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Chapter 5 — 20
The scuba tank may be negatively buoyant ant the start of a dive, and positively buoyant near the end
– after the compressed air has been lost.
Buoyancy compensator
. All divers will be impressed with the way they can rapidly inflate their
B.C. on the surface, and the noise it makes. Unfortunately, in an emergency underwater, it is much
slower as the air supply also reduces because of the Boyles Law effect. A diver in difficulties may
not realize that the B.C. air-supply button has to be depressed for longer to get the same effect. It
takes much longer to inflate 4 litres of air in the B.C. than it does to drop a 4 kg. weight belt. Also,
the 4 litres of air will expand as you ascend and produce the emergency rapid ascent – the “polaris
effect”, with all its complications and with the requirement to off-load gas. In an emergency, this
may not be easy. Release of a weight belt produces a more consistent rate of ascent throughout.
Rescue implications
. As a general rule, in assisting in a diving incident, it is preferable to ditch the
victims weights more than the rescuers, and inflate the victims B.C. more than the rescuers. As
separation often occurs, the possibly unconscious diver will still reach the surface if these actions are
performed.