Adding a valve to a soft ballast tank does change the tank from an
ambient to a potentially 1atm chamber. By closing that valve you
allow a pressure differential to be created when your sub changes
depth. So you'd have to build it much stronger than a more simple
soft ballast tank. Right?
To make a "soft" tank into a "hard"
tank, do I simply need to install a valve to let in the water as opposed to just
an open flood hole ? This valve, of course, would keep the water from
coming in while the sub gains depth (pressure).