Hysteresis is the reversible change in the nominal Ro value of a platinum temperature sensor element. Dependent upon the last temperature strain to which the element is conditioned, hysteresis occurs at both positive and negative temperature variations, with the most significant effects demonstrated in the negative temperature range.

The design and construction of the temperature resistance element itself can greatly influence its susceptibility to hysteresis, since the physical manifestation of the temperature strain is dependent upon the coupling between the resistance material (platinum) and the carrier (either ceramic or glass material). In effect, the temperature strain leads to a change not only in the crystal structure of the element, but to its electrical resistance, as well.

ABB-Sensycon wire-wound temperature resistance elements (types W60, and W86) are designed so that the platinum helix wire is embedded with a glass-ceramic filling compound, resulting in favorable coupling between the ceramic and the platinum helix wire. The elements, therefore, exhibit minimal susceptibility to the effects of hysteresis. Exposure to a temperature strain of -196°C (liquid N2) results in a lowering of the resistance value by approximately 0.01ohms at 0°C, which is equivalent to approximately 0.025K. This change is reversed if the temperature sensor is later exposed to temperatures above +250°C.

Temperature sensors constructed with the platinum measuring wire fully encapsulated in glass exhibit excellent mechanical coupling. The expansion-coefficient of the glass is adjusted to the platinum and is further stabilized through an aging process.

Thin-film temperature sensor elements are manufactured so that the platinum layer is matched to the expansion of the carrier substrate itself. However, at temperatures below -50°C, the ability of the platinum to match the expansion of the carrier is exceeded, resulting in viscous deformations. Thin-film elements exposed to temperatures of -196°C exhibit a decrease in electrical resistance of approximately 0.3ohms, or roughly 0.75K. Because this viscous deformation between -196°C and room temperature is nearly irreversible, you will see reproducibility in this temperature range within the measuring uncertainty. The viscous deformation can be almost completely reversed if the element is later exposed to temperatures of approximately +150°C or more.

Platinum thin-film temperature resistance elements, which are typically designated for low-temperature use (cryogenic -200°C), should be specially conditioned before their calibration in order to factor out the effects of hysteresis. However, after being cryogenically treated, their temperature exposure should be limited to a range of -200°C to +150°C.