07 chapters — Life-support stack
A regulator without a cylinder is a paperweight; a BCD with no suit lift to replace has nothing to do. This page puts all seven side by side: the one number to remember, what changes as you descend, what each one depends on, and one way it fails.
Rows are the seven chapters in reading order. Every chapter name opens its page; every dependency opens the chapter it points to.
The same physics keeps coming back because the kit is one system. Boyle's law alone is on three chapters; so is Archimedes. Every note links to the chapter it was written for.
Gas volume is inversely proportional to pressure. Halve the pressure and the air in a bladder, a lung or a mask doubles.
On 3 chapters
At 10 m the ambient pressure is 2 bar — double the surface — so each lungful pulls twice as many molecules from the tank. Your gas lasts half as long at 10 m and a third as long at 20 m. Pressure, not fitness, is why gas planning exists.
Air in the bladder grows by half again from 20 m to 10 m — then doubles over the last 10 m to the surface. The shallows are where runaway ascents happen; experienced divers start dumping before they even begin to rise.
At 30 m a 250 ml mask has swallowed about 190 ml of nasal air to stay equalised; a 70 ml freedive mask about 50 ml — a quarter of the cost, out of a single breath-hold. Scuba divers can afford the volume for comfort; a freediver has one breath for the whole dive.
A body in water is pushed up by the weight of the water it displaces: a litre of seawater is 1.025 kg of lift.
On 3 chapters
A tank is the one piece of gear that changes weight during the dive. An AL80 swings ~2.7 kg towards positive as you breathe it down — so your weighting has to hold you at the safety stop with a near-empty cylinder, not just at the start.
Fresh water weighs 1.00 kg/L, seawater 1.025. So a diver who is 2 kg negative needs about 2 litres of air in the wing to go neutral — you can count your buoyancy in litres.
You are weighted for the two ends of the dive: enough to sink a full wetsuit at the surface, and enough to hold a 3-minute stop at 5 m with a near-empty tank. Everything in between, the BCD makes up. A 7 mm suit loses roughly half its 5 kg of lift by 30 m; an AL80 gains 2.7 kg of lift as you breathe it down. The bladder absorbs both.
Neoprene is mostly gas, so it floats: roughly 0.7 kg of lift per millimetre of thickness for a full suit — about 5 kg for 7 mm, 2 kg for 3 mm — and it compresses away at depth. You carry lead to sink it at the surface, then add BCD air at depth to replace the lift you lost. Thermal protection is paid for in ballast.
The pressure of a gas mix is the sum of the partial pressures of its parts.
On 1 chapter
The pressure of a gas mix is the sum of the partial pressures of its parts. Air at 30 m (4 bar) delivers 0.84 bar of oxygen and 3.16 bar of nitrogen. Nitrox swaps some nitrogen for oxygen — EAN32 is 32% O2 — so you load less nitrogen and get a longer no-stop time. The trade: oxygen turns toxic above about 1.4 bar, so every mix has a maximum operating depth, MOD = (1.4 ÷ FO2 − 1) × 10 m. For EAN32 that is 33 m; for EAN36, 28 m; for air, 56 m.
Gas dissolves in proportion to its partial pressure, and each tissue loads and unloads at its own speed.
On 1 chapter
The amount of gas that dissolves in a liquid is proportional to the partial pressure of that gas above it. At 30 m you breathe nitrogen at 3.16 bar — four times the surface — and your tissues load towards that, each at its own speed. That is the whole reason a dive has a time limit. Old tables assumed the entire dive happened at maximum depth; a computer integrates the real profile, crediting every minute spent shallow — often doubling usable bottom time on a typical reef dive.
Each compartment has a maximum tolerated nitrogen pressure — its M-value — that grows with ambient pressure: M = P_amb ÷ b + a. Ascend until a tissue exceeds it and you have a ceiling you must not cross. Gradient factors dial the model back: at GF 85 you use only 85% of the gap between ambient and the M-value. This site’s simulator runs four compartments (5, 8, 12.5 and 38.3 min) with the real ZHL-16C coefficients at GF 85 — and it cannot represent repetitive diving, because it starts every dive with clean tissues.
Bühlmann, RGBM, VPM — different mathematical philosophies about how bubbles form. Conservative settings pad the margins (less bottom time); liberal ones trust the model more. Same diver, same dive, different NDLs.
Light bends crossing from water (n = 1.33) into air (n = 1.00): things look a third bigger and a quarter closer.
On 1 chapter
n1 sin θ1 = n2 sin θ2. Light leaving water (n = 1.33) for air (n = 1.00) bends away from the normal, so every ray reaching your eye through a flat lens arrives steeper than it left the object. Your brain places the object at three-quarters of its true distance and sees it four-thirds as large. The same bending narrows your view: a lens that spans 100° in air only takes in about 70° of water.
Lenses closer to your eyes mean a wider effective field of view and less internal volume — the same optics as holding binoculars tight to your face. Low-volume masks genuinely see more.
Drag rises with the square of speed; thrust is the mass of water you push back each second.
On 1 chapter
Drag F = ½ ρ C_d A v²: it rises with the square of speed, and the power your legs must deliver — force times speed — with the cube. Doubling your pace quadruples the drag and costs eight times the effort, and your breathing rate follows. Fin efficiency is really about cruising slowly with long, relaxed kicks, not sprinting.
Thrust equals the mass of water you push back each second times how fast you push it. A big blade moving a lot of water slowly wastes less energy than a small blade whipping a little water fast — the same reason a large, slow propeller beats a small, fast one.
Split blades shed water off the gap at lower angles of attack, so the drag penalty of a fast flutter kick falls. Measured studies find lower oxygen cost at cruise speed and lower peak thrust — efficiency, not free thrust. They do not “generate thrust on the upstroke”, whatever the box says.
Water conducts heat about 25 times better than air. Conduction and convection do the damage; radiation is negligible.
On 1 chapter
Underwater, heat leaves you by conduction into the water and by convection when that water moves. Radiation is negligible — so a shiny “heat-reflecting” lining cannot do what the label implies. The physics that works is thickness, dry gas, and stopping water from moving.
A loose suit flushes warm water out and cold water in with every kick — you become your own heat exchanger. Snug (not constricting) fit is worth more than an extra millimetre of neoprene.
Cylinder pressure comes down in two steps: to 9.5 bar over ambient, then to exactly the pressure around you.
On 1 chapter
Dropping 207 bar to a breath in one step would make the valve’s job impossible: the effort to crack a seat holding back full cylinder pressure would change as the tank drained, and the expansion cooling would ice it. Splitting the drop keeps delivery smooth and near-constant from full to nearly empty.
Gas cools as it expands, chilling the metal around it. In water below ~10 °C, moisture in the gas or the water itself can freeze the first stage open — a freeflow. Cold-water regs seal the spring chamber from the water and use metal fins as heat sinks to pull warmth back in.