RTD Pt100 / Pt1000 Calculator

How a platinum RTD works

A resistance temperature detector (RTD) uses the fact that the electrical resistance of platinum increases in a very predictable way with temperature. A Pt100 has a resistance of 100 Ω at 0 °C; a Pt1000 has 1000 Ω. Both follow the same curve, scaled by 10.

The Callendar-Van Dusen equation

For 0 °C ≤ t ≤ 850 °C:   R(t) = R0 × (1 + A·t + B·t²)
For −200 °C ≤ t < 0 °C:  R(t) = R0 × (1 + A·t + B·t² + C·(t − 100)·t³)

A = 3.9083 × 10⁻³   B = −5.775 × 10⁻⁷   C = −4.183 × 10⁻¹²

These IEC 60751 coefficients give the standard temperature coefficient α = 0.00385 Ω/Ω/°C. Above 0 °C the equation is a quadratic that can be solved directly; below 0 °C the calculator solves it numerically.

Worked example

A Pt100 reads 138.51 Ω. Solving the quadratic gives 100.0 °C. A quick rule of thumb, dividing by 0.385 Ω/°C, gives (138.51 − 100) ÷ 0.385 ≈ 100.0 °C here, but the linear shortcut becomes less accurate at higher temperatures.

Tolerance classes (IEC 60751)

Class	Tolerance	At 100 °C
AA	±(0.10 + 0.0017 |t|) °C	±0.27 °C
A	±(0.15 + 0.002 |t|) °C	±0.35 °C
B	±(0.30 + 0.005 |t|) °C	±0.80 °C

2-wire, 3-wire and 4-wire connections

• 2-wire: lead resistance adds directly to the reading. With a Pt100, every 0.385 Ω of lead resistance adds about 1 °C of error.
• 3-wire: the most common industrial connection; compensates for lead resistance if the leads are equal.
• 4-wire: eliminates lead resistance effects; used for laboratory and high-accuracy work.

Pt1000 sensors are ten times less sensitive to lead resistance, which is why they are popular with 2-wire connections.