Boyle's law · 0–40 m · recreational

Scroll to descend.

Scroll. You're diving. The pufferfish is every sealed gas bubble you carry — BCD, lungs, the SMB on your spool. Watch it shrink as pressure rises. The HUD tracks depth, pressure, and volume — live.

01 · 0 – 10 m · the first crush

First ten metres. Volume halves.

Your ears pop. Your sinuses tighten. By 10 m, pressure has doubled and every sealed gas pocket on your body is half what it was at the surface — the largest relative change of the entire dive. Most ear injuries happen here, not at the deep end.

Descent · 0–10 m

Equalize early

Air spaces in ears, sinuses, mask compress with depth. From 0 to 10 m those volumes halve — the biggest relative change of the whole dive. Equalize gently, early, often — Valsalva, Frenzel, Toynbee. If you can't equalize, stop, ascend a metre, try again. Pushing through it costs you an eardrum.

02 · 10 – 25 m · the gas tax

Mid-water. Every breath costs more.

By 20 m, sealed gas is one third its surface volume. By 25 m, near a quarter. Each lungful pulls more from your cylinder than the same breath at the surface — your SPG ticks down faster than your bottom-time clock. Plan accordingly.

Ascent · gas doubles

BCD venting

Your BCD or wing is a sealed gas bubble. From 10 m to surface it doubles. From 30 m to 10 m it doubles again. If you don't vent, you ride the bubble up uncontrolled — the runaway-ascent that bends divers. Vent early, vent continuously, vent before you think you need to.

03 · 25 – 40 m · the floor

Recreational floor. One-fifth.

Five times the surface pressure. Sealed gas squeezed to twenty percent. The whole ascent home is a five-fold expansion — your BCD wants to balloon, your lungs really would. This is where Boyle stops being theory.

Breath-hold · barotrauma

Lung overexpansion

Lungs are sealed gas on a held breath. One metre of breath-hold ascent can tear an alveolus. Pulmonary barotrauma — including arterial gas embolism — is one of the few diving injuries that can kill a healthy diver in shallow water. The simplest rule in scuba; the most lethal when broken. Breathe normally on ascent. Always.

One metre is enough. Lung overexpansion has been documented from breath-hold ascents in swimming-pool depths. Relative volume change matters, not absolute depth.

Expansion · 4× at 30 m

Lift bags & SMBs

A half-inflated SMB sent up from 30 m arrives at the surface four times its starting volume. Attached to the line, that becomes an involuntary express ascent. The same Boyle's law that signals the boat can tow you to it. Spool discipline. Dump valves. Never tether a lift bag to your harness.

Sources & method

Boyle's law. Robert Boyle, New Experiments Physico-Mechanicall, Touching the Spring of the Air, and its Effects, 1662. Public domain.
Diving application. PADI / DSAT, Diver Manual: Dive Theory, 2009 — pressure–volume relationship and consequences for divers. Primary teaching reference; original wording paraphrased throughout.
Pressure formula. P (bar) = 1 + depth (m) ÷ 10. Standard hydrostatic approximation matching modern dive-computer convention. Public-domain physics.
Visual scaling. V/V₀ = 1 / P_abs. The pufferfish's diameter is scaled linearly with V/V₀ (not the cube-root) — at 40 m it's 20 % of surface size. This exaggerates the visible shrinkage of a real gas; the trade-off is intentional, for legibility and drama. Real 3-D gas volume change is what the HUD's V × V₀ readout reports.
Lung overexpansion. US Navy Diving Manual, Rev 7, Vol. 1, Ch. 3 — pulmonary overinflation syndrome. Public domain (US Government work).
What this tool does not do. No deco modelling. No thermodynamics — temperature treated as constant. No real-gas correction. For dive planning use a verified computer or desktop deco software.