistors are installed on the production site and there is a certain distance between them and the control room, so the leads of the thermal resistors will have a significant impact on the measurement results.

At present, there are three main ways to lead thermal resistors:

◆ Two wire system: The method of connecting a wire at each end of a thermal resistor to extract a resistance signal is called a two wire system: This lead method is simple, but due to the inevitable lead resistance r of the connecting wire, the size of r is related to the material and length of the wire, so this lead method is only suitable for measuring lower values

◆ Three wire system: The method of connecting one lead at the root of a thermal resistor and two leads at the other end is called the three wire system. This method is usually used in conjunction with an electric bridge and can effectively eliminate the influence of lead resistance. It is a commonly used lead resistance in industrial process control.

◆ Four wire system: The method of connecting two wires at each end of the root of a thermistor is called a four wire system, where two leads provide a constant current I to the thermistor, convert R into a voltage signal U, and then lead 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 temperature detection.

The thermal resistor 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 the electric bridge, the thermal resistor's connecting wire (from the thermal resistor 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. Adopting a three wire system, one wire is connected to the power terminal of the bridge, and the other two wires are respectively connected to the bridge arm where the thermal resistance is located and the adjacent bridge arm, thus eliminating measurement errors caused by the resistance of the wire line. In industry, the three wire connection method is generally used. Thermocouples generate millivolt signals, so there is no such problem.