Mike,

Knowing that you write from experience (mine is limited to a small amount of snorkling, most recently about 30 years ago), so there are a couple of things that I wouldn't mind education on:

>So at 10meters you use twice as much air as you did at the surface, at twenty meters you use twice as much as you did at 10 meters.

At surface, total pressure is 1atm, at 10m 2atm, at 20m 3atm. Thus, assuming Boyle's law applies, at 20m why would you not use just 3x as much as you did at the surface, and need to go to 30m (4atm) to use twice as much as you do at 10m?

Also, and this probably betrays the distinction between theory and practice, at, say, 10m, where a lung-full contains twice as much air as it does at the surface, it also contains twice as much oxygen. Why does it then not last longer than a lung-full at the surface? (SOmething to do with breathing reflex being triggered by CO2 concentration in the blood?)

--

Stephen