Posted by Jason on September 23, 2001 at 22:45:47:
In Reply to: On-line nitrox calculator posted by AADIVER on September 23, 2001 at 21:58:34:
How do you figure you need to add 430? (Not that the 7psi makes a lick of difference in the real world where precision is much more coarse)
The math is (.40x1250) + (1.00xFILL) + (.21x(1750-FILL) = (.40x3000)
solving for FILL gets you 421psi of O2 added before the air topoff.
If you use 20.9% as the P02 in air, the answer is 422.5. I suspect that is their constant, as it is closest to true value.
Plugging in your 430 would suggest a P02 of air to be 20.45%.