From: Sean T. Stevenson
<cast55@telus.net>
To:
personal_submersibles@psubs.org
Sent: Wed, March 16, 2011 8:08:26
PM
Subject: Re:
[PSUBS-MAILIST] Humidity control
I would think that, with the
possible exception of very short dives, in a
small space (PSub size) the
moisture present in the surface air that gets
enclosed in the vessel is
less significant than the moisture introduced by the
occupants breath over
the course of a dive?
-Sean
On Tuesday 15 March 2011
23:00:13 you wrote:
> We should probably write a white paper on this
sometime since it seems
> to come up once or twice every year. I
always find condensation easier
> to understand in terms of dew
point. Simply put, and in all cases, if
> the temperature falls at
or below the dew point, you're going to have
> condensation. You
can also get a sense of the relative humidity by
> comparing the current
temperature against the dew point. If the air
> temp is 80F and
the dew point is 70F then you've got high humidity and a
> sticky humid
day on your hands. If the air temp is 80F and the dew
> point is
45F then you have low humidity and a nice dry day on your hands.
>
> The problem starts at the surface when the air in your submarine
is
> filled with the atmospheric conditions of whatever the current day
is
> like. Now consider that when a sub is in water the cabin air
is
> eventually going to reach equilibrium with the water
temperature. If
> the water temperature is at or below the dew
point of the atmospheric
> air, you can expect to have condensation
issues as the air temperature
> in the sub begins to drop due to water
conducting heat from the cabin
> through the hull.
>
> In a
high humidity situation (warm humid day, cool water temps), I think
>
there's a simple and cheap solution that might go a long way to
>
controlling condensation during the dive. By emptying the air from
an
> 80 cubic foot scuba tank into the submarine cabin just prior to
closing
> the hatch, the dry air from the tank should displace enough of
the humid
> air in the cabin to considerably lower the dew point inside
the cabin
> and reduce the chance of condensation. This should be
easy enough to
> test without the sub even in the water by simply
waiting for a humid day
> and monitoring the relative humidity in the
sub before and after the air
> from the scuba tank fills the
submarine.
>
> For warm climates such as Florida, if you have
room for the small
> "cooler" air conditioner that David Bartsch was
working on last year, I
> think that could be an ideal solution for not
only controlling heat in
> the cabin but also the humidity.
>
> Calcium Chloride is cheap and easily available, and is suppose to act
as
> a desiccant although I've never tried it. The southern boys
out there
> probably don't know about this stuff, but us northern boys
usually have
> plenty of it on hand for melting snow and ice in the
winter.
>
>
Jon
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