To work on it 'longer-hand' look at it this way, you need to isolate the
PN2 in you final mix as this comes from your 'air' source and its partial pressure never changes though the O2 partial pressure will 'cos you are adding pure O2.
So, a 27% mix has 73%N2 at 210 bar. Therefore 210 x .73 = 153.3 bar N2 will be in final mix. This N2 brings with it 21% O2 (in air) so, divide the PN2 by 79 (the % N2 in air) and x by 21 (% O2 in air) and you have 40.7 bar O2 that comes with the air. Add these together 40.7 O2 + 153.3 N2 = 194 bar
Air. Therefore the shortfall from 210 bar is 16, which is the pressure of 'pure' O2 that you must add. Voila - 210 bar of 27%!!!
Add it all up;
153.3 N2
40.7 O2 from air
16.0 Pure O2
210.0 bar Nitrox 27%
Berko
