Got it. Now if air is 21% Nitrox, then there is no problem. 
Alternating Nitrox and Air dives ok on ProPlus 3?
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tarponchik
Contributor
I've asked this already and I ask again: why dive on Nirox from a liveaboard? We tried this once, expecting to have more freedom for money but everyone was diving in a group; air or Nitrox, it's the same dive profile, same 3 dives a day, and nobody got DCS. Same on other liveaboards we went on except Nekton (R.I.P.) where the guys would just say, "OK, the pool is open!" and you dive as you please, groups, buddy or solo, they didn't care. But there was no Nitrox on Nekton. So, we dive liveaboards on air, cause what's the point?
Good and very relevant question:
the theory is that to dive Nitrox with air profiles, which is what you have been doing since both type of gazes were in the same group, you are adding a safety factor in your dives. Therefore minimising the chance for a DCS hit, especially after many consecutive dives and days. The other factor/theory is that you also get the positive side effect of less fatigue after the dives.
The lesson that I have taken from it: I dive nitrox when the LOB offers free nitrox- it happens for LOB in the Red Sea - and I dive air when I have to pay for it.
This would be true if you are doing 3 shallower dives per day on a liveaboard. I have never been on a liveabloard where we did fewer than 4 dives a day. On a recent one, the dives were all pretty shallow, so it did not matter. On the other hand, on most of my liveaboard trips, the dives were much deeper, and it made a huge difference. For example, I just pulled out a very old log book for my trip to Truck Lagoon, and on the day I looked at, I did 4 dives that averaged almost exactly 100 feet average depth. When I went to the Galapagos, one couple on the boat were the only ones who did not use nitrox. We dived as groups with a specific DM, and they were in my group. On every dive, they went to the surface early while the rest of us continued our dives. The farther we got into each dive day, the less time they spent with us and the more time they spent waiting in the panga for us to finish our dives. The rest of us talked about it, and we thought they were out of their minds to spend all that money for the trip and then skimp on the few bucks it would have cost for them to have probably at least 30% more dive time.
tarponchik
Contributor
This will be true if we assume that divers on air were following unsafe, risky profiles. Hardly possible, right? But about your "less fatigue" point, this is probably an urban legend.
Why would that be an assumption?
BTW, I just ran a dive sequence similar to the ones I did in Truk Lagoon through the PADI air tables and the PADI EANx 32 tables, giving each dive the same typical surface interval and taking each dive to the same level of risk on those tables. At the end of the day, the air diver would have finished with 56 minutes of bottom time, and the nitrox diver would have finished with 90 minutes of bottom time. For some people, that extra bottom time is worth the investment of a few dollars.
tarponchik
Contributor
Our last one was in the Maldives. The reef dives were shallow, so no difference here. And on the deep channel dives it could make a difference but everyone had to stay with the DM cause otherwise the strong current will drag you God knows where. Half group on air, half on Nitrox, all diving together.
Belize Agressor offered 4 dives per day except the day we did the Blue Hole but the dives were shallow. I even did 5 dives once but their night dive was 45 ft max. Same thing, that is, diving as a group, hald on air, half on Nitrox, no difference.
Your description of the Galapagos trip makes lots of sense, though. I guess, one has to do some studies before asking for Nitrox on the specific trip.
Belize Agressor offered 4 dives per day except the day we did the Blue Hole but the dives were shallow. I even did 5 dives once but their night dive was 45 ft max. Same thing, that is, diving as a group, hald on air, half on Nitrox, no difference.
Your description of the Galapagos trip makes lots of sense, though. I guess, one has to do some studies before asking for Nitrox on the specific trip.
tarponchik
Contributor
Because they dive the same sites for years and nobody gets DCS.
I still have no idea why you are talking about assuming someone is diving risky profiles on air. Where was anything like that stated?
tarponchik
Contributor
@Freewillow said "the theory is that to dive Nitrox with air profiles, which is what you have been doing since both type of gazes were in the same group, you are adding a safety factor in your dives. Therefore minimising the chance for a DCS hit, especially after many consecutive dives and days." I was responding to this.
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