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Chapter 5 — 19
BUOYANCY OVERVIEW
General
. Problems with buoyancy dominate diving accidents and deaths. To understand this vital
aspect of diving, there is no better way than to take a specific buoyancy course with one of the diving
instructor organizations - after a basic diving qualification has been obtained and before any further
open water experiences.
In over half the diving deaths there were buoyancy problems contributing to the death. Most new
divers need to understand the following facts (see above and Chapter 34 for more detailed
explanations).
1.
Most divers’ bodies are almost neutrally buoyant when immersed in sea water. They tend
to sink in fresh water.
2.
The equipment used (wet suit and buoyancy compensator,) have air within them. Weights
are needed to overcome this buoyancy effect and help the diver to descend. Novice divers
tend to use many more weights than experienced divers. One large buoyancy producing
area is the diver’s lungs (around 5 litres). Because of greater anxiety, novices
hyperventilate more and breath at higher lung capacities. As they relax, both during an
individual dive and with more diving experience, they breath less and then need less
weights. Even some experienced divers, who may be anxious, use more weights than they
need, and compensate for this by greater reliance on their buoyancy compensators
(B.C.s).
3.
If a diver is over-weighted, he assumes a more head-up position when swimming
horizontally. If underweighted, he adopts a head-down orientation. Both mean more
energy and greater air consumtion.
4.
In comparing deaths with survivors from diving accidents, the survivors ditched their
weights and inflated their B.C.s twice as often. Key factors in reducing fatalities were;
neutral buoyancy during the dive, and positive buoyancy once a problem developed.
5.
In a fatality survey on buddied divers who experienced a low-on-air or
out-of-air
(LOA/OOA) situation, it was of interest that irrespective of who became OOA first, the
over-weighted diver was the one who died – at a 6:1 ratio.
6.
Based on the formula below, 40% of divers who perished were found
to be grossly over-
weighted at the surface. Despite that, 90% died with their weight belt on and 50% did not
inflate their B.C. 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.
7.
Many cases of decompression sickness are related to loss of buoyancy control.
Wet suit effects
. How much weight is needed to overcome the gas bubbles in a wet suit on the
surface? Check it out yourself. Get rid of all extra air spaces and add weights to the wet suit until it
starts to sink. That is your answer, and it indicates how much extra weight you need to add to your
weight belt.
Weights
. You will need approximately 1 kg for each 1 mm wet suit thickness, 1 kg for "long john"
extensions, bootees and a hood, 1 kg for a filled aluminium tank, ± 1–2 kg for individual body
variations in buoyancy (fat divers are more buoyant, thin are less). Thus with a wet suit of 3 mm
thickness, you may need around 4 kg weights.
On descent, the weights needed will decrease according to Boyles Law. Thus at 10 m. in the above
example, you will need half that weight, and you will be too heavy, and sinking, until you add 2 kg
of air to your B.C. Also, gas spaces elsewhere (gastro-intestinal tract, B.C., under suit, etc) are
reduced and so more air may be needed in the B.C.