Tools

RTD & SPRT temperature calculator

Choose a conversion method, solve its coefficients from your calibration points, then generate a resistance-temperature table.

Step 1

Conversion method

Step 2 — A, B, C

Solve coefficients from calibration points

Enter the measured resistance at each calibration temperature. The point near 0 °C — the water triple point (0.01 °C) or an ice point — derives R₀, so you don't need a reading at exactly 0 °C. Add a sub-zero point to determine C for use below 0 °C.

Temp (°C)Resistance (Ω)
Near 0 °C
Point 1 (> 0)
Point 2 (> 0)
Sub-zero (< 0)
R₀ (at 0 °C)—
A (1/°C)—
B (1/°C²)—
C (1/°C⁴, t<0)—
R₀ (at 0 °C)—
α (1/°C)—
δ—
β (t<0)—

Fits R(t) = R₀(1 + A·t + B·t² + C·(t−100)·t³) (C term only below 0 °C). The A/B/C and α/δ/β forms are reciprocally related.

Step 3

Resistance-temperature table

Generated from the coefficients solved above, over −200 to 850 °C.

Temp (°C)Temp (°F)Resistance (Ω)

Step 2 — ITS-90

Solve deviation coefficients from calibration points

Enter the resistance near the water triple point, choose the subrange(s) your SPRT is calibrated over, then enter the measured temperature and resistance at each calibration (fixed) point. The deviation coefficients are solved from the difference between your readings and the ITS-90 reference function. Selecting a subrange fills its rows with the standard fixed-point temperatures and ideal-SPRT resistances — replace the resistances with your measured values.

Temp (°C)Resistance (Ω)
Near triple point

High-subrange calibration points

Temp (°C)Resistance (Ω)
Point 1
Point 2
Point 3
Point 4

Low-subrange calibration points

Temp (°C)Resistance (Ω)
Point 1
Point 2

Conversion uses W = R/R(TPW), then Wᵣ = W − ΔW(W) with the deviation function for the chosen subrange, and reads T from the ITS-90 inverse reference function. With every deviation coefficient at zero, the result is the ideal-SPRT reference curve.

Step 3

Resistance-temperature table

Generated from the solved coefficients across the selected subrange.

Temp (°C)Temp (°F)Resistance (Ω)

Frequently asked questions

What is the Callendar-Van Dusen equation?

It describes a platinum RTD's resistance versus temperature using coefficients A, B, and C (C applies below 0 °C). The coefficients can also be expressed as alpha, delta, and beta.

What is the difference between an RTD and an SPRT?

A standard platinum resistance thermometer (SPRT) is a high-accuracy primary thermometer realized on the ITS-90 scale via reference and deviation functions, whereas an industrial RTD (Pt100/Pt1000) is described by the Callendar-Van Dusen equation.

What does Pt100 mean?

A Pt100 is a platinum RTD with 100 Ω nominal resistance at 0 °C; a Pt1000 has 1000 Ω at 0 °C.