[jarlemag]

The change in ballast tank size is unrelated.

My attempt at an explanation:

Before: You put a certain amount of air in the tanks, which displaced a certain amount of water. The same amount of air and water would remain in the tank until you either put more air into the tanks, or open the vents to let air out and water in. There was no gauge for the amount of air in the tanks, but as the only relevant variable was the amount of water in the tanks, this wasn't needed.

Now: You put a certain amount of air in the tanks (measured as "kg/cm2"). At shallow depths, where the water pressure is low, this air will have greater volume and displace more water than at at greater depths. Thus you will lose buoyancy ("up-force") when descending if your tanks are not already completely full with water (the amount of water will increase), and gain buoyancy when rising if there is both air and water in the tanks (the amount of water will decrease).

Note that since the air you put in the tanks expands when the sub rises, you can put more air in the tanks than there is room for when the sub is surfaced. The air gauge shows a pressure value in kg/cm2: This is the pressure the air would have if there was no water in the tanks.. Thus, the pressure gauge shows the amount of air in the tanks, rather than the actual pressure.

A convenient unit for measuring pressure is atmospheric pressure (the air pressure at sea level), and 1 kg/cm2 equals approximately 1 atm. At the surface, the external air pressure is 1 atmosphere, and the water pressure is about 1 atmosphere for every 10 meters under the surface. The ballast tanks will be empty of water as long as the air gauge pressure is higher than the water pressure at the depth of the sub. For example, if you start at 100 m, and put 5 kg/cm2 worth of air in the tanks ("5 atm"), then the ballast tanks will be empty of water at about 50 m.

Thus, only 1 kg/cm2 of air in the tanks is necessary to completely empty the tanks at the surface, but then the tanks will not be empty before you reach the surface, and ascending will take longer time. If you put in more air, you "waste" some high pressure air, but you can surface faster.