Sideband:
I believe that is the difference between a ballanced and unballanced first stage.
Joe
Not necessarily, an unbalanced diaphragm first stage breaths easiest at low pressure; an unbalanced piston first stage gets harder to breath at low pressure. Balanced first stages don't provide much difference at all in the breathing between high and low pressure until the tank pressure reaches the interstage pressure, at which time there will be a lag in response.
The old single stage double hose regulators with an upstream first stage (e.i. the "Mistral") really started breathing nicely at less than 1000 psi. At 500 psi, it was a dream to breath, and you knew you were getting low on air
Some of the really old systems (1960s and early 1970s, with Healthways regulators especially) had what is called a "calibrated orifice" design, which caused a restriction in breathing at about 300 psi. They had a small orifice that the diver breathed through, which made the diver feel the tank was low by increasing the breathing resistance. This was an alternative to the J-valve reserve, and was called the "constant reserve" or "restrictor reserve."
New regulators have very large openings internally to increase flow at depth. This, plus the balanced first stage design, makes it very difficult without an SPG to know when you are getting low on air. It also vastly improves the air flow performance characteristics of the regulator at low tank pressure, which is desireable.
Concerning breathing off a BC, I would hesitate to do it unless i couldn't reach the surface and it was a last resort. There could be all sorts of nasty bugs (fungus, molds, etc.) growing inside the BC. But breathing off the BC is possible if there is air in there. The air will will expand, and because the BC has flexible walls, the physical act of breathing off it is no problem (although you may get some water too).
But all this talk about breathing off the BC or out of a tank with "zero" guage pressure as you ascend is not really necessary. You have all the air that you need in your lungs already. If you go back to your physics of diving, and look at Boyle's Law, it states that "at a constant temperature, the volume of a gas varies inversely with the pressure." (
New Science of Skin and Scuba Diving, Council for National Co-operation in Aquatics, 1968, page 28) We know that the pressure doubles at 33 feet, which means you have two surface lungs full of air at that depth. At 66 feet, it triples, which means you have now three surface lungs full of air. At 99 feet, you have 4, and so on. You must constantly exhale on a swim to the surface on scuba to eliminate the potential for lung overpressure injuries. This also means that you can get to the surface from almost any depth by continually exhaling as you go. If you had an OOA situation, and exhaled fully and got no air at all from your regulator at 99 feet, if you ascend to 66 feet you have a full lung full of air due to the lessening pressure and would have to exhale from there (there is residual air in your lungs that you cannot exhale).
Frederick Dumas (Cousteau's team member) taught diving to the French Navy in the early 1950s, before actual courses had been developed. Their "checkout" dive was described on page 180 of Jacques Yves Cousteau's book,
The Silent World, (Harper & Brothers Publishers, New York, 1953):
At the end of the course the honor students swim down to a hundred feet, remove all equipment and return to the surface naked. The baccalaureate is an enjoyable rite. As they soar with their original lungful, the air expands progressively in the journey through lennening pressures, issuing a continuous stream of bubbles from puckered lips.
While I would not recommend this today (today's divers are sometimes not comfortable enough in the water to "enjoy" such a ride, and today's equipment makes it difficult too), I wanted to illustrate that with compressed air and no overhead environment, OOA is not the problem it is made out to be. But we now have people routinely getting into decompression situations, where they cannot simply swim to the surface when out of air.
SeaRat
