I'll throw my two-cents in here. Condensation is the result of only two things, temperature and dew point. Dew point is the temperature at which water vapor condenses, but dew point is variable based upon the amount of water vapor. If you've got an atmosphere with 98% relative humidity and an air temp of 70(F), then the dew point might be 68(F). However, if you have only 10% relative humidity and an air temp of 70(F), the dew point may be 48(F). The exact numbers are variable, so don't take these examples as factual data points. Also, warmer air holds much more water vapor then cold air. Ever hear the expression "it's too cold to snow"? Well, it's true, although not because of some magical temperature at which it just doesn't snow, but because at very low temperatures there is so little water vapor in the air that the dew point will never be reached.
In most diving situations the water is going to be cooler than the ambient air and the surrounding water will continually pull heat from the sub until equilibrium is reached (which may never occur). Your steel psub hull is going to act as a huge heat sink, conducting heat from inside the sub out to the surrounding water, with the hull and viewports likely to reach ambient water temperature quickly because of their relatively large surface area. As the interior surfaces chill to dew point temperature, air near or at that interior surface loses heat and the water vapor within it condenses. Psubbers in hot/humid locations then, are likely to encounter more problems with condensation then psubbers in cold/dry locations. If you are diving in Florida with an air temp of 95(F), a dew point of 88(F), and a water temperature of 80(F), then you are going to have condensation as soon as a particular interior surface of the submarine cools down to the dew point temperature of 88(F). In contrast, if you are diving in a cold location with an air temp of 45(F), a dew point of 35(F), and a water temperature of 45(F), then you aren't going to have any condensation issues (at least for a certain time) because the outside water temp is higher than the dew point temperature. Of course, all these temperature variables change as you expel water vapor into the air via breathing, and the cabin temperature changes due to body and electrical component heat.
Now to get to condensation effects on electronic components. Since those components are likely attached to the hull via a bracket of some kind, the surface area of the attachment is also likely to be relatively small which means heat transfer between the inside of the sub and the outside water will be much slower than for large components such as the hull and viewports. You could delay this transfer even more by using cork or some rubber gasket between the component bracket and the hull attachment. The hull and viewports are going to be the main targets of condensation in almost any dive, and condensation on components attached to the hull are likely going to be a problem during a long dive where those components have plenty of time to cool to the dew point, or you are diving in an extreme condensation promoting environment. For example, an air temp of 90(F), dew point of 80(F), and cold water of 50(F).
Jon Jens Laland wrote:
Hi... I wonder if anyone could share their experience with condensation in relation to electrical systems inside small personal submarines. My main concern is (periodic) failures due to moisture inside electronic circuits, and how can we minimize those problems in home-built equipment? I have discussed this with David, and he told me his view on "potting" (something I will actually try out with a computer, ie. submerge it in oil). However, I'm not quite sure what do with the various electro-mechanical components such as rotary switches, etc. What I do know (from studying various midget submarine operations during World War 2) is that those guys considered the moisture/condensation to be a major problem in relation to electric switch gears. Having said this, I still think Frank is right when he says it's not a big problem if your diving a K boat and staying down for an hour or less. This is probably also more true in a milder climate than we normally have here in Norway. Best regards, Jens Laland
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