Advanced 9 minutes
Gas and depth: the window in which each mix is breathable
One equation, four constraints, and why every mix has its own range
A breathable mix isn't a good mix: it's a good mix for a specific depth. The same tank that saves your life at sixty meters knocks you unconscious at the surface, and the one you breathe comfortably up top gives you a seizure down below.
In this article you'll bring together everything this category has been building toward into a single framework: the partial pressure equation, the toxicity constraints from above and below, narcosis, and density, and how these four define a depth window for each mix.
This is the article that turns a list of mixes into a working criterion. With this framework you can look at any gas combination and work out where it's useful and where it isn't, instead of memorizing tables.
One single equation
Everything that follows comes from a single relationship: the partial pressure of a gas in a mix equals its fraction multiplied by the absolute pressure at which you breathe it. This is Dalton's Law applied to diving, already introduced in the gas laws article.
The consequence is that your body doesn't respond to the gas percentage but to its partial pressure. 21% oxygen is 0.21 bar at the surface and 1.05 bar at forty meters: the same label, five times the dose.
The reverse reading is what technical divers use: if you know what partial pressure you want and at what depth you're going, the required gas fraction is determined. Designing a mix means solving that equation for each component.
From here, it's all about setting limits on those partial pressures. Each limit cuts off a depth range, and what's left between all of them is the window in which that mix can be breathed.
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