Re: [PSUBS-MAILIST] Humidity control

Sean

Shop. Yes, the ambient air temperature in the shop started at 70.6F and ended 8 hours latter at 69.7 F. Yes, pressure hull is not insulated on inside but has a nice syntactic foam insulation on outside. Condensation was observed on viewport and inside of the sail. As boat has close quarters, I can't see aft to see if condensation was occuring. My guess is that condensation was fairly uniform throughout the inside of the pressure hull. I am not sure if the steady-state condensation rate was equal to the respiratory water production rate. It might also be that the scrubber was absorbing some of the water vapor. I concur that amount of liquid water will be related
directly to dive duration. I think conditioning the air would be great if you do it easily. I am still not convinced that for a microsub like mine that it warrants the work or the required equipment space. It would be nice not to have to worry about the impact of high RH on my boat electronics as we discussed in Vancouver.

Cliff

Cliff Redus
Redus Engineering
USA Office: 830-663-6445
USA mobile: 830-931-1280
cliffordredus@sbcglobal.com

From: Sean T. Stevenson <cast55@telus.net>
To: personal_submersibles@psubs.org
Sent: Wed, March 16, 2011 10:29:39 PM
Subject: Re: [PSUBS-MAILIST] Humidity control

Was this a shop test or a submerged test? Did you measure the external
air/water temperature?

I presume that your pressure hull is not insulated, and that the air
circulation was limited enough to permit the 85% RH air in the vessel to sit
against the hull and cool, driving local RH in that boundary layer to 100% and
condensing water on the hull, so that the steady-state condensation rate was
equal to your respiratory water production rate. As long as the consequences
of condensation in any 100% RH region are immaterial, then this may indeed not
matter; however, the amount of liquid water present will in that case relate
directly to dive duration, and you need to be wary of allowing the sub to "air
out" sufficiently after a dive to dry the surfaces where condensation was
taking place to prevent any corrosion. Where this might be a problem is in
uninsulated obscured spaces where air is poorly circulated (behind control
panels etc.). Even if you don't implement any active air conditioning, I
wonder if it would be worth the effort to insulate the hull to some extent in
all areas except for a designated region for condensation, or alternatively,
to control airflow such that circulation only slows in a designated area?

-Sean

On Wednesday 16 March 2011 12:38:45 you wrote:
> Sean, I also think this is true also. I have a relative humidly sensor in
> my boat. When I did an 8 hour test in my boat in which I do not have any
> RH control, at the time the hatch was closed, the RH was 50%. After
> about one hour, the RH climbed to around 80%. Over the next 3 hours, the
> RH climbed to 85% and stayed at this level for the duration of the test.
> This leads me to believe that the primary source of condensed water vapor
> in the boat is due to exhalation of 100% RH air. I think that purging the
> pressure hull with dry air prior to hatch closing would bring down the
> initial RH to near zero but quickly for a small psubs like mine, it would
> climb rapidly. As the chemical reaction in the scrubber needs water vapor
> to work, this concept would seem to me to be counter effective to scrubber
> operations.
>
>
> In the spirit of KISS, I am not sure it makes sense to try and condition
> the air to regulate RH anyway.
>
> Cliff
>
>
>
> Cliff Redus
> Redus Engineering
> USA Office: 830-663-6445
> USA mobile: 830-931-1280
> cliffordredus@sbcglobal.com
>
>
>
>
> ________________________________
> 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

************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003. Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.

If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
removeme-personal_submersibles@psubs.org

Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.

PSUBS.ORG
PO Box 53
Weare, NH 03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************