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What if one connected a check valve in a pipe near the boats hull (with the flow aimed outward) and used a hand operated piston pump with long enough handle to apply enough force on a short stroke thin diameter piston to expell water against outside sea pressure while at depth to reduce the amount of water contained in this tank? When the handle is again drawn back, additional water from this tank could enter this piston. Once finished, isolation valves could be closed to prevent additional water from transfering. David Bartsch > 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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