To Ken's point: C. For those who were concerned about a redundant air supply - I have an octo but no Spare Air or pony bottle - please explain to me what danger that posed and please remember I had at least 1,000psi coming out of the bow. (I dive with a Atomic T2 so don't go for "some regs won't breathe good once the air drops below 500psi.")
I guess discussing the broader issue of redundant gas supply in the context of the OP's dive (non-overhead) is not really a hijack here since this is such a free ranging discussion with so many imbedded issues. I get this has the potential to be a "dead horse tastes great less filling" circular firing squad and there will be strong feelings on the issue. Some even well though out. What follows is only my opinion but comes from lots of team dives in true overhead (cave, wreck, deco) with a minimalist (streamlined) attitude toward equipment that stipulates one should have the tools necessary for the dive (including contingencies) and no more. While solo diving (no buddy to carry redundant gas) is a special case of the non-overhead concept, even many guided group dives (no buddy teams/common training/commnon knowledge) are little more than a cluster of de facto solo divers in proximity. Guided dives potentially being spoke and hub (as opposed to team) dives where the clients are independent spokes all communicating though the guide/hub as opposed to aware/thoughful/communicating with each other.
Redundant gas/equipment is "good" because: 1) you have more gas. 2) you have a two regulators. 3) you have a bottle you can hand off to an OOG diver if needed.
Redundant gas/equipment is "bad" if it requires unnecessary gear and complexity, is a crutch for lousy gas management/situational awareness or encourages behavior beyond the divers training/comfort/abilities.
Opinions will vary as to cost benefit/what is "necessary."
Thoughts:
1. Is more gas always better. That depends on why you're carrying more gas and how. Poor gas management skill is not a good reason to carry redundant gas. You either properly manage (including planning) gas or you don't. Acquiring proper gas management skills and situational awareness is the solutiuon. Adding equipment to make up for poor skills (as popular as that notion is on SB, and among equipment manufacturers/retailers) is using a crutch and missing the forest for the trees. If you need more gas, bring a bigger tank or improve technique and conditioning to lower consumption, or both. If you don't know how to plan (properly calculate reserves/turn pressures) your gas for a dive (like a lot of recreational divers) get more/better training. Loading up on extra gear may be a retailer's/wanna be tech divers wet dream but more gear adds complexity/drag and is a bad idea if it gets in the way of streamlining/efficiency/really learning how to dive.
2. Is reg failure a risk. A reg can fail, just as can hoses and valve/DIN o rings. But are those failure modes truly catastrophic in the context of a recreational (non-overhead) dive? That is, do you lose gas/breathing ability so rapidly (instantaneously) that you can't survive the event without a redundant supply? A well maintained first stage doesn't just "seize up" and stop delivering gas. A downstream second fails open, not closed. Regs don't just spontaneously erupt into a massive freeflow if maintained and properly selected for the environment (temp). A high speed leak (blown HP seat, LP hose, o ring, free flow) is an emergency, but not an instantaneous cessation of gas. A blow out is unmistakably loud and you'll know it when you have one. A leak on the LP (high volume) side isn't going to deplete gas supply in the time it takes to do a CESA. A hose is most likely to fail upon presurization, not at the end of a dive. While equipment maintenance is key to preventing regulator/hose/o ring failure, failures can still happen (particularly with the proliferation of crappy hoses/parts from you-know-where). But gas depletion doesn't happen in seconds-more like minutes.
3. Handing off the bottle. Here again, handing off can be a good thing if donor and recipient have trained on it and keep their cool. But, in the event of an OOG diver in a non-overhead situation handing off may be worse (no better) than the donor sticking with the OOG diver during a gas sharing CESA.
As such, I'm hard pressed to see where reduandnt gas is "necessary" to safely execute a recreational/non-overhead dive provided sound diving practices (gas management/equipment maintenance) are observed.
The obvious counter to that is "what can it hurt?" That's a good question. It depends. Provided the redundant gas is truly contingent and carried in a streamlined and donatable manner sufficient to actually be useful (i.e. HEED/fixed/back mounted pony bottles need not apply), and the diver is not increasingly task loaded, is there a downside to it? Probably not, except a modest increase in drag/consumption. But there are a lot of conditions attached to that conclusion including being sufficiently practiced on the gear and trained/experienced for the plan that it doesn't do more harm than good. One aspect of potential harm is a false sense of security. Does the reduandnt gas supply encourage the diver to extend exposure beyond what they would do without the redundancy? If so that violates the condition of the gas/equipment being a true contingency. It becomes "wag the dog." That is "I'll do something I'm otherwise not comfortable/confident doing/trained for because if I f*** up/get in over my head I have my redudant gas to bail me out."
