You know, I'm pretty much on Nemrod's side of this. Has anyone ever thought of doing a buoyancy check on a near-empty tank? You should be neutral at 500 psig at the water's surface. That would make you a bit heavy on a full tank, but not so heavy that you cannot swim up from just about anywhere (with your fins on, of course).
Having started diving before BCs (in cold Oregon water, with a 1/4 inch full wet suit), and having helped in the development of BCs, I now see that this tool is being abused. Buoyancy compensation is just that, compensation for the loss of buoyancy of the wet suit. But now, even instructors teach being overweighted so that it is not necessary to swim down to the bottom. Simply deflate the BC and down you go.
The emergency situation is not the loss of the BC's ability to hold air, but the situation where the diver is dependent upon the BC holding air to stay out of on emergency. If the diver is weighted correctly, loss of the BC should not be an emergercy, just an inconvenience.
When the AtPak first arrived on the scene in the 1970s, there was a death in Clear Lake, Oregon when a diver descended from a boat with his air turned off. He couldn't inflate his BC, and went straight to the bottom in 90 feet of 37 degree F water, with a non-functioning BC and regulator. That diver was highly overweighted too (weighted for salt water in a fresh water lake, and usually heavy in salt water). That is an emergency situation which would have justified ditching the entire scuba unit, but for some reason that did not happen either.
In short, you should never be in a situation where loss of the BC is an emergency. This is simply an aid to compensate for the loss of buoyance of your diving suit (usually a wet suit, as dry suits now have power inflation and don't loose buoyancy).
Many moons ago, when I was in high school, our dive club took a week-long trip to Thetis Island, BC. While there, a missionary ship came in with a fishing net around one of its two props. They asked if we could cut it off. My dive buddy and I were eager to help, and (after ensuring that there was no way they would start their engines) dove in the docks to cut the netting off. What we didn't know was that there was four-inch diameter rope around this netting too. So we hacked away with our dive knives. I found out that day why serrated edges are better than straight edges for some things; the straight edge of my knife, though sharp, would not cut that rope. We finally hacked through the last rope and netting, and were quickly dragged to the bottom by the weight of the netting and rope. It was a pile on the bottom about four feet in diameter. But we were determined to lift that off the bottom, and the two of us swam that glob of net and rope to the surface, about twenty feet above. It took an extreme amount of effort, as it probably weighed 35 pounds underwater (over a hundred on the dock). But we got it up there where about four people pulled it out of the water.
What I'm now seeing is that divers are weighting themselves almost this badly as a normal practice for a dive, and depending upon their 60 pounds of buoyancy in the BC to get off the bottom. If the BC fails, this is like lifting that net-rope mess we had at Thetis Island in 1963 off the bottom and to the surface. I can see how that would be an emergency, but it is caused by a fundamental lack of understanding of the physics of diving.
Concerning "redundent" buoyancy in the form of a lift bag, I think it is foolish. First, you already have "redundant buoyancy" by ditching weights the wet suit's buoyancy (what there is of it--in salt water, there is some), or the lack of the weights to allow a controled, swimming ascent. "But weights are expensive, and I don't want to loose them" some might say. Well, carrying around another piece of equipment is also expensive, in time, effort, and the need to care for it. Putting air in a lift bag raises the very real risk of a buoyant, out-of-control ascent that is faster than currently recommended by the instructional agencies. Having it attached to you at the time is dangerous too.
You might want to practice this loss of BC buoyancy, after doing a buoyance check described above and becoming properly weighted, by going down and swimming to the surface sans air in the BC. It's really not that difficult for a correctly weighted diver.
John
Ex-NAUI Instructor #2710
