swankenstein:
I think that the relatively warm air can hold a certain amount of moisture. When the housing is immersed in colder water, the air in contact with the now cold port cools and can no longer hold the same amount of moisture. This moisture condenses on the port. A dessicant pack in the housing can lower the humidity to a level where even when the air cools on the submerged port, no vapor will condense because it's still below it's saturation point for that temperature. Putting your camera in the housing in an air conditioned room ensures that the air is cold and dry to begin with. I use dessicant packs sometimes. I just make sure that they are newly-recharged (just sitting in a room can cause them to be "saturated" in normal temperature air) and I try to leave them in the housing for at least an hour to let them do their work.
Hi, Swankenstein,
You are correct, as the temperature of the air goes up, the amount of water vapor it can contain goes up, too. We usually call it humidity. Scientists call it the partial pressure of water.
Dessicants have the capability of absorbing water--more strongly than, say cotton--and can hold onto it even at elevated temperatures. However, their capacity is limited and when exceeded, they can be dripping wet. There are a variety of dessicants and most rely on a chemical bond being formed between water molecules and the dessicant structure or from adsorption into molecular "tunnels". Sometimes this is easily reversible and they can be regenerated, usually by heating. Since the water molecules have to come into physical contact with the dessicant, air circulation and time are needed. Hence, the wisdom of your letting your closed housing sit for awhile.
What I read was that this fogging phenomenon required that the camera operate. And I have never had such fogging occur with my SLR or my DSLR, whether I closed the housing in an a/c'ed room or a non-a/c'ed room. I have not had it happen to either of my old Nikonoses. Further reading seems to indicate that this is a phenomenon related to P&S cameras heating up.
One more thing...even at low temperatures...such as, say, 80 deg F and lower, air can contain a significant amount of water vapor without condensation occurring. That is why my housed cameras have never fogged: I have never gone past what is called the dew point.
At this point, you might have guessed that I am a chemist
...and at this point I think I need to stop with all the lingo and jargon.:no Suffice it to say that condensation in housed SLR's is darn rare. So, what we are looking at is something related to the P&S cameras in a humid environment.
I think I understand what is going on, but I need to talk with some P&S camera experts to see just what the temperature rise is when they operate.
Fun stuff...
joewr
