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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. > > > > > ************************************************************************ > ************************************************************************ > ************************************************************************ > The personal submersibles mailing list complies with the US Federal > CAN-SPAM Act of 2003. 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