Diving Gas Laws: Boyle, Dalton, Henry

ℹ️ Educational content for informational purposes. Test your knowledge with the Nitrox calculator (Dalton's law) and the consumption calculator (Boyle's law).

Why physics in diving?

Underwater, gases behave differently than at the surface. Understanding gas laws prevents accidents (barotrauma, narcosis, O₂ toxicity, DCS) and is key to mastering decompression. These laws are part of all FFESSM curricula from N2 onwards.

Pressure in diving

Pressure increases by 1 bar every 10 metres of water. At the surface: 1 bar. At 10m: 2 bars. At 20m: 3 bars. At 40m: 5 bars.

Boyle's Law

P × V = constant

At constant temperature, gas volume is inversely proportional to pressure. At 10m (2 bars), an air volume is halved. At 30m (4 bars), quartered.

Diving applications

Ears — Middle ear air compresses on descent → Valsalva maneuver to equalize
Mask — Mask air compresses → mask squeeze if you don't exhale through nose
Lungs — DEADLY DANGER: never hold your breath while ascending. Expanding air can cause pulmonary overexpansion
Air consumption — At 20m (3 bars), you consume 3× more air than at surface

Dalton's Law

P_total = P₁ + P₂ + ... + P_n

Total pressure of a gas mixture equals the sum of partial pressures of each gas. Partial pressure = total pressure × gas fraction.

Diving applications

Nitrogen narcosis — N₂ partial pressure increases with depth. Beyond ~3.2 bar (≈30m on air), nitrogen has a narcotic effect
O₂ toxicity — Beyond pp O₂ of 1.6 bar, seizure risk (Paul Bert effect). This limits Nitrox and pure O₂ maximum depth (see Nitrox calculator)

Henry's Law

C = k × P

At constant temperature, the amount of gas dissolved in a liquid is proportional to the pressure of the gas above the liquid. This is THE fundamental principle of decompression.

Diving applications

Nitrogen dissolution — At depth, nitrogen dissolves into tissues proportionally to ambient pressure
Decompression — On ascent, pressure drops. Dissolved nitrogen must be eliminated gradually. Too fast = bubbles → DCS
Tables and computers — MN90 tables and Bühlmann model are direct applications of Henry's Law

Summary for exams

Law	Formula	Phenomenon	Risk
Boyle's	P×V = const	Volume change	Barotrauma, lung overexpansion
Dalton's	Pp = P × f	Partial pressures	N₂ narcosis, O₂ toxicity
Henry's	C = k × P	Gas dissolution	Decompression sickness

FAQ

Why must you never hold your breath while diving?

Boyle's Law: on ascent, pressure decreases and lung air expands. With blocked airways, alveoli can rupture → pulmonary overexpansion, arterial gas embolism, pneumothorax. This is the most serious diving accident and can occur from just 1 metre depth.

What exactly is nitrogen narcosis?

Nitrogen narcosis (Dalton's Law) is a narcotic effect of nitrogen under high partial pressure, typically appearing around 30-40m on air. Symptoms: euphoria, slowed thinking, impaired judgment. It resolves on ascent. Deep diving uses helium instead of nitrogen to avoid it.

Which levels cover these laws?

Boyle's is introduced at N1 (ears, lungs). Dalton and Henry are deepened at N2 and mastered at N3/N4. Instructors (E2+) must be able to teach them with calculation exercises.