Scuba Diving Physics, Explained Simply
Dive physics sounds intimidating, but it comes down to a handful of simple ideas about pressure and gases. Understand these and a whole section of your exam takes care of itself.
Pressure and depth: the one idea everything builds on
Every 10 metres (about 33 feet) you descend in seawater adds one more atmosphere of pressure — the same weight the whole atmosphere presses down with at the surface. So at the surface you're under 1 atmosphere; at 10 m you're under 2; at 20 m, 3; at 30 m, 4. This is 'absolute' pressure, and it's the number that matters for almost every physics question you'll see.
Two things follow. The biggest proportional pressure changes happen near the surface — going from 10 m up to the surface halves the pressure — which is exactly why the last few metres of an ascent demand the most care.
Boyle's Law — why you never hold your breath
Boyle's Law says that when temperature stays constant, the volume of a gas changes inversely with pressure: double the pressure and you halve the volume; halve the pressure and the volume doubles. Underwater, that governs every air space — your lungs, mask, ears, BCD and wetsuit.
On the way down, pressure rises and air spaces shrink, which is why you equalise your ears and mask. On the way up, pressure drops and that air expands. Hence the single most important rule in diving: never hold your breath, because expanding air with nowhere to go can injure your lungs. Breathe continuously and ascend slowly.
Buoyancy and Archimedes' Principle
Archimedes' Principle says an object is pushed up by a force equal to the weight of the water it displaces. Displace more water than you weigh and you float (positive buoyancy); less and you sink (negative); exactly the same and you hover (neutral) — the goal of good diving.
Boyle's Law sneaks back in here: as you descend, the tiny air bubbles in your wetsuit compress, so it displaces less water and you become less buoyant. That's why you add a little air to your BCD as you go deeper and release it as you rise — keeping your total displacement steady.
Gases under pressure: Dalton's and Henry's Laws
Air is a mixture — roughly 21% oxygen and 79% nitrogen. Dalton's Law says each gas contributes its own share of the total pressure (its 'partial pressure'), and those shares rise with depth. As the partial pressure of nitrogen climbs it can cause nitrogen narcosis — a temporary, reversible 'intoxicated' feeling at depth; as oxygen's partial pressure climbs it can eventually become toxic, which is why depth limits exist.
Henry's Law explains decompression: the more pressure a gas is under, the more of it dissolves into a liquid — here, your tissues absorbing nitrogen. Ascend too fast and that nitrogen comes out of solution as bubbles, causing decompression sickness. Slow ascents and safety stops let it leave gently, the way a fizzy drink opened slowly doesn't foam over.
Temperature and your tank
Gay-Lussac's Law links temperature and pressure in a fixed volume like a scuba cylinder: heat the tank and the pressure reading rises; cool it and it falls. A cylinder filled and left in the sun can read higher than expected, and the same tank reads lower once it's in cold water — handy to know when a 'full' tank seems to lose pressure before you've even dived.
How this shows up on your exam
Physics is one of the most predictable parts of any diving exam, because the rules never change. Expect questions on pressure at a given depth, how a balloon or lift bag changes size as it rises, why you must never hold your breath, and what causes narcosis and decompression sickness.
- Work in absolute pressure (add the 1 atmosphere at the surface) unless told otherwise.
- For volume questions, remember volume and pressure move in opposite directions.
- When a question mentions ascending, think 'gas expands' and 'go slow'.
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Frequently asked questions
Why can't you hold your breath while scuba diving?
Because of Boyle's Law: as you ascend, the surrounding pressure drops and the air in your lungs expands. Hold your breath and that expanding air has nowhere to go, which can overexpand and injure your lungs. Always breathe continuously and ascend slowly.
How much does pressure increase as you go deeper?
In seawater, pressure increases by about one atmosphere for every 10 metres, or roughly every 33 feet. At the surface you're under 1 atmosphere of absolute pressure; at 30 metres you're under 4.
What causes nitrogen narcosis?
As you descend, the partial pressure of nitrogen in the air you breathe rises (Dalton's Law). At depth that has an anaesthetic-like effect, causing a temporary, reversible impairment often described as feeling intoxicated. It eases as you ascend to shallower water.
What causes decompression sickness?
Under pressure, extra nitrogen dissolves into your body's tissues (Henry's Law). Ascend too quickly and it comes out of solution as bubbles, which can cause decompression sickness. Slow ascents and safety stops let the nitrogen leave gradually and safely.
Do I need to be good at maths to pass dive physics?
No. Most exam physics is conceptual — understanding pressure, buoyancy and how gases behave. The little arithmetic that appears is simple once you know to add the atmosphere at the surface. Practising a handful of questions is the fastest way to get comfortable.