We have strayed a bit from the "dangerous gear" thread title, but this is kinda important so I'll try to bring it back together.
I've been thinking back over my diving history, and remember that when I was trained by LA County in 1963 (give or take a year), we had an open water emergency swimming ascent that we needed to do to complete our course. We had already been in a pool session where the instructor took a weighted net over the top of us (as a buddy team) and dropped it onto both of us. We needed to get away from that net by helping each other. So the emergency swimming ascent was not a big deal from about 30 feet. I've already mentioned the buoyant ascent in the Navy, but not the pool harassment that we went through to graduate too. The Navy instructors would do almost anything to get us to surface--the first thing both me and my buddy did was to hand them our masks just so they wouldn't have them to pull off. This is a far cry from today's diving instruction, and I'm reminded that in the U.S. Navy School for Underwater Swimmers, we needed to complete: one dive to 50 to 70 feet for 10 minutes; one dive 70-130 feet for five minutes; and two day scuba compass swims (1,000 yards and 1500 yards). We had a lot of practice swims that were shorter, starting at 500 yards. We had to do zero visibility dives, and put a bunch of bolts and nuts together while on that dive. We were at the U.S. Navy School for Underwater Swimmers for three weeks continuously (10 hours/day, 5 days/week for 3 weeks is about 150 hours of training). Compare this to a NAUI 1975 course, in which the "Minimum course duration is 27 hours. Of this time, 16 hours or more are to be in-water activities, and the remainder is to be spent in classroom/lecture activities. Of the time spent in water activities, at least 2 hours are to be in open water; the balance of the water time may be in open water or pool..." Open water skills included "...3) With scuba equipment: clear mask and mouthpiece, buddy breathe, alternate between snorkel and scuba and make a controlled emergency swimming ascent" (NAUI Instructor Manual, 3/1973, page 2.1d-1-3 8/75). So the emergency swimming ascent was still taught in 1975 when I was an instructor.
However, contrast this with the U.S. Navy, whose main purpose was not to provide an "enjoyable" experience, but to produce Navy divers by selecting out those who were unfit, who could not stand the physical and psychological harassment (ever done pushups with twin 90s on your back, or laying on your twin 90s face up, doing flutter kicks with a mask full of water?), or who decided they just did not want to be Navy divers. We swam and ran for hours, usually under some psychological harassment (the physical pain was self-evident). Now, I still do my doff and don drills in our local pool, which is a competition diving pool 18 feet deep. I routinely doff and surface, which is the equivalent of an emergency swimming ascent. So I completely agree with Thalassamania that these skills can and should be taught.
Duckbill, the emergency swimming ascent physics does not change. It is different, however, from a buoyant ascent. In an emergency swimming ascent the diver controls how rapidly (s)he ascends. In a buoyant ascent, there is no such control. The physiology is more important than the physics, in that the "blow and go" concept can actually cause a problem. With 'blow and go" if too much air is blown out, the risk is to shut down the broncheoli (SP?--the small tube running to the air sac). If this is pinched off, there can be an over-inflation of the lung in this particular area even though the main lungs are exhaling air. This is one reason the Navy wants a doctor available and a chamber available for this training. The other difference is in the personnel that the Navy has to work with, who are young, passed a rigorous diving physical, have been physically conditioning for weeks (perhaps, like us USAF types, months), and were in pretty much top condition to dive. Dr. Stanley Miles wrote a book some years ago titled Underwater Medicine. In it he gave the following formula for a diving accident:
A = CE (prf/tms), where A = accident, C = Chance, E = Environment, p = accident proneness, r = risk acceptance, f = physical factors, t = training, m = maturity, and s = safety measures.
While this is not from any safety professionals (we have other models), this is a useful construct. For the factors, the numerator is the ones which would tend to help cause an accident. Accident proneness is not an accepted safety area now, but some people are simply not as self-aware as others. Risk acceptance deals with what diving conditions we are willing to dive into, and how we dive. Here, for instance, we can talk about running out of air, which means that the diver has allowed the dive to progress to a point where low air is a possibility. This is what the DAN 2010 Fatality Workshop called the "trigger" to an emergency ascent, which can lead to air embolism and death. Physical factors are obvious; the regulator I mentioned involved in a fatal accident earlier in this thread was owned by a guy who was well over 250 pounds, and very, very out-of-shape. Both contributed to his accident and death.
On the other side is training, maturity and safety measures, decreasing the potential for an accident. This is the denominator of the equation, which when divided into the numerator decreases the factor. I noted above the difference in training time for the Navy divers verses the NAUI 1975 standard (150+ hours verses 27 hours minimum). This would tend to make the Navy divers safer divers, with more experience in the water and more self-awareness than a basic scuba diver. The maturity level is also greater in a U.S. Navy diver due to what that person has been through to get through the course. The instructors don't put up with much, and in order to graduate you must be pretty good in the water. Now we came to safety measures, and this is where the two types of divers can find parity. Recreational divers can have as many safety measures as Navy divers, and sometimes (combat, for instance) the Navy divers have even less and depend upon their training and watermanship to get through situations. But, recreational divers can have dangerous equipment too.
It is here that we can begin to get back to this thread's "Dangerous Equipment" area. Running out of air is one of the main dangers to recreational diving. The J-valve was used early on as a signal to divers that their air supply was about exhausted and they needed to surface. It could be bumped or inadvertently knocked down, thus giving the diver a false sense of security. This is why many of us who use J-valve reverse the lever action so that the lever is pointed forwards and not to the rear--it's harder to knock down in that position. Healthways came out with a restrictor orifice for their double hose SCUBA regulator, and also used in in their Scubair single hose regulator. At about 500 psig it would restrict the flow of air so the diver would know that he had to get to the surface when breathing got harder. But the restriction became worse when the diver had to dive deeper, so overhead diving was not allowed for this valve configuration. Then we got to the use of a submersible pressure gauge, and people either did not use the J-reserve valve anymore (electing to dive it with the reserve "down"), or whet to K-valves. But the SPG had a problem; in blacked-out water you could not see it (silted in, zero visibility, night), and did not then know how much air you had left. The use of a watch and depth gauge to determine air consumption rates was not taught anymore (from what I've heard), so there was no backup method of knowing about how much air was left in a tank. What I'm getting to, with a lot of words (my apologies) is that most gear, if not well understood, can become dangerous when a diver simply "assumes" that it works and doesn't understand why it works, and possible failure modes.
SeaRat
