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Chapter 34 — 4
greater likelihood of emergency ascent and salt water aspiration. The dive should always be
completed with at least 50 ATA remaining.
A description of the methods of preventing and coping with an OOA and LOA situation is in
Chapter 11.
Fig. 34.1
In some cases the diver was using a smaller cylinder than a 2000 litre (72 cu.ft) tank. A 1400
litre (50 cu.ft) cylinder has much less endurance than a conventional cylinder, and allows
fewer breaths once a LOA situation develops at a significant depth. Also, a diver using a
smaller cylinder will usually run out of air sooner, encouraging separation from his group.
Buoyancy
In the ANZ survey, half
the diving victims (52%) encountered buoyancy problems.
Most
of these were due to inadequate buoyancy, but some (8%) had excessive buoyancy. The DAN
survey buoyancy problems were the commonest adverse event leading to death.
The
buoyancy changes peculiar to wet suits
were a significant factor. The considerable
buoyancy offered by a wet suit at the surface needs to be compensated by weights. An
approximate formula for this is:
• 1 kg for each 1 mm thickness,
• 1 kg for "long john" extensions and a hood,
• 1 kg for an aluminium tank,
•
±
1–2 kg for individual body variations in buoyancy.
Based on the above formula,
40% of divers who perished were found to be grossly
overweighted
at the surface. This factor would have been greater at depth. When weighted
according to this formula, a diver should be neutrally buoyant at or near the surface. In this
state, descent or ascent are equally easy.
During descent, the wet suit becomes compressed, making the diver negatively buoyant. This
is where the
buoyancy compensator
(B.C.)
comes in. It is inflated just sufficiently to restore
neutral buoyancy. This is why it is called a buoyancy compensator.
Evidently,
some divers deliberately overweighted on the surface
, using this excess weight
to descend more easily and were then using the B.C. to maintain depth and then later to return
to the surface. This places excessive reliance on the B.C.. This dangerous practice is