Principle
In metals, higher temperature increases lattice vibration and electron scattering, so resistance rises. Over small ranges R = R₀(1 + αΔT). Siemens proposed the platinum resistance thermometer in 1871 and Callendar established precise methods.
Pt100 and the Callendar-Van Dusen equation
A Pt100 is 100 Ω at 0 °C. IEC 60751 specifies α = 0.00385 /°C and the Callendar-Van Dusen equation R(T) = R₀(1 + AT + BT²) above 0 °C, with tolerance classes AA, A, B and C. Platinum is stable and linear, so it serves as a reference sensor; nickel and copper elements also exist.
Wiring
- 2-wire: lead resistance adds directly to the error.
- 3-wire: compensates lead resistance; the industrial standard.
- 4-wire: separate current and sense leads virtually eliminate lead effects for precision work.
Comparison
RTDs are more accurate and stable than thermocouples but cover a narrower range and respond more slowly. Thermistors are more sensitive but nonlinear. Excitation current is kept small, around 1 mA, to limit self-heating.
References
- IEC 60751:2022. Industrial platinum resistance thermometers and platinum temperature sensors. International Electrotechnical Commission.
- Callendar, H. L. (1887). On the practical measurement of temperature. Philosophical Transactions of the Royal Society A, 178, 161-230.
- Fraden, J. (2016). Handbook of Modern Sensors (5th ed.). Springer.