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Pt100 resistance ↔ temperature calculator (IEC 60751)
Convert between resistance and temperature for Pt100 / Pt1000 sensors, with class AA, A, B and C tolerances at that temperature.
Standard-curve conversion for an uncalibrated sensor. A calibrated PRT has its own coefficients (Callendar–Van Dusen or ITS-90) on the certificate — use those, not the nominal curve, for measurement.
Result
| Class | Tolerance at 50.0 °C | In ohms |
|---|---|---|
| AA | ±0.185 °C | ±0.0712 Ω |
| A | ±0.250 °C | ±0.0963 Ω |
| B | ±0.550 °C | ±0.2118 Ω |
| C | ±1.100 °C | ±0.4236 Ω |
Classes are defined over limited ranges (e.g. AA -50 to +250 °C (wire-wound); +600 °C for the thin-film variant); values outside are formula extrapolations.
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Pt100 reference points
How the calculation works
- 01t ≥ 0 °C: R(t) = R₀ (1 + A t + B t²); t < 0 °C: R(t) = R₀ (1 + A t + B t² + C (t − 100) t³), with A = 3.9083 × 10⁻³, B = −5.775 × 10⁻⁷, C = −4.183 × 10⁻¹².
- 02Resistance → temperature: closed-form quadratic above 0 °C, Newton iteration below.
- 03Tolerances (IEC 60751:2022): AA ±(0.1 + 0.0017|t|), A ±(0.15 + 0.002|t|), B ±(0.3 + 0.005|t|), C ±(0.6 + 0.01|t|) °C.
Limitations
- Uses the standard curve, not a calibrated sensor's own coefficients; a calibrated PRT should use the coefficients on its certificate.
- Tolerance classes apply over ranges that depend on the element type (wire-wound vs film) — consult the standard for validity limits.
Frequently asked questions
- What resistance does a Pt100 have at 100 °C?
- 138.5055 Ω on the IEC 60751 standard curve (α = 0.003851), usually quoted as 138.51 Ω.
- What is the Class B tolerance of a Pt100 at 0 °C?
- ±0.3 °C, growing to ±0.8 °C at 100 °C: ±(0.3 + 0.005|t|) °C.
- Does the calculator work for Pt1000?
- Yes — set R₀ to 1000 Ω. The temperature coefficient and tolerance classes are the same.
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Pt100 tolerance classes explained (IEC 60751)
Class AA, A, B and C define the maximum permissible deviation of a platinum resistance thermometer from the standard curve as a function of temperature. They are tolerances, not uncertainties.
How to calibrate a Pt100
Calibration by comparison against a reference thermometer, and the factors that decide the uncertainty.
Self-heating in resistance thermometers
The measuring current warms the sensing element. The effect depends on the medium — so calibrate at a current and condition representative of use.
Readouts and bridges for reference thermometers
A reference PRT is only as good as the instrument reading it. Resolution, linearity, measurement current and calibration of the readout all enter the budget.