While driving a friend, a novice diver, to a dive site, she said something so shocking that I almost pulled over. "So, just to be clear," she asked, "when I want to go up I push the inflate button, right?" My friend had completed her open water certification some time ago, and had already enjoyed several recreational scuba dives when I caught her error. Frankly, I am disgusted that her instructor never explained proper use of the buoyancy compensator's (BCD's) power inflator to her. Unfortunately, she is not the first certified recreational diver that I have encountered who was confused about proper buoyancy control.
What is wrong with inflating a BCD to ascend? The answer can be found with Boyle's Law and the behavior of air in a diver's BCD as he ascends. According to Boyle's Law, a gas (such as the air in a diver's BCD) will expand as he ascends areas with a lower pressure. A diver who ascends will become increasingly buoyant as the air in his BCD expands unless he releases air from his BCD. A diver who inflates his BCD to ascend will begin to float upwards rapidly, and will continue to ascend at an increasing rate unless he vents air from the BCD. Inflating a BCD to ascend is an effective way to send yourself into an uncontrolled ascent - one of the worst things a diver can do because a fast ascent increases a diver's risk of decompression sickness, pulmonary barotrauma and ear barotrauma.
Underwater, a diver should inflate his BCD for one reason and one reason only - to stop himself from sinking.
Divers wear weight belts to offset the natural buoyancy of their bodies, wetsuits, gear, and tanks (as they empty). BCDs compensate for negative buoyancy caused by weights and the compression of air on descent. The deeper a diver goes, the more the air trapped in his gear and wetsuit compresses and the more negatively buoyant he becomes. Without a BCD, a diver sinks slowly at first, and then plummets downwards as his buoyancy decreases with depth. Divers add air to their BCDs on descent to stop themselves from sinking. This is the only use for a BCD underwater.
More information on BCD's and buoyancy:
• Styles and Features of Common Buoyancy Compensators
• Boyle's Law and Scuba Diving
• Buoyancy Basics for Scuba Divers
To properly ascend, a diver should begin neutrally buoyant, and then use his fins or lung volume to slowly draw himself upwards. As he rises, the air in his BCD and other gear will expand, causing him to become a slightly positively buoyant. Once he begins to move upwards without using his fins or his lungs, the diver should vent a small amount of air from his BCD to reattain neutral buoyancy, then kick or use his lungs to rise again. The goal is to stay as close to neutral buoyancy as possible during the entire ascent.
Divers inflate to maintain neutral buoyancy on descent and deflate to maintain neutral buoyancy on ascent. This may seem counterintuitive at first, but it makes sense when the effects of water pressure are taken into account.
