RTD Temperature Explained
A resistance temperature detector (RTD) measures temperature via the change in resistance of a platinum element. Learn the Pt100/Pt1000 linear approximation, default alpha, and limitations.
Definition
A Resistance Temperature Detector (RTD) measures temperature by exploiting the predictable change in electrical resistance of a metal element — most commonly platinum — as temperature changes. The most widely used RTD is the Pt100, which has a resistance of 100 Ω at 0 °C. As temperature rises, resistance increases in an approximately linear fashion. The simple linear approximation R = R0 x (1 + α x T) provides a practical way to convert between resistance and temperature for moderate ranges.
Why it matters
RTDs are the preferred temperature sensor in process instrumentation where accuracy, stability, and repeatability matter more than cost or response speed. Converting between the measured resistance and the corresponding temperature is a routine task during commissioning, calibration, and troubleshooting. The linear approximation is sufficient for quick field checks; full IEC 60751 polynomial tables are used for high-accuracy applications but are beyond the scope of this guide and calculator.
Formula
Units involved
- •R (ohms, Ω) — measured resistance of the RTD element
- •R0 (ohms, Ω) — resistance at 0 °C (100 Ω for Pt100, 1000 Ω for Pt1000)
- •α (1/°C) — temperature coefficient of resistance (0.00385 °C⁻¹ for standard platinum)
- •T (°C) — temperature
Concept diagram
Worked example
A Pt100 RTD reads 138.5 Ω. What is the approximate temperature? (Using α = 0.00385 °C⁻¹)
- 01R = 138.5 Ω, R0 = 100 Ω, α = 0.00385 °C⁻¹
- 02T = (R / R0 - 1) / α
- 03T = (138.5 / 100 - 1) / 0.00385
- 04T = (1.385 - 1) / 0.00385
- 05T = 0.385 / 0.00385
- 06T = 100 °C
A Pt100 reading 138.5 Ω corresponds to approximately 100 °C.
Common mistakes
- •Using the wrong R0 value. Pt100 has R0 = 100 Ω; Pt1000 has R0 = 1000 Ω. Using the wrong base resistance gives temperatures off by a factor of 10.
- •Assuming perfect linearity over wide ranges. The linear approximation R = R0(1 + αT) is accurate within a few tenths of a degree over moderate ranges (roughly -50 to +200 °C) but diverges at temperature extremes. The full Callendar-Van Dusen equation (IEC 60751) adds quadratic and cubic correction terms.
- •Ignoring lead-wire resistance. In a 2-wire RTD connection, cable resistance adds to the measured resistance and appears as a temperature offset. 3-wire and 4-wire configurations compensate for this.
- •Confusing RTDs with thermocouples. RTDs measure absolute resistance; thermocouples measure a voltage difference. They use different instruments, wiring, and conversion tables.
When to use the calculator
Use the RTD Temperature calculator to convert between resistance and temperature for Pt100 or Pt1000 elements using the linear approximation. Enter the measured resistance to find temperature, or enter a temperature to find the expected resistance.