Also, your pony bottle is YOURS. If a diver is in a real OOG, they are going for the regulator that they have the easiest access to: the one in your mouth (an argument for the long-hose setup that I dive).
In reality they will be going for the first breathing source that they can reach. It might be the one in your mouth ... it might be your octo or the backup you have bungeed around your neck ... or it might be your pony bottle. It all depends on your configuration and the direction they are coming from ... but if someone's not breathing, I can assure you that they will go for the first one that they see ... which will not necessarily be the one you are breathing from.
If you really want to have some control over which reg they take, the best way to do that is to hold the reg you WANT to donate out there where it will certainly be the first one they can get to. This, of course, requires you to see them coming. For that reason, whenever you see someone swimming in your direction in a manner that suggests they are having a problem, get prepared to donate your reg ... hold it out there where they can practically open their mouth and swim right into it. That might not turn out to be the problem ... but if it is, you have resolved it in the most efficient manner, and if it isn't the worst thing that happens is that you have to restow your equipment after you've dealt with whatever the problem happens to be.
At that point you should be switching to your backup regulator, then possibly opening your pony bottle up and begin breathing from it (especially if you are near rock bottom, play it safe and let them have your backgas). I'm in the "carry your pony charged, but closed" group, so if a real f**kup occurs you have a breath in your hose while you manipulate the valve to get full access to your gas.
This all works really well for Internet diving ... but in the real world how the incident plays out will depend on a great many variables ... not least of which is how stressed out the OOA diver is, and how well-practiced you are at dealing with the emergency. Switching regs, turning on pony bottles, and dealing with someone who MIGHT be so agitated they're trying to undress you in their haste to obtain a breathing source will make the actual underwater scenario anything BUT the orderly process one can imagine while typing about it. Chaos and unpredictability will have you reacting faster than you can think about it. So the bottom line is, no matter how it plays out, the best way to be prepared to deal with it is to practice the scenario over and over until you can recover from the initial surprise without having to think about it ... because in the real world, you may not have time to.
I agree with your logic on this one, as I would also carry 40% in a pony and not simply air. Emergency's do not happen often so you basically carry a pony bottle just to carry a pony so you might as well give it a second purpose; the basic nitrox cert allows for up to 40% and at least some of the courses touch on using higher % mixes in order to accelerate decompression, which I am assuming you are doing. So you are getting 2 uses out of your pony bottle.
If you are going to use your pony to practice accelerated deco, please get some training and understand what you're doing first. Carrying EAN40 as emergency gas so that you can "dual-purpose" your pony bottle has more downsides than upsides ... mostly because using EAN40 as deco gas isn't going to "accelerate decompression" very much at all. My big question here would be, why are you even thinking in those terms? If you're diving within NDLs ... which you SHOULD be doing unless you have some deco training, or at least some comprehension of how to do it properly ... then there is no need whatsoever to accelerate decompression. Just do your ascent as you've been trained, and "deco" on your backgas.
Why introduce risks or complicate your dive unnecessarily?
If you're going to use your pony bottle as an emergency supply, then it should contain a gas that you can use without additional risk at any point in your dive. If you're going to use it as a deco bottle, then don't consider it an emergency supply. There IS NO "dual purpose" about it ... that's just learning a bad habit that will, eventually, lead to unnecessary complications.
Even at the recreational limit of 130 you (or your out of air diver) will not be exposed to a high enough PPO for a long enough duration to lead to OXTOX. A PPO of 1.4 is extremely conservative as it is, and you will definitely be at or below a PPO of 1.4 within 1-2 minutes as you will be heading to the surface. OXTOX is cumulative and not instant at recreational depths on recreational mixes (<40%). I disagree with the flaming going on around you using 40%, a minute's exposure to a PPO of 2.0 will not likely kill you, but drowning definitely will.
I'd be really uncomfortable either giving or taking this advice ... for the reason you stated in the part I bolded above.
Conservatism is built into a great many diving practices for a very practical reason ... which is because even the experts don't truly understand all the variables that affect your physiology when you dive. Oxtox ... like DCS and nitrogen narcosis ... affects different people in different ways ... and affects even the same person in different ways under different circumstances or on different days. There's just too many variables to say that "this level is safe" and "that level is not safe".
What we do know about oxtox is that the liklihood of a successful recovery underwater is amazingly slim ... if it happens to you, it will almost certainly kill you.
So why take chances? Those "extremely conservative" limits exist for a reason ... and even within those limits people are sometimes affected. Under no circumstances should a recreational diver consider that a PPO2 of 2.0 is "OK" ... not even for a short period of time. It probably will be ... but if it isn't, you're dead.
Is the risk worth it? Not to my concern.
As my old friend Uncle Pug used to tell me ... "You really need to rethink your approach to that situation".
... Bob (Grateful Diver)
Y'all are funny . . . I asked a question to find if there were any legal or moral issues with sharing Nitrox with an unknown diver . . .
