The Boyle's law describes the effects of the pressure on gases. Indeed gases are compressible, and during scuba diving the volume of a gas decreases during the descent (while the surrounding pressure increases) and increases during tht ascent (while the pressure decreases).

Experimental test :

A ballon containing a fixed quantity of gas is submerged into water at 20 meter of depth. At this depth, the ballon has a volume V1 = 1 litre and the surrounding pressure is P1 = 3 bar. While ascending, the ballon inflates and we can observe that at 10 meters depth, its volume is V2 = 1.5 x V1 = 1.5 litres and the surrounding pressure is P2 = 2 bars. At the surface, the balloon has a volume V3 = 3 x V1 = 3 litres and the surrounding pressure is P3 = 1 bar.

That is to say :

- At 20 m depth P1 x V1 = 3 x 1 = 3

- At 10 m depth P2 x V2 = 2 x 1.5 = 3

- At 0 m depth P3 x V3 = 1 x 3 = 3

At constant temperature, the variation of the volume, V, of a gas is inversely proportional to the absolute pressure, P, acting upon it.

The equation of the Boyle – Mariotte Law is: P x V = k

where P is the pressure exerted on the gas, V is the volume of the gas and is the constant for a given mass and temperature.

That is to say P1 x V1 = P2 x V2 = P3 x V3 = a constant

The Boyle-Mariotte law has many application to diving :

- barotrauma : the variation of the volume of a gas in the body during the descent or the ascent may cause injuries.

- diving time : the air that divers breathe underwater is compressed, so for a given volume of air inhaled the quantity of air consumed increases with depth.

- effects upon equipments : use of a stabilizing jacket, compression of diving suit...