Submarine Ballast

Schematic of a submarine ballast system.

I watched a movie about the U.S. Navy this morning. The movie had several scenes with a submarine. While watching the submarine travel through the water, diving and surfacing I was mindful of the fluid mechanics at work in the submarine's ballast tanks. The ballast tank is a tank that can be filled with air or flooded with water. The ballast tank allows the submarine to control it's buoyancy. When the tanks are filled with air the submarine weighs less than the water that is displaces and vice versa when the tanks are filled with water.