Boyle’s Law

The Gas Laws are very important to our understanding of decompression sickness, the way they work together in the body enables us to understand how nitrogen works when under pressure and how it effects divers.

Boyles’ Law

Definition: ‘The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system’.

boyles law

For every ten meters of descent the pressure increases by one atmosphere absolute (ata or atm). Therefore total lung volume during a breath hold dive at ten meters is half of what it was at the surface. At 20m it is one third, at 30m it is 1/4, at 40m it is 1/5.

When a diver surfaces or ascends these figures are reversed. But when breathing compressed gases, as in scuba diving, the mass of gas in the lungs is increased to fill the normal volume. An ascent to the surface without exhaling would cause the gas already in the lungs to expand further, but with minimal capacity to do so. If a diver rushes to the surface with full lung capacity at 20m the lungs would expand a further three times normal with the largest pressure change of all happening at the last ten metres when it would double.

This is an important law to bear in mind to explain pressurisation and depressurisation issues and injuries.

Depth Pressure Air Volume Air Density
0m/0ft 1bar/ata 1 x1
10m/33ft 2bar/ata 1/2 x2
20m/66ft 3bar/ata 1/3 x3
30m/99ft 4bar/ata 1/4 x4
40m/131ft 5bar/ata 1/5 x5


The graph above is in all PADI Open Water Manuals. Although no pupil is told that this is in fact Boyle’s Law, it is a table many divers understand.

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