Tools · Bubbles

A breath of air won't hold still.

Here's the one piece of physics every diver carries around. A pocket of air doesn't stay the same size as you move up and down: take it deeper and the pressure climbs, squeezing the gas into less space; let it rise and the pressure drops, so it swells back out. That's Boyle's law, and it's behind everything from why your ears need equalizing to why you never, ever hold your breath on the way up.

Drag the slider and watch the bubble — but keep an eye on the two numbers, because they don't move together.

Notice the gas bar drops off a cliff while the width barely budges. That's the whole trick: diameter is the cube root of volume, so a bubble can carry a quarter of its surface gas and still look two-thirds the size. The same maths runs in reverse on the way up — and it's fiercest in the last few metres, where a lungful taken at 10 m doubles by the surface. The same expanding gas is what makes a BCD a runaway on the way up. Slow ascents and "never hold your breath" aren't fussiness; they're this graph.

Here's the part that costs you bottom time. You always take a full breath — roughly the same lungful whether you're at the surface or at 30 m, because your body breathes by volume, not by counting molecules. But your regulator hands you that air at the surrounding pressure, so at 30 m (four atmospheres) the same lungful is packed with four times the gas — the same crowding that makes deep gas thick to breathe, all of it drawn from your tank. Same volume, four times the molecules — so you empty the tank about four times faster and your bottom time shrinks to roughly a quarter. That pressure reading above isn't only the bubble's squeeze factor; it's also your air-use multiplier.