Posted by CalAbDiver on August 18, 2001 at 17:46:11:
I am surprised that I have not heard more about the use of 50/50 Heli-Air for diving deep. Here is the data for its use [10.5%O2, 50%He, 39.5%N2]:
depth ATA ppO2 ppN2 EAD N2
0 1 0.105 0.395 --
33fsw 2 0.21 0.79 -0-
66fsw 3 0.32 1.19 17fsw
99 4 0.42 1.58 33fsw
132 5 0.53 1.98 50
165 6 0.63 2.37 66
198 7 0.73 2.77 83
231 8 0.84 3.16 99
264 9 0.95 3.56 116
297fsw 10 1.05 3.95 132fsw
For a travel mix and a deco mix, any EAN mixture could be used. Obviously, the leaner the O2 the deeper it could be breathed. EAN45 with a ppO2 of 1.6 at 84 fsw would be perfect as both, because it could be used as a travel mix to 35 fsw, and as a deco mix from 80 fsw back to the surface. Then the first deep stop could be one-half of the maximum depth for 2 minutes using the Heli-Air mix still, and the next stop would be one-half of that for 4 minutes using the EAN45, etc. until the 50 ft / 40 ft / 30 ft etc Navy tables for EAD was reached and followed back to the surface.
Even a dive to 300 fsw on 50/50 Heli-Air would be no more narcotic than and EAD dive to 132 fsw, and the ppO2 would be no worse than an EAD of 132 also, significantly less than the NOAA limit of 1.6 ATA.
Granted that 50% O2 or 100% O2 would work faster as a deco gas since it has a steeper N2 gradient, but that would necessitate carrying more slung bottles rather than just one with EAN 45. The only issue with EAN45 would be the size of your bottle, 40 cu ft, 65 cu ft, 72 cu ft, or 80 cu ft.
I am not sure I see any advantage in mixing exotic proportions of O2, He, and N2 for TriMix when 50/50 Heli-Air works just as good if not better than anything else for deep diving.
My definition of deep diving is anything deeper than 132 fsw on EAN32. Beyond 132 fsw the N2 effect of air is intolerable, in my opinion, and beyond 132 fsw the ppO2 of EAN32 is too dangerous according to NOAA rules, even though it is conceivably possible to survive a ppO2 of 2.0 for up to 5 minutes for a rescue scenario, according to the U.S. Navy diving manuals (past & present).