RTD chips ( Platinum Thin Film Elements ) are manufactured from metals whose resistance increases with temperature. Within a limited temperature range, the resistivity increases linearly with temperature. This resistance is directly proportional to a metal wire's length, and inversely proportional to the cross-sectional area. The element is coated with ceramic that can withstand high temperature. Some Rtd are wire wounded with ceramic casing.

The chemical stability, availability in pure form, and highly reproducible electrical properties, has made Platinum the metal of choice for RTD's which are made of either IEC/DIN-grade platinum or reference-grade platinum. The difference lies in the purity of the platinum. The IEC/DIN standard is pure platinum that is intentionally contaminated with other platinum.

To measure the resistance of an RTD, a small electric current (about 1 mA) must flow through the sensor to create the necessary voltage drop. The current causes the platinum element in the RTD to heat up above the temperature of the RTD's environment. The heating is proportional to the electric power (P=I sq. R) in the RTD and the heat transfer between the RTD sensing element and the RTD environment. If the RTD is in a poor heat transfer medium (e.g., air), it will heat up more than if it is in a fluid, such as water. The electrical current will heat the sensor and may influence the measurement.

Tolerances for RTD's should meet the standards of ASTM E1137 Grade A or B and IEC 751 Class A or B.