I thought I'd correct a slight misunderstanding that some folks seem to
have.

The amount of  ballast does not determine the depth that the sub will
reach.  An object that is neutrally bouyant at the surface will also be
neutrally bouyant at 30 feet, 30 meters, or 1000 meters (assuming it
maintains constant mass and volume).  Varying the amount of ballast will
change ascent/descent RATE, not the depth.

Assuming constant mass and volume is a big assumption.  Unfortunately,
there's a positive feedback system working here.  As the depth increases,
the sub tends to be compressed.  This means it is more dense, and has
more of a tendency to sink.  As it rises, it expands, and has more of a
tendency to rise.  I deal with this effect all the time as a diver.
Various gas pockets within the body as well as the air bubbles in the
neoprene of my wetsuit compress as I descend.  Almost all divers wear a
Bouyancy Compensator Device which allows adjustment to neutral bouyancy
at any depth.  Of course, the BCD is really just a big bag of air, so it
makes the problem worse as you ascend/descend.

Andy wants to carefully trim for neutral bouyancy once, then remember the
ballast level so he can quickly and automatically reach that same
(neutral) bouyancy the next time he puts the sub in the water, or wants
to dive from a surfaced (highly bouyant) position.

Don

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