@halocline,
This is how I'm thinking about this:
6.5 lbs of air weighs 6.5 lbs, correct?
Suppose a diver weights himself to be neutral with an "empty" cylinder and a completely deflated BC at 3 [sic] fsw (say). If his full cylinder contains 6.5 lbs of air at the
start of his dive, then, if he is struggling at the surface, maybe by getting rid of ~6.5 lbs (
of something) he or his rescuer will increase the likelihood of a positive outcome.
Does this make sense?
NOTE: I chose 80 cu ft for no special reason--except that it's in the ballpark of what many recreational divers will be using (namely, 77.4 cu ft in a Luxfer Al 80).
rx7diver
Yes, I understand that, but nobody uses 6.5 lbs of air diving with an AL80. First, an AL80 holds 77 cft, which weighs roughly 6.2 lbs, not 6.5. Second, people do weight checks to be slightly positive on the surface (water surface at eye level) with 500 PSI. (If they are following the generally accepted recreational training guidelines as I remember them) This brings the weight of the air compensated for to around 5.2 lbs, and keep in mind that divers are slightly positive, maybe about 3 lbs, I suspect that's low, but just for arguments sake. The amount positive is equal to the weight of the portion of the diver's head that remains above the surface during the weight check.
So that means that a diver properly weighted using that standard check, with a full AL80, will actually be around 2 lbs negative on the surface, not anywhere near 6.5 lbs.
I've been diving a long time, most of it in warm water with AL tanks; single tanks, manifolded doubles, and sidemount. I have a lot of experience dialing in my own buoyancy and helping others do so in those scenarios. Most recreational single tank divers are carrying more weight than they need. There are a few reasons for that which we don't need to go into in this post. But my point is that overweighted divers might need ditchable weight on the surface, and technical divers using much larger volumes of air and/or heavy gear certainly benefit from it, but single tank warm water recreational divers don't, as long as they are properly weighted and reasonably competent. Don't forget, we're talking about divers that are using BCs with 25-40 lbs of lift. BC failure at the beginning of a dive? Whose fault is that....part of a pre dive check is making sure your BC works.
Actually, I have been guilty, more than once, of jumping into the water with my tank valve off. I used be a DM and from time to time I would embarrass myself by carefully checking everyone else's tank and forgetting about my own. In that case, I did have a full tank on the surface with no functioning BC. But, since I can swim a few lbs to the surface and maintain positive buoyancy, and I can reach my valve, those scenarios were a little embarrassing but not dangerous.
Sorry for this long post, but it is an interesting topic.
