Okay, lets talk buoyancy control, a subject I've written papers about in years past (many years past, by the way--IQ6 & 7). If a person starts out neutral, and by neutral I am saying neutral on a full breath, at the surface at the beginning of a dive, what is that going to do to you at the end. It has a lot to do with what types of tanks you are diving, and how many. I usually dive either my twin steel 42s (1800 psi service pressure--the "old" LP tanks), or a steel 71.2 (64.7 cubic feet at 2250 psi). The buoyance characteristics of my twin steel 45s is approximately 2.4 pounds per cylinder, or 4.8 pounts total (negative, they don't become positive, even in salt water). The published data I got on a single 71.2 cylinder is that they are negative 2 pounds full, and positive 3.5 pounds when empty. These figures are for salt water, and the difference is about the same (4.8 pounds and 5.5 pounds). I was given two AL 80s, and they go from a negative 3.4 pounds to a positive 2.5 pounds), about a six pound difference.
Now, if I am at eye level on a full breath at the beginning of a dive, I will be at eye level at the end of a dive at the surface with a full exhale (I have a 5 liter vital capasity). If I inhale completely (remember Archimedes' Principle), and fill my lungs with 5 liters of air, I have displaced:
5 liters = 0.176573334 cubic feet
Sea water weighs 64 pounds per cubic foot.
0.17657 cubic feet times 64 pounds per cubic foot equals 11.3 pounds
(0.17657 ft3 x 64 lbs/ft3 = 11.3 lbs)
This means that I can more than compensate for the gain in buoyancy of 5 pounds from breathing alone. This is the art of diving, to be neutral at all times during a dive.
A wet suit looses buoyance at depth, and hence we developed buoyance control devices (BCs; I say "we" because I was one of those doing the experimentation and publishing on the subject of buoyancy control in those days). This was done in response to wet suit diving, especially in colder waters. Wet suits loose almost all their buoyance by 33 feet in fresh water (probably a bit deeper in salt water). How do I know? I took off my weight belt in Clear Lake in the 1970s, and tied it to the boat anchor. I was at perfectly neutral buoyancy. When I wanted to surface, I simply went up to my weight belt, and put it back on. Those were the days when I wore 16 pounds of weight in fresh water, and 22 in salt water (it's a bit more now, with 20 pounds more body weight).
Dry suits were a different story. For years none of us used a buoyance control device when diving dry suits. We put on a CO2 life vest in case of a leak, and loss of the suit's buoyancy, but BCs were not used in the early days of dry suit diving.
This is how Sport Diving was conducted early on, and how we taught buoyancy control. I emphasize that this is "Sport Diving," and not "Technical Diving."
Let's look at DIR technical diving, as was posted above:
Web Monkey said:
Anybody can make it to the surface. The trick is to do it without getting DCS.
Can you swim 60 feet in 2 minutes then stop for three minutes and wait while still underwater?...
A couple of steel tanks, an aluminum tank or two, a few regulators, a manifold, a couple of dive lights lights, a backplate, wreck reel, and maybe a camera and a couple of strobes while diving in fresh water would probably do it...
I will answer these questions, but first let's talk about buoyancy of tanks. His "couple of steel tanks" are probably HP120, 3500 psi (overfill) tanks, which are -18.1 pounds each when full, and -9.2 pounds when empty, a swing of 8.9 pounds each, or 17.8 pounds for twins. There is no way of compensating this weight swing with breath control--you must depend on the BC (Cousteau put floats on the triple tanks they used when he was in a similar situation diving deep). Add to this the other tanks, and the "wing" becames a must. Also, because the tanks are now a part of your weighting system for your suit, you no longer have the option of taking them off. Note the huge difference in pressure between full and empty; when you do this, you get greater buoyance changes.
This is why I have stayed with the older cylinders--I like their buoyance characteristics and I don't like the newer cylinder's buoyancy characteristics. I have seen new students with these new cylinders so overweighted in a pool, wearing only the scuba and no wet suit, that performing simple tasks such as staying afloat for an extended time with no air in the BC was nearly impossible for a small woman in the class.
Now, Web Monkey asked how to do an emergency swimming ascent without getting DCS? It is easy--you don't go near the "knife edge" of the no-decompression limits for Sport Diving. Web Monkey is obviously a technical diver, and there are different parameters for technical diving. Talking about DCS, I still have not had an adequate answer to my original question--why don't technical divers insist on a decompression chamber for their decompression dives? If you cannot surface without risking DCS, you are not Sport Diving anymore, and technical divers are at high risk of DCS and dying because they have no chamber available. Can I swim 60 feet in two minutes and complete 3 minutes of a precautionary stop in an emergency swimming ascent? No, I cannot, but I also don't need to either. Technical divers, who either are decompression diving or at the edge of the no-decompression tables, do need to. They probably should also have periodic long-bone x-rays to see whether they have experienced "silent bubbles" and may have asceptic bone necrosis from it (non-symptomatic).
Now, about the original purpose of this thread, using a pony bottle. If you want to Sport Dive, you probably don't need one. If you get into technical diving, you must have one (or one of the several alternatives). It simply depends upon what type of diving you are doing.
One last piece of information. In order for diving instructors to keep their exemption from the Commercial Diving Regulations, they are required to dive with either J-valves or a pony bottle (per the OSHA Commercial Diving regs, Appendix C). These instructors are also required to do no-decompression dive profiles to maintain this exemption. If they do not, then a decompression chamber is required under the Commercial Diving Regulations. The requirement for the pony bottles for diving instructors is probably the reason the charter boats now require them. Only diving instructors who are employees come under these regulations, which is why probably most DIR instructors are independent contractors. If they aren't independent contractors, and teach decompression diving, then they are probably in violation of OSHA regulations. You can check them out at:
http://www.osha.gov/SLTC/commercialdiving/
http://www.osha.gov/SLTC/commercialdiving/standards.html
SeaRat
