How can I measure still air in 90-100% RH range with constant 25°C temperature?
I may be missing something here but I have used humidity sensors in the past that can measure ambient humidity and will go up to 100% RH, with 2-4% accuracy when over 90% humidity.
Could you use one of these sensors? There are several companies building them, such as Sensirion, Honeywell, TE among others.
So I think I've solved my own problem. There is a dew sensor called SY-DS-1, which starts changing it's resistance way before condensation occurs - in fact, its operational range starts below 70% RH and is specified to work in high humidity for extended time. Datasheet specifies testing 1000 hours in 95% humidity. I expect it to be designed for prolonged contact with liquid water - it's a dew sensor after all (duh). Information in the datasheet is scarce, but I've already ordered two of them - one for torturing, other as reference for later.
http://samyoungsnc.com/wp-content/uploads/2019/03/Datasheet-_-SY-DS-1-Series-Ver.1.8-Standard.pdf The plot doesn't come from the datasheet, I've found it in a dew sensor project based on the same device.