Scuba diving theory, explained in plain words

Your tank empties faster at 30 m (99 ft), and your computer gives you less no-stop time on the second dive. Each page below starts from a moment like these and explains the physics or physiology behind it.

• Boyle’s law in diving: why every air space changes with depthBoyle’s law for divers: why your ears, mask and BCD change with depth, the formula, worked examples in meters and feet, and the mistakes to avoid.
• Charles’s law in diving: why your full tank reads lower in the waterCharles’s law for divers: why a hot fill reads lower once it cools, why tanks don’t belong in hot cars, the formula in kelvin, and examples in bar and psi.
• Dalton’s law in diving: partial pressures of oxygen and nitrogenDalton’s law for divers: what partial pressure is, why oxygen and nitrogen effects grow with depth, and how to work out your own nitrox MOD, step by step.
• Henry’s law in diving: nitrogen in, nitrogen outHenry’s law for divers: why your tissues absorb nitrogen at depth, why it takes time and how a slow ascent lets it out, with examples in meters and feet.
• Buoyancy in diving: why you sink, float or hoverBuoyancy for divers: Archimedes’ principle, how much lead to carry in salt or fresh water, why buoyancy changes with depth, and examples in kg and lb.
• SAC rate and RMV: calculate your air consumption and plan your gasHow to calculate your SAC rate and RMV, why depth multiplies your air use and how long your tank will last, with examples in bar and liters, psi and cu ft.
• Barotrauma and equalizing: ears, sinuses, mask and lungsHow to equalize your ears, sinuses and mask, what to do when an ear won’t clear, and why you must never hold your breath on scuba. Sourced from DAN.
• Decompression sickness: signs, prevention and first aidWhat decompression sickness is, why it happens, the signs to watch for after a dive, how to prevent it with your computer, and the first aid divers learn.
• Dive computers and NDL: what the numbers on your wrist meanWhat your dive computer’s no-decompression limit means, why it shrinks with depth, how fast to ascend, the safety stop, and what to do if you overstay.
• Nitrogen narcosis: when deep air feels like a drinkNitrogen narcosis explained for scuba divers: why air at depth dulls your thinking, when most divers notice it, the warning signs and how to handle it.
• Oxygen toxicity and nitrox: ppO2 limits and MODNitrox cuts nitrogen but adds an oxygen limit. The ppO2 limits of 1.4 and 1.6, the warning signs of oxygen toxicity, and the MOD worked in meters and feet.
• Underwater navigation: compass, patterns and natural cluesHow to navigate underwater: hold and set a dive compass, find your reciprocal heading, measure distance in kick cycles and read natural clues on the site.

Studying for a course? Each course page shows which topics your level covers. Check yourself with the practice questions, keep the dive theory formulas cheat sheet at hand, and look up any term in the glossary.