PT100 RTD
A standard Pt100 follows a specified resistance-temperature curve. Using the Callendar–Van Dusen form, R(T) = R₀[1 + AT + BT²] for T ≥ 0 °C. Below 0 °C, add C(T − 100)T³ inside the brackets. For Pt100, R₀ = 100 Ω. Use the specified coefficients and the actual probe's rated temperature range, not one linear coefficient across all temperatures.
A measurement circuit applies an excitation current and measures voltage to infer resistance. A four-wire connection separates current delivery from voltage sensing to reduce lead-resistance error. Three-wire compensation depends on the circuit and matching of the leads; it is not automatically perfect cancellation. Convert resistance with the appropriate curve, and include excitation, reference, ADC and sensor errors.
In plain terms
A platinum thread that grows slightly fatter with heat: measuring how fat (its resistance) tells you the temperature, and platinum's growth curve is so consistent it's defined by international standard.
Why designers use it
- Provide reference-grade temperature in pharmaceutical reactor and bioprocess monitoring.
- Measure gas-turbine inlet air down to −60 °C with consistent calibration over decade lifetimes.
- Calibrate other temperature sensors — Pt100s are the secondary standard in temperature metrology.
- Replace thermocouples in rotating-machinery bearings where milli-volt thermocouple wiring is too noisy.
Best for
- Pharma
- HVAC reference
- Calibration
Key specifications
- Nominal resistance: 100 Ω at 0 °C
- IEC Class AA tolerance example: ±(0.1 + 0.0017 × |T|) °C (Only over the applicable sensor class and construction range; not a full-system error budget.)
- Lead compensation: Two-, three- or four-wire measurement arrangements
- Excitation: Trade signal size against self-heating
When not to use it
- Above ~600 °C in oxidising or contaminating atmospheres — the thin film slowly drifts; use a Type K thermocouple in a sheath.
- When power dissipation in the sensor must be near zero — even 1 mA causes self-heating in still air, biasing readings high.
Common mistakes
- Applying the below-zero polynomial's extra term at positive temperatures.
- Treating ±0.1 °C as a constant accuracy across the complete temperature range.
- Ignoring lead resistance or assuming any three-wire connection cancels it perfectly.
- Using too much excitation current: self-heating depends on I²R and the probe's thermal environment. At 1 mA through 100 Ω, dissipation is 0.1 mW, not 0.1 mW from I×R.
- Confusing a standard curve's mathematical range with the rated range of the chosen sensor construction.
Where you will find it
- TI's RTD guide compares two-, three- and four-wire measurement circuits and ratiometric ADC arrangements. These examples show how the measurement circuit contributes error in addition to the sensor's own tolerance.
A short history
Platinum resistance sensors turn a predictable resistance change into a temperature measurement. The designation here means a nominal 100 Ω at 0 °C. Accurate instruments must also account for lead resistance, excitation-current self-heating and the sensor's specified tolerance class; the nominal value alone does not set accuracy.
Good to know
- Pt100 and Pt1000 specify different nominal resistances at 0 °C.
- A precise ADC cannot by itself remove lead-wire resistance or probe self-heating.