Thermistor & RTD

An NTC thermistor falls steeply in resistance as it heats — the Beta model gives 1/T = 1/T₀ + (1/B)·ln(R/R₀) (T in kelvin). An RTD rises almost linearly: R = R₀(1 + α·T), with Pt100 using R₀ = 100 Ω and α ≈ 0.00385.

The Beta model is a two-point fit — good over a modest span. For wide-range accuracy use the Steinhart-Hart equation, and for RTDs above ~200 °C use Callendar-Van Dusen.