Regarding fills. 21/35 is the "standard gas" for use down to 57 metres / 170'. If you're not diving to that depth and aren't GUE person (or other dogmatic agency), you're allowed to use other gas fills that better matches the depth you'll be diving to, i.e. far cheaper mixes given that helium is so expensive.
For example, diving to 40m. You could do this on 28%, but with narcosis. Add enough helium to bring up the
END*** (Equivalent Narcotic Depth) to whichever depth you're personally happy with. Lets just say you'll tolerate an END of 30m.
Thus you could use a mix of 28% oxygen (MOD = dose over mix = 1.4 / 0.28 = 5ata = 40m)
Add 20% helium the formula using ATA is
END = (1 − fraction of He) x Depth_in_ATA – 1
= (1 - 0.2) x 5 – 1
= 0.8 x 5 – 1 = 30m
So your "cheap" mix is simply 28/20 for an END of 30m when diving to 40m.
Again, for a 45m dive:
O2 = 1.4 / 5.5 = 25.5%, round to 25%
He at 27%
END = (1 - 0.27) x 5.5 – 1 = 30m
Thus dive with 25/27 mix at 45m.
As
most dives the you're diving above the max depth, you could reduce your Helium a tad (don't mess with the O2 MOD!). For 45m, many people consider the right gas is 25/25.
Or, to put it another way, you can dive with expensive mixes. Or you could do more diving! When you do more diving you'll be a lot less bothered about narcosis and expensive gasses; but that's for you to decide based upon your experience and the type of dive; if it's particularly complex (penetration, current, workload) then use more helium. If it's looking at the pretty fish, cheap may be better!
*** END; do or don't you count O2 as narcotic in END calculations. A topic for another thread.
