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Diving gas laws

Boyle, Dalton, Henry: understanding physics to dive safely

TheoryOpen WaterAdvancedRescue

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

Pressure when diving

Pressure increases by 1 bar every 10 meters of water. At the surface, atmospheric pressure is 1 bar (1 atm). This increase is the basis for ALL physical phenomena in diving.

← low pressurehigh pressure →

Boyle's Law

P × V = constante

At constant temperature, the volume of a gas is inversely proportional to its pressure. The deeper you go, the more air compresses. The shallower you go, the more it expands.

1.0
Pressure (bar)
12.0
Volume (liters)
×1.0
Compression
×1.0
Air consumption
👂Ears — Valsalva maneuver to equalize compression
🤿Mask — Exhale through the nose to prevent mask squeeze
🫁Lungs — NEVER hold your breath on ascent!
⛽Consumption — You breathe 1× more air than at the surface

Dalton's Law

Ptotal = P1 + P2 + ... + Pn

The total pressure of a gas mixture is the sum of the partial pressures of each gas. Partial pressure = total pressure × gas fraction. In air: 79% N₂, 21% O₂.

Partial pressures by depth (air)

ppO₂ (oxygen) ppN₂ (nitrogen) ⚠ Danger thresholds

Diving applications

  • Nitrogen narcosis — ppN₂ increases with depth. Beyond ~3.2 bar (≈30m on air), nitrogen has a narcotic effect ("rapture of the deep")
  • O₂ toxicity — Beyond a ppO₂ of 1.6 bar, risk of seizures (Paul Bert effect). This is why each oxygen-enriched mix (Nitrox, pure O₂) has a Maximum Operating Depth (MOD): e.g., 30m for Nitrox 40%, 6m for pure O₂.
  • Nitrox — By increasing the O₂ fraction and reducing N₂, you decrease nitrogen loading but limit 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 that 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. If ascent is too fast, nitrogen forms bubbles → decompression sickness
  • Tables and computers — MN90 tables and the Bühlmann model are direct applications of Henry's Law

Summary for exams

🫧

Boyle's Law

P × V = constant

Pressure compresses gas volumes. On descent, volumes shrink. On ascent, they expand.

⚠ Barotrauma, lung overexpansion
🧪

Dalton's Law

Pp = P × fraction

Partial pressures of each gas increase with depth, even though their proportions remain the same.

⚠ N₂ narcosis, O₂ toxicity (Paul Bert effect)
💧

Henry's Law

C = k × P

The higher the pressure, the more gas dissolves into tissues. This is the foundation of decompression.

⚠ Decompression sickness (DCS)

Frequently asked questions

Why should you never hold your breath while diving?
Boyle's Law: on ascent, pressure decreases and the air in your lungs expands. If your airways are blocked, alveoli over-expand and can rupture → lung overexpansion injury, gas embolism, pneumothorax. This is the most serious diving accident and it can happen from just 1 meter depth.
What exactly is nitrogen narcosis?
Nitrogen narcosis (Dalton's Law) is a narcotic effect of nitrogen under high partial pressure. It typically appears around 30-40m on air. Symptoms: euphoria, slowed thinking, impaired judgment, sometimes anxiety. It disappears on ascent. For deep diving, helium is used instead of nitrogen to avoid it.
Are these laws part of all certification levels?
Boyle's Law is introduced from the first level (ears, lungs). Dalton and Henry are covered more deeply at Advanced and Rescue levels. Instructors should be able to teach them with calculation exercises. Our Nitrox calculator is a great tool to practice Dalton's Law.
What is the link between Henry's Law and decompression tables?
Decompression tables and algorithms like Bühlmann model Henry's Law directly: they calculate how much nitrogen has dissolved in your tissues based on depth and time, then determine the ascent rate and stops needed so that this nitrogen is eliminated without forming dangerous bubbles. That's why exceeding table limits exposes you to decompression sickness.