First I want to point out that I am not a chemist, sorry for bad description.
I have CO2 incubator. CO2 concentration is 10-20% and the humidity level is around 90%. I was using a humidity sensor to measure humidity and some kind of transparent substance have formed around the sensor. Sensor becomes unprecise and had a slow response time. Substance almost looks like plexiglass.
Is it possible that carbonic acid which is forming from CO2+H20 -> H2CO3 is somehow reacting with integrated circuit case or sensor and forming some kind of glass-like substance?
Does it stay like thay when you remove the pressure and take it to the normal air? if yes, then it is likely co2 is being absorbed into the plastic part, and that plastic part is swelling. You can avoid it by using a glass window for optical sensor. Or use H2O permeable but not CO2 permeable membrane if this sensor needs contact with the gas. Or use another type of sensor that does not contain this plastic. Or reduce CO2 pressure. I dont know any CO2+H2O compounds that would stay stable and solid at normal pressure. Methanol could be forming from those materials if energy is present, but it needs catalyst and is very unlikely to happen on its own.