What a PT100 is
A PT100 is a platinum resistance temperature detector (RTD) with a resistance of exactly 100 Ω at 0 °C. A PT1000 has 1,000 Ω at 0 °C. Platinum’s resistance rises very regularly with temperature, so these sensors are the standard for accurate industrial, laboratory and HVAC measurement from about −200 to 850 °C.
The Callendar–Van Dusen equation
IEC 60751 defines the resistance for 0 °C and above as:
R(t) = R0 × (1 + A × t + B × t²)
with A = 3.9083 × 10⁻³ and B = −5.775 × 10⁻⁷. Below 0 °C an extra term, C × (t − 100) × t³ with C = −4.183 × 10⁻¹², is added. The calculator solves the quadratic for temperature; below 0 °C it ignores the C term, which shifts the result by less than about 0.2 °C down to −100 °C.
Example
A PT100 that reads 138.51 Ω is at 100 °C. At 25 °C, a PT100 reads 109.73 Ω and a PT1000 1,097.3 Ω.
The average slope between 0 and 100 °C is 0.385 Ω/°C for a PT100. That is the “alpha” of 0.00385 printed on many sensors.
Tolerance classes
| Class | Tolerance | At 100 °C |
|---|---|---|
| AA | ±(0.1 + 0.0017 t) | ±0.27 °C |
| A | ±(0.15 + 0.002 t) | ±0.35 °C |
| B | ±(0.3 + 0.005 t) | ±0.80 °C |
Here t is the temperature in °C, taken as a positive number. Class B is the most common and cheapest. Class A and AA suit calibration and laboratory work.
Wiring: 2, 3 or 4 wires
The lead wires add resistance. With a PT100, 0.385 Ω of lead resistance already means a 1 °C error.
- 2-wire: the leads are measured with the sensor. Only for short runs or PT1000 sensors.
- 3-wire: the most common in industry; the instrument cancels the lead resistance if the wires are equal.
- 4-wire: the most accurate, separating the current and measuring pairs.
PT1000 sensors are ten times less sensitive to lead resistance, which is why they are popular in heating controls and battery-powered loggers.
Tips
- Keep the measuring current low, around 1 mA for a PT100, to avoid self-heating.
- Check the sensor’s alpha: a few older or non-European sensors use 0.00392 instead of 0.00385.
- Response time depends on the probe sheath. Thin-film elements in a thin sheath react in seconds, heavy thermowells in minutes.