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[PSUBS-MAILIST] Compensating for Buoyancy Loss in Larger Subs at Depth





Sean, 
 
This is a very interesting point to me, since my later goals involve building a live aboard submarine.  I wonder what Carsten's figures for dealing with that are.


Regards,

Szybowski




> Date: Tue, 21 Jul 2009 23:09:04 -0700
> From: cast55@telus.net
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] water level
>
> Don't forget to account for the possibility of having to adjust in the
> other direction - i.e. adding buoyancy to compensate for reduction in
> volume as the hull deflects at depth. For a small, shallow sub, this
> won't be significant, but as depth and the size of vehicle increase, the
> change in volume may become significant.
>
> -Sean
>
>
> ShellyDalg@aol.com wrote:
> > Hi David. A sight glass is the easiest to read on a tank, but it's not
> > much use to us for variable ballast tanks. The reason being, it would
> > need to be able to withstand a lot of pressure and they are hard to
> > keep clean.
> > The variable ballast tank, often called "hard ballast tanks" are
> > pressure vessels that will withstand being opened at depth to adjust
> > buoyancy when you're at your deepest operating depth, plus what ever
> > safety factor you wanted.
> > Although a simple "trim tank" can be used to adjust buoyancy at
> > or near the surface, and then valved off with a set amount of water
> > ballast in it, no adjustment can be made when you're deep unless the
> > tank is able to withstand the pressure at your deepest dive.
> > Once you open a hard ballast tank at depth, and close the valves
> > to maintain that level of water, the pressure is "locked" into that
> > tank. Then, when you get back to the surface, you release that
> > pressure by opening the valve and letting the pressurized bubble blow
> > out the tank.
> > If you want to adjust for more buoyancy at depth, due to changes
> > in salinity or maybe you picked up something from the bottom, you must
> > blow high pressure air into the tank until the correct water volume is
> > achieved, then close the tank off to maintain that level of water.
> > Knowing just how much water is in your hard ballast tank is a
> > problem.and one that I considered a while back. The "sight glass"
> > method would require a very heavy walled clear tube with valves in
> > case it developed a leak. I'm not sure how risky that would be if it
> > blew out but it should be fairly easy to build.
> > Another method would be to use a "see through"tank. There's a
> > filament wound fiberglass/composite tank available called "Lite"
> > propane tanks and they come in a few sizes, the largest being about 8
> > gallons for use on warehouse type propane fork lifts. These will
> > withstand 250 psi ( with a safety factor) and you can tell at a glance
> > how much liquid is in it. They come with threaded fittings molded in,
> > and a metal "basket" that protects it from bumps. They run about $300
> > each I think.
> > I'm leaning toward the see through tank right now, and have a
> > place inside designed for it. That only gives me 64 pounds of variable
> > ballast though. ( 8 gallons at 8 pounds per). It would be nice to
> > have more adjustment, like if you found something and wanted to pick
> > it up and bring it back to the surface. It's possible to blow a little
> > air into the "soft" ballast tank for lifting purposes, but that bubble
> > will become much larger as you near the surface, making it difficult
> > to control buoyancy as your depth changes.
> > Most psubs have a few lead bricks that get adjusted at the
> > surface for how much payload you're carrying on any given dive, like
> > an additional passenger, some added machinery or cargo, or what ever
> > will change the total weight of the sub.
> > Buoyancy is based on water displacement and total weight in water,
> > so when you add a passenger, you must ask them how much they weigh and
> > adjust the lead bricks accordingly to achieve "neutral" buoyancy. A
> > hard ballast tank is required for small changes in buoyancy like
> > salinity and temperature changes. For instance, if I launch my boat in
> > the harbor, which is a river mouth with a small amount of dissolved
> > salts, and then motor out into the ocean where the salinity level is
> > higher, the sub is displacing more weight ( the water is saltier so is
> > heavier ) so additional ballast must be added to maintain "neutral. It
> > runs about 4% from fresh to salt water while temperature can add about 1%.
> > These are percentages of "displacement" or the total volume of water
> > displaced by the submarine. Several "high dollar" subs I've read about
> > have as much as 240 pounds of "variable" ballast. Not sure how they
> > achieve that, but it's useful if you plan on bringing back samples or
> > stuff from down deep.
> > Frank D.
> >
> > ------------------------------------------------------------------------
> > What's for dinner tonight? Find quick and easy dinner ideas
> > <http://food.aol.com/dinner-tonight?ncid=emlcntusfood00000009> for any
> > occasion.
>
>
>
>
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