Dive Physics Refresher

As you go down, the water squeezes every pocket of air you're carrying — the air spaces behind your eardrums, the bubble in your BCD, even the air in your lungs. That squeeze is Boyle's Law: the same amount of gas simply takes up less room as the pressure around it goes up. Go back up, and the reverse happens — that same gas wants to expand again.

That's why you equalize your ears on the way down: the air space in your middle ear shrinks first, and if you don't add air to match, the pressure difference across your eardrum starts to hurt. Equalize early and often, before it hurts — not after.

It's also why you never hold your breath on ascent: the air in your lungs expands as pressure drops, and if it has nowhere to go, that's a real lung injury risk. Breathing out continuously and normally on the way up lets that expanding air escape safely.

And it's why your BCD needs small top-ups and vents throughout the dive, not just at the start: the same bubble of air in your BCD is smaller and less buoyant deep, and bigger and more buoyant shallow, even though you haven't added or dumped a thing. Buoyancy is a moving target as depth changes — that's normal, and it's why good buoyancy control is a skill, not a one-time setting.

The gas in your tank has a fixed percentage of oxygen and nitrogen — that never changes once it's filled. But what your body actually experiences from that mix does change as you descend, because pressure effectively concentrates it. That's Dalton's Law: total pressure is shared between the gases in a mix, so each gas's partial pressure rises with depth even though the percentages on the tank label stay the same.

This is exactly why nitrox has a maximum depth. Oxygen is essential, but it becomes toxic once its partial pressure gets high enough — and that risk is driven by depth, not by how long you've been down. Nitrox's extra oxygen is a genuine benefit (less nitrogen exposure), but it also means oxygen hits that risky threshold at a shallower depth than plain air does. The Maximum Operating Depth calculated below isn't a formality — it's the real depth where that trade-off flips against you.

Think of the nitrogen in your body the way you'd think of the fizz in a sealed soda can: under pressure, more gas dissolves into the liquid than would at normal pressure. The deeper and longer you dive, the more nitrogen quietly dissolves into your blood and tissues. That's completely normal and happens on every single dive — it's not dangerous by itself.

What matters is how fast the pressure comes off. Open a soda slowly and the fizz stays mostly dissolved. Shake it and crack it open fast, and it foams everywhere. Your body behaves the same way: ascend slowly, and dissolved nitrogen has time to leave gradually through normal breathing — no bubbles. Ascend too fast, and that gas can come out of solution as actual bubbles in your blood and tissues, which is what decompression sickness ( "the bends") is.

That's the whole reason slow ascents and safety stops exist: a controlled ascent rate plus a pause around 5 m / 15 ft for a few minutes gives your body extra time to off-gas gently at low pressure, well before you're back to full surface pressure. It's cheap, unglamorous insurance on a process that's easy to get right if you just don't rush it.