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Dissolved Gases and Saturation: The Physics of Decompression
What it means for a tissue to be saturated, and why the ascent has to be slow
Throughout the entire bottom phase, you feel absolutely nothing. And yet that's where your ascent gets decided: every minute at depth changes the amount of gas dissolved in your tissues, silently, with no warning sign at all.
In this article you'll see what it means for a gas to dissolve into a tissue, how fast that happens, what saturation and supersaturation are, and why the ascent rate and the safety stop both come directly from this physics.
It's the bridge between physics and decompression. Understanding it turns your dive computer's limits into something you actually grasp, instead of an arbitrary number you just have to obey.
Dissolution and Diffusion
When a gas is in contact with a liquid under pressure, part of that gas ends up dissolved in the liquid. It doesn't form bubbles or occupy space of its own: its molecules end up dispersed among the liquid's own molecules, the same way sugar dissolves in water.
The amount that dissolves is determined by Henry's Law, which you saw in the previous article: it's proportional to the gas's partial pressure. More pressure, more dissolved gas.
The rate at which this happens depends on another phenomenon: diffusion, the spontaneous movement of molecules from where there's more concentration toward where there's less. Diffusion needs no energy and no pump; it happens on its own as long as a difference exists.
Metabolic gases, oxygen and carbon dioxide, are constantly consumed and produced, so their balance stays relatively stable. Nitrogen isn't: the body doesn't use it for anything. It just goes in, accumulates, and comes back out. That's why it's the main character of decompression.
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