Temperature–Resistance Characteristics of Platinum RTDs

Overview

A platinum RTD measures temperature through the predictable change of platinum resistance. PT100 sensors have 100 Ω at 0 °C; the resistance–temperature relationship is defined by IEC 60751 and is close to linear, with a small, well-defined curvature above 0 °C.

Key Points

Classes. AA, A and B define the permitted deviation; class AA is the tightest and costs the most.
Lead resistance. Two-wire construction adds lead resistance to the reading; three-wire and four-wire circuits cancel it.
Self-heating. The measuring current warms the element; keep it low, especially for thin-film sensors.
Range. Platinum RTDs cover roughly −200 to 850 °C, with the best stability in the middle of that range.

Summary

RTDs are the right choice where accuracy and repeatability matter more than response time or maximum temperature; above roughly 600 °C a thermocouple is usually the more robust option.

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