1) First, you have twice as much equipment to monitor ... three is very little possibility that the cylinder will loose pressure during the dive.
2) Second, there is an issue with streamlining. I dive a high current river, and any extra equipment adds drag. In pristine environements without current, this probably is not much of a problem, but in a river, it can make quite a difference.
3a) Not if your are properly trained. 100 feet is 4 atmospheres absolute, meaning that the air density at 100 feet is 4 times denser than at the surface. That also means that upon a CESA, the air in one’s lungs expands to four times its original volume.
3b) I did this as an experiment in my local pool
3c) So not only is a CESA possible from 100 feet, Jacques Cousteau described it as an “enjoyable experience” for highly trained divers.
4) What we have now is divers who are not highly trained, have minimal water skills, and are not relaxed in the water. Agencies responded with equipment “redundancy” rather than training. Carry this tank and simply switch, rather than not carrying a spare tank and relying upon water skills. The equipment manufacturers and dive shops jumped on this, as they could sell more equipment, and make more money.
SeaRat
1) At this time, I put SPGs on every cylinder/reg, even if they have AI. Sometimes that's a button SPG. Maybe you don't technically need it, but it can be helpful for knowing you need to cut your safety-stop a little short.
2) I'm always a fan of streamlining.
3a) That makes sense. However, like you mentioned, as a lightly-experienced (about 200 dives?) scuba-diver, CESA at 100ft isn't something I would know how to practice safely, nor would be highly confident in my ability to perform. I would simply head for the surface, breathe out slowly, and hope I make it. There are some obvious safety limitations with actually doing a 100ft CESA, just for practice.
3b) Interesting exercise.
3c) Perhaps if I could safely practice it.
4) I definitely understand the general critique of the dive industry. I'd even say classes are setup in such a way as to instill learned dependency. I fully understand there can be unanticipated and unintuitive hazards of types of diving beyond one's training level, however one doesn't need a "wetsuit flattuance" certification to fart in a wetsuit.
(I'm still skeptical about the whole solo-certification. Cary a pony bottle, be extra careful, and consider other forms of redundancy.... got it)
The difficulty of "get good" type advice, is we're still back to step-1, in terms of where or how does a diver learn these skills you talk about? We have the dive-agency-training issues mention, and others not mentioned, but that's ultimately where most people have to go for training these days. So, as a diver, I dump $250 to $1000 on some class, where only a little makes it into the dive-instructor's pocket. Meanwhile,
(I've heard) some agencies apparently don't allow instructors to train anyone outside a formal class, and the formal classes of course follow a specific curriculum.
For the moment, I'm okay with being a "scrub" with a pony-bottle. It's rigged up nicely, hoses bungeed, etc. I feel safer with that, than the idea of air-sharing with a buddy-hazard. Though if I can practice a 60-100ft CESA safely, I won't complain about having a 3rd form of redundancy.
