How to avoid condensation inside housing in hot humid weather?

Condensation in underwater cameras housings.

There are many posts on this subject with only bits of information that don't always seems to add up or is just a bit bitty. So I hope this helps.. This generally applies If the housing "does not" have "dry" air inside, and has a higher humidity level, i.e particularly a issue in the tropics.

POLY carbonate housings often have more front port glass fogging issues than aluminium housings, the poly case doesn't condense much moisture onto its inner surfaces as it a poor conductor of heat, the inner surface inside the housing therefore remains warmer and any moisture that is present inside, doesn't readily condense on the housing walls, resulting in more vapour staying air borne longer which finds it way to the glass port. The glass port generally being the coldest surface, the moisture condenses, similar to your windows at home.

A METAL housing inner surface temperature is normally cooler as the case conducts heat way into the cold water, and the inner surface temperature is therefore much closer to the cooler water temperature, (i.e water cooled) then acts like a "moisture sponge" so to speak, moisture condenses on the housing sides, this may not be visible, resulting in less moisture being "air borne", (air is now a bit dryer) resulting in less moisture vapour reaching the glass port window, therefore less fogging issues. This process has a similar effect as putting silica gel packs inside the housing to help soak up vapour.

In either housing type, a hot camera, heating up the inside of the housing can in some circumstances re-evaporate some moisture, that is (if any) off the case inner sides to some degree.

So if a glass port is clear with the camera off, Turning on the camera in some circumstances can cause fogging to occur.

As you dive deeper the water temperature often cools and you can find fogging occurs more at depth and can also clear nearer the surface.

The answer to all these issues is to have dry air inside your housing at all times.

Ways to achieve this is to always assemble camera in a air conditioned room. Carefully blow dive cylinder dry air into housing as you close it, or put open prepared camera housing in large plastic bag and fill with dive cylinder dry air, and close it whilst in the dry atmosphere. Least reliable is using silica gel packs inside the housing.

If you would like to know about the physics, google "dew point" and "relative humidity".

To help you with this, remember these following points..
Water condenses on a glass port when the RH reaches 100% at that point.
An air conditioned room may have a RH of about 40%.When vapour condenses on a glass surface, it has reached its dew point temperature.
Dew point of dry dive air is in the minus figures. i.e Extremely dry.

As an example: An aircon room at 25 degrees, with RH=40% has a dew point of 10 degrees. Seal up the camera and trap this air inside, and condensation will not occur on the port until the water temp reaches 10 degrees. This is always going to stop fogging in the above example.

Andy Willett