20250603 Resistance Thermometer Temperature Measurement
A resistance thermometer (such as a Pt100) is a temperature sensor that converts temperature into an electrical resistance by exploiting the principle that its resistance varies with temperature.
2025-06-03
Measurement principle
A resistance thermometer (such as a Pt100) is a temperature sensor that converts temperature into a resistance value by exploiting the principle that its resistance varies with temperature. The Temperature Transmitter obtains the resistance value (voltage/current) by applying an excitation current to the resistance thermometer and measuring the voltage across it, then converts this resistance value into a temperature reading, thereby achieving temperature measurement. There are three wiring configurations between the resistance thermometer and the Temperature Transmitter: 2-wire, 3-wire, and 4-wire.
Two-wire system
The Temperature Transmitter applies an excitation current I to the thermal resistor via leads L1 and L2, and measures the potentials V1 and V2.
Since the resistances of the connecting leads, RL1 and RL2, cannot be measured and are instead incorporated into the resistance value of the thermal resistor, this introduces an additional measurement error. For example, at 100°C, the temperature coefficient of a Pt100 thermal resistor is 0.379 Ω/°C; if the lead resistance is 1 Ω, the resulting measurement error would be 2.64°C.
3-wire system
A common wiring configuration in practical applications. As shown in Figure 2, an additional lead is added to compensate for measurement errors caused by the resistance of the connecting wires. The three-wire configuration requires that the three leads be made of the same material, have identical wire gauges and lengths, and operate at the same temperature, ensuring that their resistances are equal—i.e., RL1 = RL2 = RL3. An excitation current I is applied to the RTD via leads L1 and L2, and the resulting potentials V1, V2, and V3 are measured. Lead L3 is connected to a high‑input‑impedance circuit, so IL3 = 0.
Thus, it can be concluded that the three-wire connection method compensates for measurement errors caused by the resistance of the connecting leads.
4-wire system
The ideal wiring configuration for resistance temperature measurement. A current I is applied to the resistance thermometer via leads L1 and L2, and the resulting potentials V3 and V4 are measured. Leads L3 and L4 are connected to a high‑input‑impedance circuit, ensuring that IL3 = 0 and IL4 = 0; consequently, V3 − V4 equals the voltage across the resistance thermometer.
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