🧪 Gas Laws in Diving – Dalton & Henry (Narcosis & Decompression)
When you dive open-circuit scuba, you breathe compressed breathing gas at ambient pressure. That single fact ties together narcosis, oxygen exposure, nitrogen loading and why your dive computer is obsessed with depth and time.
Two classical laws frame almost everything else in “theory class”:
- Dalton’s Law → how each gas in the mix “pushes” as partial pressure
- Henry’s Law → how gas dissolves into your blood and tissues at pressure
Pair this with pressure & Boyle’s Law (volume vs pressure in your BCD and lungs) and you have the core physics behind recreational limits.
Educational only: follow your agency limits, instructor and computer — not a blog — for real dive planning.
🧪 Dalton’s Law – partial pressure
In a mixture, each gas behaves as if it alone occupied the volume: each contributes a partial pressure. Totals add up to ambient pressure.
What divers use in practice
Partial pressure ≈ fraction × total ambient pressure
(Using the usual teaching model: depth in sea water, pressure in bar absolute.)
Example: air at 30 m (~4 bar)
Dry air is often approximated as:
- Oxygen ~21%
- Nitrogen ~79%
| Gas | Fraction | Partial pressure at ~4 bar |
|---|---|---|
| O₂ | 0.21 | ≈ 0.84 bar |
| N₂ | 0.79 | ≈ 3.16 bar |
Try it: partial pressures (Dalton)
Optional: set ambient from depth (classroom models)
Uses the same simple teaching approximation as our Boyle / depth article: absolute pressure ≈ 1 bar + (depth ÷ m per bar). Type any ambient pressure directly above — decimals (2.3 bar) and comma decimals work.
Each partial pressure ≈ fraction × total ambient pressure. Nitrogen fraction is taken as 1 − O₂ (standard two-gas teaching model for air and nitrox — not trimix).
The percentages on the tank sticker do not change — but the potency of each gas, expressed as partial pressure, does change with depth. That is why “normal air” still becomes more narcotic and loads more inert gas deeper down.
😵 Nitrogen narcosis – Dalton in the real world
Inert gas narcosis is depth-related; on air, nitrogen partial pressure is the usual focus, so people say nitrogen narcosis.
Typical reported symptoms (vary by individual and day):
- Slower thinking, fixation on a single task
- Overconfidence or anxiety
- Tunnel vision, impaired judgment
Training, depth discipline, gas choices (where certified) and conservative planning matter. For context on deep profiles, see our deep diving notes — and remember narcosis is not reliably “trained away” only by experience.
🔥 Oxygen – essential, but not “more is always safer”
Oxygen partial pressure sets oxygen exposure. As depth or oxygen fraction rises, PO₂ rises — helpful within limits for decompression and nitrox, but hazardous if you pass agency-prescribed maximum PO₂ or exposure times.
Many recreational programmes use conservative planning values such as about 1.4 bar PO₂ as a working ceiling and 1.6 bar only for short, controlled exposures — exact numbers belong to your course materials.
Excessive PO₂ raises risk of acute oxygen toxicity; convulsions underwater are a serious hazard. That is why Nitrox divers calculate MOD and monitor PO₂.
🧬 Henry’s Law – gas in your tissues
Henry’s Law links partial pressure of a gas to how much can dissolve into liquid — here, plasma and tissues — at equilibrium.
Simple mental model
Higher inspired partial pressure → more gas on-loading (over time, tissues approach equilibrium). During ascent, pressure falls and gas comes out of solution; the body eliminates it mostly via the lungs if ascent is controlled.
On-gassing / off-gassing
- Descend / stay deep: inert gas partial pressure in the lungs is higher → tissues take up more gas.
- Ascend slowly: gradient favours elimination via breathing; bubbles that form are kept small enough to filter harmlessly when you stay within models.
⚠️ Decompression sickness (DCS) – when elimination can’t keep up
If you exceed dive computer or table limits, or ascend too fast for your tissue loading, bubbles can grow large enough to cause symptoms — joints, skin, nervous system, circulation — we summarise that as decompression sickness.
It is not “one bubble always equals pain”; it is a statistical, model-based sport with real consequences — which is why agencies teach maximum ascent rates, safety stops, and no-decompression limits.
Recreational note: a safety stop at ~5 m is recommended best practice on many profiles; it is not the same as a mandatory decompression stop after exceeding the NDL on technical schedules.
🔗 How Dalton and Henry work together
- Dalton tells you how strong each gas is acting at that depth (narcosis, oxygen, how much “push” drives diffusion into tissue).
- Henry describes how loading and unloading trends work over time at those partial pressures.
Your computer’s algorithm is a wrapper around those ideas plus empirical testing — which is why using it correctly matters more than memorising numbers.
The same pressure idea drives gas use and SAC planning: deeper means denser gas per breath, not just “harder breathing”.
Practical takeaways
- Depth limits exist because partial pressures climb fast.
- Slow, controlled ascents reduce supersaturation stress.
- Nitrox reduces nitrogen fraction → lower N₂ partial pressure at the same depth → often more conservative nitrogen loading for a given mix and PO₂ cap — learn this properly in a Nitrox course.
❓ FAQ
What is Dalton’s Law in scuba diving?
Dalton’s Law states that in a gas mixture each component gas exerts its own partial pressure, and the total pressure is the sum of those partial pressures. For divers, the important idea is: partial pressure ≈ fraction × ambient pressure (P × F).
What is Henry’s Law in scuba diving?
Henry’s Law relates the amount of gas that dissolves in a liquid to the partial pressure of that gas. At higher pressure, more gas can dissolve into blood and tissues; during ascent, pressure falls and gas comes out of solution — which is why controlled ascent and dive computer limits matter.
Why does nitrogen narcosis increase with depth on air?
On air, nitrogen fraction is fixed, but ambient pressure rises with depth, so nitrogen partial pressure rises. Higher partial pressure of narcotic gases correlates with stronger inert gas narcosis — often called nitrogen narcosis when diving air.
What does oxygen partial pressure have to do with toxicity?
Oxygen toxicity risk depends on inspired oxygen partial pressure and exposure time. Training programmes publish maximum PO₂ values for planning; exceeding those raises risk of acute oxygen toxicity including convulsions underwater, which is why nitrox and technical divers track PO₂ carefully.
Does Nitrox “cure” narcosis?
Not magically. At the same depth, common nitrox blends lower nitrogen partial pressure compared to air, which can reduce narcosis somewhat — but you still have oxygen and other factors, and you must respect PO₂ and MOD.
Is dissolved gas the same as bubbles in your veins?
Dissolved gas is on a molecular level in solution. Problems escalate when local supersaturation allows free-phase gas that the body cannot tolerate — models aim to keep you on the safe side of that line.
Where does Boyle’s Law fit?
Same dive, different question: Boyle explains volume changes as pressure changes — critical for buoyancy and breath control. See the companion article.
