What is the wiring method for thermal resistors?

Thermistor (such as Pt100) is a temperature sensor that converts temperature into resistance based on the principle that its resistance value changes with temperature. A temperature transmitter obtains the resistance value (voltage/current) by applying a known excitation current to a thermal resistor to measure its voltage at both ends, and then converts the resistance value into a temperature value to achieve temperature measurement.
There are three wiring methods between thermistor and temperature transmitter: two-wire system, three wire system, and four wire system.

Two line system
As shown in Figure 1. The transmitter applies excitation current I to the thermistor through wires L1 and L2, and measures the potential V1 V2.

Calculated Rt:

Due to the inability to measure the resistance RL1 and RL2 of the connecting wires, they were included in the resistance value of the thermistor, resulting in additional errors in the measurement results. If the thermal resistivity of Pt100 thermistor is 0.379 Ω/℃ at 100 ℃, and the resistance value of the wire is 2 Ω, the measurement error caused will be 5.3 ℃.

Three wire system
It is a common connection method in practical applications. As shown in Figure 2, add a wire to compensate for the measurement error caused by the resistance of the connecting wire. The three wire system requires that the material, diameter, length, and operating temperature of the three wires be consistent, so that the resistance values of the three wires are the same, i.e. RL1=RL2=RL3. Apply excitation current I to the thermal resistor through wires L1 and L2, and measure the potential V1 V2,V3. Wire L3 is connected to a high input impedance circuit, IL3=0.

The resistance value Rt of the thermistor:

From this, it can be concluded that the three wire connection method can compensate for measurement errors caused by the resistance of the connecting wires.