- Abrupt internal failure of the 1st stage regulator (on the now nearly-universal upstream opening diaphragm regulators, this can be caused by delamination of the HP seat, breakage of the pin that opens the HP valve, or clogging by debris)
- Clogged passages in the dip tube or tank valve
- Mouthpiece separates from 2nd stage (you or I could switch to a secondary, or breathe from the 2nd stage without a mouthpiece, but PADI doesn't train people to do either of those things)
- Ripped diaphragm, folded exhaust valve, or damaged housing in the 2nd stage, an astute diver would switch to a secondary or ride the purge button, again not taught and not something everyone will figure out
I have seen three separate firsthand reports of abrupt 1st stage failures on SB that led to complete inability to deliver air, although one was a reg that was far overdue for service.
It sounds like you (already) have a better handle than I about what failures could possibly cause a single hose regulator to STOP (completely) delivering air to the diver at depth.
If you read discussions on this forum, one of the most common scuba failures is a mouth piece falling off. Of course you can use a reg with no mouhpiece but if you inhale water during the event you will be choking and trying to cough and in very cold water it will be difficult for the diver to effectively seal their numb lips on a reg with no mouthpiece (one reason why I use aircraft safety wire or a constrictor knot and string rather than zip ties.)
However there is another potential failure that has given me nightmares for a long time. Let's say you are catching lobsters and you are low on air and should be going up, but you see one more bug and he goes far back in a small cave or ledge. As you drop down, you vent your BC because you want to be heavy and stable and be able to work the lobster without swimming etc. So you have 600 psi and are at 80 feet or so, you have been swimming for most of the dive and have been trying to not use excessive air, so your heart rate is up a little, but you are controlling your breathing.
You reach for the bug but he shoots further under the ledge and you instantly and instinctively lunge forward and try to catch the lobster. Unfortunately, you just slammed the reg into the roof of the ledge and you ripped the hose at the collar at the first stage connection. Alternatively, you could simply snap the hose off at the threaded connection.. it is only brass. Now you are wedged a little in the hole, and air is screaming out of the hose.
You wiggle out of the ledge, you now have 200 lbs of air and as you look at your pressure gage it gets hard to breath, You get one more breath as the final bits of air exit the hole in your hose and you slam the BC inflator into the UP position.. It farts a little and all air stops coming from your reg. You are out of breath, scared to death and you are 8 or 10 lbs negative, have no means to inflate the BC (orally or otherwise) and are essentially pinned to the bottom with a bag of heavy lobsters clipped to your BC.
If you don't have a pony bottle, dont have a buddy, and don't have ditchable lead and can drop the bag of lobsters super fast.. you just are NOT going to be able to swim 80 or 90 feet in that condition. You are screwed.
I think a similar failure could occur on a yoke regulator if you slam it on the ceiling and the pressure is low and the reg shifts and the o-ring extrudes at the valve face.. I think that is possible, when the tank pressure is low.
In any regard, it is ridiculous to dive in such a manner that we assume that we can
always sip our air for the ascent. It just might NOT be there when you really need it.
Also, an observation from my long list of "stupid human tricks".. if you do have to make an ascent and you have near zero air in the tank and it is just dribbling out on ascent.. it is much, much better to press the purge button and "accept" the slow delivery of the air and inhale it slowly, rather than sucking super hard and fast and pulling a huge vacuum that does not serve to deliver air at an increased rate. It is so much easier to accept a slow stream of air compared to SUCKING super hard and just getting a tiny bit of air. I guess it is more psychological than anything, but the difference is dramatic.
If you don't believe me, here is a simple at home game to play.. walk briskly for maybe 90 seconds, enough to get your breathing rate up a tiny bit and you can feel your heart pumping more than resting... then take a normal straw, stick it in your mouth and continue walking. Breath only through the single small straw and try to inhale really fast and hard .. sucking a vacuum. I think you will agree that it will quickly become uncomfortable..
Then rest and do it again, but this time concentrate on taking a really long slooow relaxed inhalations. Don't try to inhale too hard.. often you can calm down and continue to walk for quite a while on this reduced air flow. This is analogous to passively accepting the limited air flow from purging of a near empty tank.
As for some of the other comments, a CESA from 30 or 35 feet, really should be pretty easy for anyone if they're not diving in a super negative condition.
.
