How it's calculated

R(t) = R0 × (1 + A·t + B·t²) for t ≥ 0 °C R(t) = R0 × (1 + A·t + B·t² + C·(t − 100)·t³) for t < 0 °C A = 3.9083×10⁻³, B = −5.775×10⁻⁷, C = −4.183×10⁻¹²

A PT100 reads 100 Ω at 0 °C and about 138.5 Ω at 100 °C, rising roughly 0.385 Ω per °C. A PT1000 is ten times higher.

Fault-finding tips

Measure at the sensor with the wires disconnected. Lead resistance adds error on 2-wire connections; 3- and 4-wire connections compensate for it.
Open circuit reads very high or overrange. A short reads near 0 Ω, which shows as a very low temperature.