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Archimedes Principle
In the 3rd century B.C., Archimedes theorized that buoyant force exerted on an object is equal to the weight of the amount of fluid or air displaced by that object. If the density of the object is less than the weight of the fluid or air it displaces, the object rises. If density exceeds the displacement weight, it sinks. Neutral buoyancy occurs when the density and displacement are roughly equivalent. In air or water, a neutral buoyancy craft floats in a state of equilibrium, neither rising nor sinking.
Blimp Theory
Lighter-than-air flight was limited to uncontrollable hot air balloons until powered airships with rigid frames evolved in the early 20th century. Vast amounts of hydrogen were required to displace enough air to lift the floating behemoths. In more recent years, lighter, nonrigid blimps have supplanted airships. A blimp's gas envelope is filled with inflammable helium. It generates positive buoyancy, or lift, equivalent to the weight of the air it displaces. To get off the ground, positive buoyancy must exceed the blimp's cumulative weight, including its gondola, engines, envelope and the helium in the envelope. But if lift is the sole force, a blimp would simply rise uncontrollably for thousands of feet, necessitating venting expensive helium to offset the buoyancy. Instead, blimps balance lift force with negative buoyancy by pumping air in and out of sealed compartments inside the gas envelope called ballonets.
Blimps in Flight
Blimps on the ground first pump sufficient air into the ballonets to induce a state of neutral buoyancy. A balance is struck between lift generated by the low density helium and the negative buoyancy caused by heavier air in the ballonets. Power from the engines propels the blimp into the air at a sharp angle. Aloft, neutral buoyancy is maintained for most of the blimp's cruising flight. If more altitude is desired, air is released from the ballonets, tipping the equilibrium in favor of the less dense helium. When it's time to descend and land, dense air is pumped back into the ballonets, sending buoyancy into the negative range and causing the blimp to sink. A blimp usually touches down in a state of slightly negative buoyancy.
Submarine Theory
Submarine designs date to the 1500s. During the U.S. Civil War, a Confederate sub sank a Union ship. But a viable, modern submarine did not appear until after World War I. A submarine floats on water because the amount of water displaced exceeds the density of the hollow vessel, generating buoyancy. Submarines explore the depths by altering that density through the use of ballast tanks and trim tanks. The tanks are situated between the inner and outer hulls of the submarine.
Submarines in Water
A submarine is in a state of positive buoyancy on the surface. To submerge, vents on the ballast tanks open, admitting water into the tanks and inducing negative buoyancy. As the submarine sinks to the desired depth, controlled amounts of compressed air from on-board tanks are pumped back into the ballast tanks, forcing water out of the tank. A state of neutral buoyancy is achieved and the sub hovers at that depth. Buoyancy in the trim tanks is also adjusted to alter the forward and aft trim of the submarine and maintain straight and level attitude. To surface, compressed air is introduced into the ballast tanks and water is forced out, incrementally returning the sub to a state of positive buoyancy and causing it to rise.