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I was having a conversation with a buddy of mine and he was telling me about a "friend" of his that supposedly had regulator issues at depth, but his BCD would still inflate. He told me his friend said that he was able to make a swimming ascent by adding air into his BCD, then immediately putting his oral inflator into his mouth, pressing the deflate button, and breathing, but with some minor difficulties due to water. My friend told me that he "called his bull****", but I personally think that it may be *possible*, assuming you could crane your head around to get the hose in an upright position to breathe
It is theoretically possible, assuming you had simultaneous malfunctions that rendered both second stages unable to deliver gas (which requires very unusual failures, and here you're positing two of them). Somebody recently posted, asking about failure modes that leave regulators unable to deliver gas -- they exist, but they are very unusual. If you can deliver air into the BC, you can theoretically breath it back out. There may be quite a bit of biologic crud in the bladder, though; this is truly a last ditch strategy.
I prefer receiving a nice, clean working reg from my buddy, myself.
I think I found a picture of your buddy's friend telling another story. I think if I had been listening to that story, I would have had a couple more questions as to the "Why". Based on the little you've said, I expect this was a recreational dive within NDL's so there should be several options that are better choices than breathing the air from a BC/Wing. I am curious why they all got passed over for the BC.
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Spoken by the arresting Officer: "If you take your hands off the car, I'll make your birth certificate a worthless document."
Hold down your inflator and deflator button and you get a mixture of fresh air and what ever air deflates out of your BC. If you're completely deflated already you just get fresh air.
A different method from the story but if you're asking if you can continuously breathe out of an inflator the answer is yes you can.
Can you get enough air? That is a question I've never tested. I just starred at the bubbles and went "meh, not worth it."
I personally would ditch weight (if I had to) fully deflate then cram on both buttons if I had to breath out of my inflator. I've heard lung infections aren't the biggest craze.
If your regulator is done with (presumably stage 1, since if it is stage 2 both of your regulators would need to be dead for this event to occur), your BCD inflation would not work since it connects to the same low pressure ports as your regulator does.
Assuming that by some weird alignment of stars, a lottery ticket and a lightening strike BOTH (primary / secondary regulator) low pressure ports on stage 1 went bad but BCD port remained functional consider the physics:
You would need to inflate and breathe in at the same rate else you will go up and very quick. You could do inflate / deflate at the same time, but I doubt it would work as your defoliator port needs to be above your BC bladder and if you are breathing from it it is about the same level, this means you would get positive buoyancy before you would get any air sucked out ...
I think your friend's BS comment is spot on.
If you are in open water -- you can
1) Grab air from your buddy
2) Emergency ascend from as low as about 100' simply by continuously exhaling (remember compressed air in your lungs expands as you go up, so if you exhale at the rate that it expands you are OK, and this should have been covered in the open water class)
3) Pull out your quick release or belt weights, exhale, dump them and kick, you will be up on the surface VERY quickly (try this on land and you will be see that if needed you can move quite a bit after full exhale before you have to put some air back in)
4) If you are in an overhead environment -- well, you are screwed, but again, that is what buddy system or pony bottle is for...
Lets re-think this....
His questions is, is it possible to breathe off of a working inflator, and the answer is yes, it is possible.
Now, lets take it a little bit further. Lets say you and your dive buddy really screw up on a cave dive. You've gotten severely lost, and by the time you find your way, you KNOW for a fact that you don't have enough air to make it back to land using the traditional method SCUBA intended. Is it possible to use your wing as a rebreather with the remaining air in your tanks? You inhale from the reg, exhale into the wing. Recycle the gas in the wing 3 times, then discard that gas, and again Inhale through your REG, exhaling through your wing.
Just FYI for those that don't know, we breathe in 21%, we exhale 16% Oxygen. Granted the CO2 levels are growing, but if the crap hit the fan, this is a better alternative than dying.