Dive Physics Made Simple: The Gas Laws, Explained
The gas laws are the backbone of the theory exam, and they feel like four unrelated equations until you see what each one actually governs underwater. Here they are in plain language, with the "so what" attached.
Boyle's law: pressure and volume
As pressure goes up, volume goes down, and vice versa. Descend to 10 metres and the pressure doubles to 2 ATA, so a flexible air space squeezes to half its surface volume. Come back up and it expands again.
Why it matters: it is the reason you never hold your breath on ascent (expanding lung gas can cause serious injury), the reason your BCD and ears behave the way they do, and the reason a tank of air lasts a fraction as long at depth.
Charles's and Gay-Lussac's laws: temperature
Heat a gas and it wants to expand or push harder; cool it and the opposite. Together these explain why a tank filled warm reads lower once it cools in cold water, and why a cylinder left in the sun reads higher.
Why it matters: a "short fill" is often just a warm fill that cooled down. Plan gas with temperature in mind.
Dalton's law: partial pressures
In a mix, each gas exerts pressure in proportion to how much of it is present. At depth the total pressure rises, so the partial pressure of every gas you breathe rises with it.
Why it matters: this is the engine behind oxygen toxicity limits (and your maximum operating depth) and behind nitrogen narcosis. PO2, PN2, narcosis, gas density: same idea, just depth turning the dial.
Henry's law: gas into tissue
The higher the partial pressure, the more gas dissolves into a liquid, including your tissues. Go deeper or stay longer and your tissues take on more nitrogen. Ascend and that nitrogen comes back out of solution, which must happen slowly.
Why it matters: this is the whole basis of decompression and why ascending too fast risks decompression sickness. Dalton sets the pressure; Henry decides how much soaks in.
How they connect
| Law | Governs | Shows up as |
|---|---|---|
| Boyle | Pressure vs volume | Air spaces, ascent rules, gas consumption |
| Charles / Gay-Lussac | Temperature effects | Tank pressure changes hot vs cold |
| Dalton | Partial pressures | Oxygen toxicity, narcosis, MOD |
| Henry | Gas dissolving in tissue | Nitrogen loading, decompression, DCS |
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Free cheat sheet Get the guide ($29)Frequently asked questions
Which gas law is most tested?
Boyle, Dalton, and Henry carry the most exam weight, because they connect directly to safety: ascent, oxygen limits, and decompression.
Do I need to memorize the equations?
You need to apply them, which is easier once you understand what each governs. Memorizing P1V1 = P2V2 is useless if you cannot see that it means "double the pressure, half the volume."
Why is temperature in kelvin?
The temperature laws only work on an absolute scale that starts at absolute zero. Convert Celsius to kelvin by adding 273 before you plug in.
Independent study aid for exam preparation only. Not affiliated with or endorsed by any certification agency, and not a substitute for certified training or proper dive planning.