Haldane's Decompression Tables

The whole mechanism is visible in a bottle of fizzy water, and the analogy is exact rather than loose.

1. Take a sealed carbonated bottle and look at it: clear liquid, no bubbles, gas held in solution by pressure.
2. Open it slowly and watch bubbles form throughout the liquid.
3. Reseal, shake, and open fast — note how much more violently it degasses.
4. Now open a second bottle slowly, in stages, closing the cap between each release.

Gas stays dissolved while the pressure holds it there, and comes out of solution when the pressure drops — and how fast it comes out depends on how fast you drop it. The staged release produces far less foaming than the sudden one.

That last observation IS decompression practice. The diver is the bottle, nitrogen is the carbonation, and ascending is opening the cap.

The treatment for the bends is the disease reversed, and it is worth seeing why.

1. Take a clear rigid pressure vessel with a sealed carbonated liquid inside that has begun to bubble.
2. Raise the external pressure with a compressor and watch the bubbles.
3. Hold the pressure, then release it slowly in stages.

Raising the pressure makes the bubbles shrink and redissolve; releasing it slowly lets the gas leave without re-forming them. That is precisely what a recompression chamber does to a bent diver — push them back down, dissolve the bubbles, then bring them up on a schedule slow enough that the gas leaves through the lungs instead of forming bubbles again.

Oxygen is given during treatment because it does two jobs at once: it displaces nitrogen in the breathing mix so the gradient out of the tissue is steeper, and it perfuses tissue that bubbles have starved.

Decompression is not the only thing depth does to a diver, and the other two arrive without warning.

1. Compute the partial pressure of nitrogen at 30, 40 and 60 m.
2. Compute the partial pressure of oxygen at the same depths on air.
3. Mark where each crosses its accepted limit.

Nitrogen narcosis begins around 30 m and worsens steadily — it is an anaesthetic effect, and its danger is that impaired judgement cannot assess itself. Divers describe a false sense of wellbeing while doing something foolish.

Oxygen becomes toxic above roughly 1.4 bar partial pressure, which on air is about 57 m, and the first symptom can be a convulsion. A seizure underwater with a mouthpiece is usually fatal regardless of how much gas remains.

Both are solved by changing the mix rather than the schedule: helium replaces nitrogen to remove narcosis, and the oxygen fraction is REDUCED for deep work so its partial pressure stays tolerable. A deep trimix is deliberately hypoxic at the surface — it would not sustain you on the boat, and only becomes breathable at depth.