Resistance Thermometer Devices

Resistance temperature detectors (RTDs) are robust, highly stable sensors widely used in industrial and laboratory applications, including freeze-drying. They operate on the principle that the electrical resistance of certain metals (most commonly platinum) increases predictably with temperature. Platinum RTDs (Pt100, Pt1000) are the most widely adopted because of their reproducibility, stability under sterilisation, and wide operating range, with tolerance classes (A and B) defined by IEC 60751.
RTDs are manufactured using fine platinum wire or thin films deposited on ceramic or glass cores, typically housed within stainless-steel sheaths. Their accuracy is determined by both the intrinsic tolerance class of the sensing element and the wiring configuration used to connect the element to the measurement system.
In a 2-wire configuration, the simplest arrangement, lead resistance adds directly to the sensor resistance and introduces error (e.g. ~0.09 °C for 20 cm of AWG 24 copper leads in a Pt100). A 3-wire configuration compensates for most of this effect by assuming equal resistance in all wires, and is the most common solution in freeze-drying systems where a balance of cost, robustness, and accuracy is required. A 4-wire configuration eliminates lead resistance entirely by separating current supply and voltage measurement, and is used in calibration or high-accuracy laboratory applications.
RTDs offer high accuracy, reproducibility, and long-term stability, but their relatively large thermal mass means they respond more slowly than thermocouples. This is advantageous for monitoring shelf or oil temperatures in freeze-drying, where strong thermal coupling ensures the probe equilibrates rapidly, but presents challenges in vial-based measurements during secondary drying, where the low heat capacity of solids can lead to sensor-dominated readings.