Tools · Henry's law

What dissolves, and what fizzes

Henry's law: the amount of a gas that dissolves into a liquid is proportional to the partial pressure of that gas above it.

A sealed soda is bottled under pressure, so a lot of carbon dioxide stays dissolved and invisible in the liquid. You are not so different. Under the pressure of depth, more nitrogen from every breath dissolves quietly into your blood and tissues — the deeper you are, the more they can hold.

The trouble isn't the dissolving; it's the un-dissolving. Open the soda slowly and the gas seeps out gently. Crack it fast — or shake it first — and it erupts into foam, because the dissolved gas can't leave smoothly when the pressure drops all at once. Ascend slowly and your nitrogen leaves the same gentle way, breathed out through your lungs. Ascend too fast and it comes out of solution as bubbles, right inside you. Set a depth, then choose how fast to come up.

Notice what the numbers say: at 30 metres your blood sits at four atmospheres, so it holds about four times the nitrogen it did at the surface. That extra load isn't dangerous while you're down there — it's dissolved and quiet, in equilibrium with the pressure around you. The danger lives entirely in the trip up, when the pressure that was keeping it dissolved is removed. Remove it gently and the gas has time to find its way out through your lungs; remove it fast and it has nowhere to go but out of solution, in place.

That single fact — gas in solution scales with pressure, and wants out when the pressure falls — is the whole reason for slow ascents, safety stops, and decompression schedules. None of those are arbitrary rules; they're just giving Henry's law the time it demands. This toy shows you the why. It is emphatically not a dive plan — the real limits come from a computer, a table, and training.

In practice you don't judge any of this by eye — your dive computer is tracking the dissolved load in the background and showing you a safe ascent rate, usually a bar that turns green or a warning that flashes when you climb too fast. Every training agency's first rule is the same: follow the computer. The old line — "never rise faster than your smallest bubbles" — is a poor guide on its own, because bubble speed depends on size and shape, and even the smallest ones tend to rise faster than the rate you actually want. Its one real use is as a last resort. If the computer has failed and you've nothing else to measure with, the smallest bubbles become a ceiling you simply never overtake. Think of driving with a dead speedometer behind a police car: you can't read your exact speed, but you know that whatever happens, you don't pass the cruiser — and it may be going a little fast itself, which is precisely why it's a limit and not a target. The bubble is that police car.