Instructor Manual
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Ocean Diver
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Adapting to the underwater world
Copyright © BSAC 2017
05
The compression of the air volume for every 10m water depth is easy to
remember.
At 10m, at an absolute pressure of two bar, the volume of air reduces to half of
its original surface volume.
At 20m, three bar, it reduces to one-third of its original volume.
At 30m, four bar, it reduces to one-quarter of its original volume, and so on.
• As pressure decreases, volume increases
If the surrounding pressure decreases, such as on an ascent, then the volume
of a body of air will increase.
Impact on divers
Fortunately, as human bodies have a high-water content of 70-85 per cent, our
bodies can readily adapt to the increase in water pressure that recreational divers
will normally experience. However, any air spaces within the body cannot adapt so
readily. The most important of these are the lungs.
• Lungs
The lungs are not a rigid air container like a bucket, but are a flexible air space
more like two balloons. As we breathe in and out, the elasticity of the lungs
allows expansion and contraction. If a swimmer takes a breath and dives down,
immediately the water pressure squeezes the air volume in the lungs and they
reduce in size. We do not feel this reduction unless we dive down deep.
• Diving equipment
To survive underwater we need equipment that enables us to adapt to the
underwater environment. This equipment is also subjected to the effects of
depth and pressure. The following sections covering snorkelling and scuba
equipment will explain the effects in more detail.
