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The difference is critical. A buoyancy compensator, filled with a fixed quantity (not volume) of air, does decrease in volume as you descend and the pressure increases. You'd sink faster and faster if you did not add air as required. Therefore, you must actively manage a BCD! On the way up, the opposite happens. The quantity of air in your buoyancy compensator has an increasing volume of air as you rise and the pressure decreases, providing you with increasing buoyancy. Uncontrolled, this would rocket you towards the surface. Therefore, you must actively manage a BCD! Lungs do not change in volume in SCUBA as they do when free (breath hold) diving, because your regulator is allowing you to constantly breathe air which is at the same pressure as the water around you. When free diving, you carry weights to be neutrally buoyant at around 30 feet. Above the neutral buoyancy depth, you float towards the surface, and must swim down to descend. Below the neutral buoyancy depth, you become negatively buoyant, sink increasingly quickly, and must swim up to ascend. This is also important to be aware of; you don't want to strap weights to yourself that you can't drop: You may think you're capable of swimming up against your negative buoyancy, but if you only test at 10 feet under you may not be capable when you get deeper! |