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[Leaky Bottom]

As someone who knows nothing about subs this may seem a daft question to those who know, so, when a sub dives I presume you pump water into the ballast tanks which expells the air and the sub loses it's bouyancy and dives, now when you want to surface you pump the water out of the ballast tanks but where does the air come from to replace it as the  sub is still underwater, I would of thought a vacuum would be created in the tanks and not give any positive bouyancy.
 
I know there's a simple answer because they work, it's just that I can't think of it

[Subculture]

As you fill the tank air is compressed inside the boat. There are various ways of doing this depending on the system used.

Some systems are total loss, in that the air is not recirculated, but expelled to the atmosphere as the tank fills. With such a system you cannot empty the tank whilst underwater unless you have an auxilliary system like gas or compressed air stored in a bottle to blow the tank clear.

[warspite]

The different methods are, a bladder like swimming arm bands that is filled with air from a reserve tank - this displaces the water in the dive tank and she returns to normal buoyancy, same principle but using a air canister like those used for spraying paint with an artists spray gun, and then the more common type - the syringe or WTC method, as the syringe moves back it sucks water in and fills the cavity while the air is compressed within the WTC chamber - when in reverse the water is expelled and the air reoccupies the chamber behind the syringe.

In all, like all subs keeping the water out of critical areas is the priority.

[dreadnought72]

a vacuum would be created in the tanks and not give any positive bouyancy.

Just to make it clear - a vacuum, if you could achieve one, would give you maximum positive bouyancy. It weighs nothing per cubic metre, compared to air's 1.2 kg/m³

Andy