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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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