I am trying to do some evaporation experiments inside a vacuum chamber. Naively I thought that humidity inside a vacuum chamber would always be close to zero as the water vapors would be immediately sucked out to the pump. But it seems that the evaporation rate inside a vacuum chamber is greatly influenced by the amount of liquid lying inside the vacuum chamber, which implies that the water vapor is not sucked out, but stays inside the chamber and builds up humidity. Is this true? I am not getting a clear physical picture of it. And if this is the case, could anyone suggest a simple way to control the humidity inside a chamber? (Besides desiccants.)
you have plenty of adsorbed water molecules in your chamber if it has no load-lock chamber. heat your chamber to desorb it. flush with nitrogen the next time you open it. have some shutter in front of your sample so that you can have some pre-evaporation to decrease the base pressure (if you have the possibility, dual crucible, have some Ti evaporated). pump and pump. if your pumping system is not very powerfull, you can make some dilution steps before the experiment, flush dry nitrogen and pump back to base pressure (repeat 10 times..or so)...
have clean polished chamber walls (and everything else inside), that is why mostly aluminium foils are used and replaced from time to time.
While sending air into your vacuum chamber it is better to process the air before entrance by means of dehumidifier. My view of analysis is that the moisture present in the intake air may be condensing because of pressure changes
In case you wish to control the humidity level in vacuum, it is actually not that difficult. You just stream a small flow of gas into your chamber which you humidify in a controlled way. Building your own system, you can have a vacuum approved humidity sensor inside your chamber and then based on its feedback mix dry and humid gas into the chamber. Cellkraft AB (which I am affiliated to) offers standard products for humidification in vacuum systems.
https://cellkraft.se/humidifiers/flow-through-humidifiers/
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