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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.
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