Resistance thermometer Stability
Stability is long-term drift in thermometer readings.
A typical specification would limit drift to 0.1ºC per year for rated operation. Normal services at points well within the temperature rating typically cause much less drift.
Drift is a consequence of the element material, with platinum being the most stable; encapsulating materials, which could contaminate the element; and mechanical stress placed on the element by expansion of winding bobbins or other supporting structures.