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Re: [PSUBS-MAILIST] Humidity control
Operating electronics generate heat, so in general airflow through an
electronics enclosure will experience a reduction in RH as it is warmed. More
of a concern IMO would be ensuring that condensation on cold surfaces such as
your hull can not lead to water dripping on your electronics, and that you
should avoid a situation where you have for example a metal electronics
enclosure in thermal contact with the cold hull. RH shouldn't be an issue
provided you keep below the 100% condensation threshold. I can't imagine many
scenarios in which your electronics would be colder than the surrounding air.
-Sean
On Wednesday 16 March 2011 14:49:14 you wrote:
> 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
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