How a platinum RTD works
Platinum's electrical resistance rises with temperature in a highly repeatable way, which is why the PT100 is the workhorse of accurate industrial temperature measurement. IEC 60751 standardises the exact relationship as the Callendar–Van Dusen equation:
and below zero, one extra term:
with the standard coefficients A = 3.9083×10⁻³ °C⁻¹, B = −5.775×10⁻⁷ °C⁻², C = −4.183×10⁻¹² °C⁻⁴ and R₀ the resistance at 0 °C (100 Ω for PT100, 1000 Ω for PT1000). Converting resistance back to temperature uses the quadratic solution above 0 °C and a numerical solve below — exactly what this calculator does.
Worked example
Your multimeter reads 119.40 Ω across a PT100. The temperature:
- R/R₀ = 1.1940, above 1 so t > 0 °C — use the quadratic solution
- t = [−A + √(A² − 4B(1 − R/R₀))] ÷ 2B
- t = 50.0 °C (type 119.40 into the resistance field to confirm)
PT100 reference points (IEC 60751)
| °C | PT100 (Ω) | °C | PT100 (Ω) |
|---|---|---|---|
| −200 | 18.52 | 200 | 175.86 |
| −100 | 60.26 | 300 | 212.05 |
| −50 | 80.31 | 400 | 247.09 |
| 0 | 100.00 | 500 | 280.98 |
| 50 | 119.40 | 600 | 313.71 |
| 100 | 138.51 | 850 | 390.48 |
Multiply by 10 for PT1000 values.
Field notes
- Lead-wire resistance adds directly in 2-wire circuits. Each ohm of cable reads as ≈ 2.6 °C of error on a PT100. Use 3-wire (compensates matched leads) or 4-wire (eliminates leads entirely) connection — or a PT1000, which shrinks the same error ten-fold.
- Quick health checks: ~100 Ω near 0 °C, ~110 Ω at room temperature. A reading of a few ohms means a shorted element; open circuit means a broken one.
- Self-heating: the measuring current warms the element. Keep it ≤ 1 mA and this stays negligible.
- Check the alpha. This tool implements the IEC/DIN curve (α = 0.00385). Some US-made sensors use α = 0.00392 — their tables differ slightly, so confirm the standard on the datasheet.
Frequently asked questions
What resistance should a PT100 read at room temperature?
At 25 °C a PT100 reads about 109.73 Ω. At 0 °C it reads exactly 100 Ω by definition, and at 100 °C, 138.51 Ω.
What is the difference between PT100 and PT1000?
Only the base resistance: PT1000 reads 1000 Ω at 0 °C, ten times the PT100. The temperature behaviour is identical, and the higher resistance makes lead-wire resistance errors ten times smaller.
What is the Callendar–Van Dusen equation?
The IEC 60751 standard equation relating platinum RTD resistance to temperature: R = R0(1 + At + Bt²) above 0 °C, with an extra C(t−100)t³ term below 0 °C. A = 3.9083×10⁻³, B = −5.775×10⁻⁷, C = −4.183×10⁻¹².
How accurate is a Class A or Class B PT100?
Per IEC 60751: Class AA ±(0.10 + 0.0017·|t|) °C, Class A ±(0.15 + 0.002·|t|) °C, Class B ±(0.30 + 0.005·|t|) °C, Class C ±(0.60 + 0.01·|t|) °C. At 100 °C a Class B element may be off by ±0.8 °C.
Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.