Boyle's Law in Scuba Diving

The law in one line

Boyle's law says that, at a constant temperature, the volume of a gas is inversely proportional to its pressure. Double the pressure and the volume halves; halve the pressure and the volume doubles. Written as a formula, P1 × V1 = P2 × V2, where P is absolute pressure and V is volume. The Irish scientist Robert Boyle published the relationship in 1662.

What it means underwater

Pressure underwater rises by about 1 bar for every 10 metres (33 feet) of seawater, as explained in water pressure at depth. A balloon filled at the surface with 6 litres of air would shrink to 3 litres at 10 m, 2 litres at 20 m and 1.5 litres at 30 m. Bring it back up and it grows again by exactly the same steps.

Never hold your breath

When you breathe from a regulator, each breath arrives at the pressure around you, so your lungs stay their normal size at any depth. The danger comes on the way up. If you hold your breath while ascending, the air in your lungs expands with nowhere to go. A rise of only a metre or two can be enough to overstretch the lungs in shallow water, where the relative pressure change is biggest. This can tear lung tissue and push air into the chest or bloodstream, causing an arterial gas embolism, one of the most serious diving injuries.

The rule is simple: breathe continuously and never hold your breath on scuba. If you ever have to make an emergency ascent without gas, you exhale steadily the whole way up.

The squeeze on descent

Boyle's law works in the other direction on the way down. Any air space that can't take in more air gets squeezed:

• Ears and sinuses feel pressure within the first metre or two. Adding air through ear equalization keeps them comfortable.
• Your mask presses against your face unless you breathe a little air out through your nose into it.
• Your exposure suit is full of tiny gas bubbles that shrink, so a wetsuit gives less warmth and less buoyancy at depth (see wetsuits and drysuits).
• Your buoyancy control device loses volume, so you sink faster unless you add air.

Buoyancy and air use

Because gas volume changes with depth, so does your buoyancy. Air added to a buoyancy control device at depth will expand as you rise, which can turn a slow ascent into a fast one unless you vent it. Good divers release a little air from the BCD all the way up. Boyle's law also explains air consumption: at 30 m each breath takes four times as much gas from the tank as it would at the surface.

The shallow zone

The volume of a gas doubles between 10 m and the surface but only rises by a quarter between 40 m and 30 m. That is why the last 10 metres of an ascent are the most critical, why ascents should be slowest near the surface and why the safety stop sits at 5 m (15 ft).

Frequently asked questions