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I was thinking about car gas gages as well. Perhaps even a digital one. I like the idea of having the bladder VBT inside that can be pumped out by hand, with a high pressure electric pump, of perhaps with one of those pneumatic vacuum generators. With the bladder fuel tank type VBT, you can more or less see how much water is in one or more of them. If you have a bigger one you might not even need to have the extra lead around for dealing with different cargo, pilot, or passenger loads. For larger ones, I was thinking of using a spring strap system to keep the water from sloshing back and forth, like I worked up for RC plane bladder fuel tanks. BDH To: personal_submersibles@psubs.org Subject: Re: [PSUBS-MAILIST] water level Date: Wed, 22 Jul 2009 08:28:53 -0400 From: vbra676539@aol.com Frank and David, You can use a regulator to compensate externally mounted hard tanks so they are pre-set for X-psi over pressure at any depth. For deeper boats, it keeps you from having to carry the weight of pressure vessels capable of whatever your maximum depth is. The JSLs, for instance, use two pairs of vertical aluminum tanks built into the MBTs. Alternatively, when you get to MUCH greater depths, the system would use too much air. The Pisces class has three tanks, two hard up in the brow and a soft tank in the aft buoyancy sphere. They all operate at (more or less) surface pressure and are pumped overboard. You could use site glasses on the Sea-Link style, but the Pisces used electric level gauges calibrated for the shape of the tank (think fuel gauges in a car, only stainless steel). Perry PC-15 class boats had VBTs mounted externally and used for minor changes only (they held about 50# of water each). And they were beefy! Of course the sub weighed 14 tons, so you had some payload to play with. Most Perry's carried soft tanks and pump systems which is slow but precise. Take your pick. Vance -----Original Message----- From: David Bartsch <dbartsch2236@hotmail.com> To: personal_submersibles@psubs.org Sent: Wed, Jul 22, 2009 1:04 am Subject: RE: [PSUBS-MAILIST] water level
Frank,
I wish now I had shown more interest in the inner workings of the sub I was stationed on while in the service. Thanks for taking the time to clear up some of these small but important tank level details. David Bartsch From: ShellyDalg@aol.com Date: Tue, 21 Jul 2009 11:20:28 -0400 Subject: Re: [PSUBS-MAILIST] water level To: personal_submersibles@psubs.org 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" p ropane 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 sali nity 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.
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