Andy, you assume that the tank is open to the water and that nothing
opposes its movement in and out. This is not the way it works on tiny
subs afaik (but maybe you know better ;-).

The only ACCURATE way to know the volume of water in the tank is to use a
volumetric pump to fill and empty it and count turns. Assuming there are
no leaks (hahaha) that will be accurate. But not to 0.3%.

If this is some sort of slow research sub then this will be adequate from
all points of view, excepting for the pressurized volumetric pump which is
a chapter for itself. If you want to go fast, use the navy way, blow air
into the tanks to raise, and vent air out the top to go down. And use
a valve to close off the water when not changing pressure. Andy, you
are into this afaik, what kind of ballast control do you use on model subs
?

Peter

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