It depends on how clean the environment is around the capacitor.
Pure water/humidity is not conductive however it can cause oxidation/corrosion damage when combined with corrosive contaminants.
If conductive contamination is allowed to build on the capacitor there will be current leakage between the terminals which will affect the operation of the capacitor and may cause failure.
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Humidity is the gaseous ,state of water and has little effect on electronic components. The problems arise when temperature and or pressure changes cause the gas to condense to liquid. Liquid water causes reliability problems in many electronic devices including capacitors.
A wet ceramic capacitor will fail in minutes. While they will function in a humid environment, any condensation will cause failure within minutes, reliability cannot be guaranteed. The automotive, aeronautics and military electronics industry all require some sort of coating to meet reliability in high humidity. Industry has used everything from wax to vacuum deposited man-made coatings.
" Industry has used everything from wax to vacuum deposited man-made coatings."A universal problem is that the coatings that have been devised almost always display some gas permeability to water vapor. This means that water can enter the component even if it does so very slowly. When environmental conditions are proper, the vapor turns to liquid and the device fails. One thing that makes these failures so insidious is that when the temperature becomes very low (less than 0 deg C, the parts don't fail because ionic activity pretty much ceases below that temperature). Most failures are clustered between 0 and 25 degrees C. One might suspect this temperature range to be benign. but that's where most moisture related failures occur.
The individual who has done the study to produce the <...reliable information...> might, as might the individual(s) in the organisation that published it.
Technical libraries, capacitor manufacturers and learned technical journals would be good places to start as would an internet search.
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What about an air dielectric tuning capacitor? Does water vapor change the permittivity of air?
I have often wondered if a two plate air capacitor changes reactance, when the two plates are rotated in opposite directions. But it might take a faster rotation than we are capable of, to see it.
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