Instructor Manual
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Ocean Diver
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Adapting to the underwater world
Copyright © BSAC 2017
22
During a dive
• Compensate with BC or drysuit
Having left the surface, the increasing ambient pressure during descent will
compress the bubbles in the wet/semi-dry suit material or the gas held within
the drysuit resulting in buoyancy loss.
To balance this loss of buoyancy, gas is introduced to either the BC if wearing a
wet/semi-dry suit, or the drysuit.
• Fine tuning with lungs
Moving around or over underwater features can be affected by using the lungs
to fine tune neutral buoyancy.
Ascent
• Gas will expand
Gas introduced at depth into a BC or drysuit will expand as the diver ascends
and the ambient pressure reduces.
• Controlled dumping of gas equalises diver’s buoyancy
Removing or dumping the gas in a controlled manner will equalise the diver’s
buoyancy as they ascend.
Buoyancy check – correct weighting
Divers need to set the amount of additional weight they wear not just to assist the
descent, but for the control that will be required on an ascent.
Gas in the cylinder has weight and as it is used during the dive, both the cylinder
contents and its weight will reduce. Buoyancy checks before diving should take
account of this by adding one to two kilos to allow for gas consumed during a dive
so that neutral buoyancy can be maintained at the end of the dive.
The salt content, or salinity, of water can also vary and affect a diver’s buoyancy.
Divers weighted for fresh-water diving in quarries or lakes will need to add
additional weight for sea-water diving. Conversely, divers weighted for sea diving
will need to remove some weight for diving in fresh water.
