Thermal resistance temperature measurement principle (two-three-four-wire system)

The principle of temperature measurement The thermal resistance (such as Pt100) is a temperature sensor that uses the principle that its resistance value changes with temperature and converts the temperature into the resistance. The temperature transmitter obtains the resistance value (voltage/current) by applying a known excitation current to the thermal resistance to measure the voltage across it, and then converts the resistance value into a temperature value to achieve temperature measurement. There are three wiring methods between the thermal resistance and the temperature transmitter: two-wire system, three-wire system, and four-wire system. The two-wire system is shown in Figure 1. The transmitter applies excitation current I to the thermal resistance through wires L1, L2, and the measured potentials V1, V2......
Principle of temperature measurement
Thermistor (such as Pt100) is a temperature sensor that converts temperature into resistance, which is made using the principle that its resistance changes with temperature.
The temperature transmitter obtains the resistance value (voltage/current) by applying a known excitation current to the thermal resistance to measure the voltage across it, and then converts the resistance value into a temperature value to achieve temperature measurement.
There are three wiring methods between the thermal resistance and the temperature transmitter: two-wire system, three-wire system, and four-wire system.
two-wire system
  Figure 1. The transmitter applies an excitation current I to the thermal resistance through the wires L1 and L2, and the electric potentials V1 and V2 are measured.

calculated Rt:
Because the resistance RL1 and RL2 of the connecting wires cannot be measured, they are included in the resistance value of the thermal resistance, which causes additional errors in the measurement results. For example, the thermal resistivity of the Pt100 thermal resistance is 0.379Ω/°C at 100°C. If the resistance value of the wire is 2Ω at this time, the measurement error will be 5.3°C.
three-wire system
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 materials, diameters, and lengths of the three wires are the same, and the operating temperature is the same, so that the resistance values of the three wires are the same, that is, RL1=RL2=RL3. Excitation current I is applied to the thermal resistor through the wires L1 and L2, and the electric potentials V1, V2, V3 are measured. Wire L3 is connected to the high input impedance circuit, IL3=0.

The resistance of thermal resistance Rt:
Therefore, the three-wire connection method can compensate the measurement error caused by the resistance of the connecting wire.
four-wire system
is the ideal wiring method for thermal resistance temperature measurement. As shown in Figure 3, an excitation current I is applied to the thermal resistance through the wires L1 and L2, and the potentials V3 and V4 are measured. The wires L3 and L4 are connected to the high input impedance circuit, IL3=0, IL4=0, so V4-V3 is equal to the voltage across the thermal resistance.

The resistance value of thermal resistance:
It can be obtained that the four-wire measurement method is not affected by the resistance of the connecting wire.