The question as stated is moronic.
First off, it's idiotic to give 3 digits of precision for cylinder volume and pressure, and
not give 3 digits of precision for air pressure on the surface (not gonna be 1.00 bar) and water density (not gonna be 1.00 kg/L). That's a science 101 fail. Especially if you're then going to nitpick for 4 bar
which is the same order of error as said water density.
Second, anyway, if you hope to get more than 2 digits of precision, you'd better
start by ditching the ideal gas laws and taking into account
the non-linearity of gas laws. See section 4 "Cubic equations of state" and following. Of course at this point, you need temperature, too.
Third, you can get all the gas. Just remove the first stage, fill the cylinder with water. Not realistic? Then first you need to subtract the 10-ish bar until your first stage stops delivering (could be 8, could be 12, Poseidons are 15, I seem to remember, please correct?).
Then, finally, at this point you can subtract 4 bar.
Me? I'd go 210*12/4=630. That's 210 as in 230 minus 20, 30 if the water's cold. Easy enough that I can do it on the boat. Good enough for diving. Probably more accurate than the official "correct" answer. Accommodates a bit of temperature related pressure drop. And safer to boot.
And if someone disagrees, he'd better be good at solving 3rd order polynomials on the spot...
But then again I'm not taking Mon1.
If your answers is the "correct" one, mark, I'm really confused. It's neither good physics nor good diving.
Cheers,
Matthieu
