Intermediate 9 minutes
The gas laws applied to diving
Boyle, Charles, Gay-Lussac, Dalton, and Henry, explained by what they do underwater
Five laws formulated between the seventeenth and nineteenth centuries, by people who never dove with a tank, explain practically everything that happens to a diver. You don't need to memorize them as formulas: you just need to understand what each one does.
In this article you'll see the five gas laws relevant to diving, what relationship each one describes and, above all, what practical consequence it has underwater.
Each of these laws sits behind a specific safety rule. Understanding them turns those rules from something you have to memorize into something you can simply work out.
Boyle: pressure and volume
Boyle's Law says that, at constant temperature, the volume of a gas is inversely proportional to pressure: if pressure doubles, volume is halved, and vice versa.
Applied to diving, using the figures from the previous article: a volume of gas that takes up one liter at the surface takes up half a liter at 10 meters (2 atmospheres), and a quarter of a liter at 30 meters (4 atmospheres). On the way up, the process reverses and that gas regains its original volume.
This is the law most present in everyday practice. It explains why you need to equalize your ears, sinuses, and mask on the way down; why your BCD needs more air at depth to maintain the same buoyancy; why the bubbles you exhale grow visibly as they rise; and why you never hold your breath during ascent.
It also explains something you've already seen from another angle: because the relationship is proportional rather than linear in terms of meters, the biggest volume changes happen near the surface.
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