Principle and Common Wiring Methods of Thermal Resistance Temperature Measurement

Thermistor temperature measurement principle and common wiring method 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. The two-line system is shown in Figure 1. The transmitter applies excitation current I to the thermistor through wires L1 and L2, measuring potentials V1 and V2. Calculated Rt: Due to the inability to measure the resistance RL1 and RL2 of the connecting wires, it is included in the resistance value of the thermistor, resulting in additional measurement errors. 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 ℃. The three wire system is the most 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. The 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 thermistor through wires L1 and L2, and potentials V3 and V4 are measured. Wires L3 and L4 are connected to a high input impedance circuit, IL3=0,IL4=0, Therefore, V4-V3 is equal to the voltage across the thermistor. The resistance value of the thermistor: From this, it can be concluded that the four wire measurement method is not affected by the resistance of the connecting wires