Bubbletrubble
Contributor
@PatW: I agree with your reasoning, although I believe you misunderstood what Wantonmien was saying. Wantonmien said that when he assembles his camera + UW housing in an A/C room and then splashes into warm water (77°F or warmer) then he experiences condensation inside the case. In his example, the temperature inside the case is colder than the ambient temp outside the case.
@Wantonmien: You know, I don't normally have a problem trapping air into the UW housing that's colder than the temperature of the water. Bottom temps where I dive are usually in the 50-65°F range. Perhaps this is a case of different practices for different diving conditions.
As PatW stated, in order for condensation to occur inside an UW housing, there needs to be a source of water vapor (humid air inside housing) and a temperature gradient (between air inside housing and water/air outside of housing). In general terms, warmer air has a greater capacity to support water vapor than colder air.
Scenario #1: When the skin of a container is cooled to a temperature below that of the dew point of air inside the container, condensation occurs on the inside of the walls of the container.
Scenario #2: Alternatively, if something inside the container (camera) heats up and there's cold air with some water vapor surrounding the camera inside the UW housing, condensation can occur on the camera itself. It can look like the camera is "sweating."
OK, so let's see why assembling the camera + UW housing in an A/C room works for me. Cold, dehumidified air = low water vapor inside housing. Since water bottom temps are in the 50s-60s (°F), having that cold A/C air inside the housing decreases the temperature gradient between outside and inside the housing. In this way, I avoid the condensation that occurs in Scenario #1.
Without doing any tests, I can't be sure why assembling the camera + UW housing in an A/C room didn't work for you. After all, cold, dehumidified air = low water vapor inside housing. However, a significant difference is that water bottom temps are higher than 77°F (25°C). I suppose that it's possible that condensation forms on the camera itself (inside the housing) as it heats up - Scenario #2. What happens if you assemble the camera + UW housing in an A/C room and then don't turn on the camera at all? I suspect that you might not see condensation on the camera or on the inside walls of the UW housing. I'd be really interested in finding out what happens. In any case, decreasing the temperature gradient between outside the case and inside the case is a good thing. Eliminate the temp gradient and you eliminate condensation problems.
Let me know if you can do the above experiment. I'm pretty curious about the results.
that to me would be like an ice bath !! 
