RTD and Pt100 sensors
How platinum resistance thermometers work, the IEC 60751 characteristic (Callendar–Van Dusen), tolerance classes, 2/3/4-wire connection and typical measurement errors.
Principle
The resistance of platinum rises with temperature. Pt100 has 100 Ω at 0 °C, Pt1000 1000 Ω. IEC 60751 defines the characteristic (α = 0.00385 /°C):
R(t) = R0 · (1 + A·t + B·t²) for 0 … 850 °C
R(t) = R0 · (1 + A·t + B·t² + C·(t − 100)·t³) for −200 … 0 °C
A = 3.9083·10⁻³, B = −5.775·10⁻⁷, C = −4.183·10⁻¹²
| t, °C | −50 | 0 | 25 | 100 | 200 | 500 |
|---|---|---|---|---|---|---|
| Pt100, Ω | 80.31 | 100.00 | 109.73 | 138.51 | 175.86 | 280.98 |
Around room temperature the slope is ≈ 0.385 Ω/°C for Pt100 (3.85 Ω/°C for Pt1000). Convert with the RTD calculator.
Tolerance classes (IEC 60751)
| Class | Tolerance |
|---|---|
| AA | ±(0.10 + 0.0017· |
| A | ±(0.15 + 0.002· |
| B | ±(0.30 + 0.005· |
| C | ±(0.60 + 0.010· |
Class B at 100 °C: ±0.8 °C. The validity range of each class depends on the sensor construction.
2-, 3- and 4-wire connection
The measuring instrument sees the sensor plus the lead resistance.
| Connection | Lead resistance effect |
|---|---|
| 2-wire | Fully added: 2 × 0.5 Ω leads ≈ +1 Ω ≈ +2.6 °C error for Pt100 |
| 3-wire | Compensated if all three leads have the same resistance |
| 4-wire | Eliminated: current and voltage on separate pairs |
Use 3-wire as minimum for Pt100 in the field, 4-wire for accurate measurement; Pt1000 is less sensitive to lead resistance.
Other error sources
- Self-heating by the measuring current (keep current small, ~1 mA).
- Thermal contact: immersion depth, thermowell, response time.
- Module setting: wrong sensor type (Pt100 vs Ni100, α 0.00385 vs 0.003916) gives systematic errors.
Connections
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Sources
Updated 2026-10-02