RiverRat:
Nope, most divers, including even some with 2000+ dives, are OVERWEIGHTED at the beginning of the dive. So they compensate by adding air at depth to make up for suit compression, lack of breath control, etc. by adding air to a BC. How do you think the early divers did it before the BC was invented? The only way to get a tank on your back in those days was a harness and backplate. When you dial your weight in to less than a pound you should be able to exhale, sink then use only LUNG VOLUME to control your buoyancy. I'm NOT saying I NEVER put air in my BC, I just usually don't and if I do it's a small blast on a deep dive, current on a wall or something if I need a little extra "boost". But if you really know how to dive you don't need much if any air in a BC. You only need it to float you on the surface. Use your lungs to control your dive.
As far as tank slap, I have an Oceanic BC I use for training, pool work. If I use that BC it has some inherant "flop" to it and only one strap located at the top of the tank. At the end of the dive it's more noticable that the rear end of the tank goes positive.
My backplate has 2 Scubapro tank straps. Tank can't "float" on one end like in my poodle jacket. And the plate is more secure to my back than any vest could ever be.
I don't buy it.
Don't agree. Although air in the wing, or any BC for that matter, affects trim, it does not mean that if you have NO air in the wing you'll have "bad" trim. Unless, of course, you have on a heavy weightbelt, then "more" air in the wing will really make you look like the Concorde on takeoff. Or if you have "too much" weight on your back, from too much lead in BC trim pockets, or a too heavy backplate, you may have a feet high trim. I could see air in the wing to balance that out. But then you're not really diving a "balanced rig" then are you?
Not to brag, but in my avatar I'm at about 60fsw. in G. Cayman, look at my trim and wing. Am I not flat? If you look at my wing it's flapping around flat as a pancake, no air in it.
I would agree with you statement that air/no air means nothing with regard to trim.
There are, however, a couple of aspects that seem to be missed in your comments:
1. With a 80 cubic foot aluminum, from start to end of dive, you are talking about 5 lbs of difference - so if you start at zero, you are 5 lbs light at the end. That was the physics I was talking about.
2. If you wear any sort of wetsuit, you will have to add that to the neutral point - at 60 feet it could be a couple of pounds (shorty, small person), to over 10 lbs.
3. If you start at the surface, at the beginning of the dive neutral, wearing a wet suit, you had better be overweighted, or making a safety stop at the end will be very, very difficult, if not down right unsafe.
4. You need to read people's profiles, I learned to dive, all the way though my instructor cert, before either BC's or BP/W's were invented. Guess what, correctly weighted then is correctly weighted now. With a two piece, 1/4 wetsuit, boots, gloves and hood (what I dove with during my instructor class at San Diego State), I started every dive 12 lbs negative. That is not an amount one can correct for by breathing.
5. Your avatar shows a person - who knows how trimmed you are or are not, but I assume you are trimmed (honor system here), but if that is a shorty, and it is the start of your dive and you are neutral at 60 ft with no air - you are underweighted.
6. Regarding two straps versus one - most likely you mount your tank too low, but it is possible you have a really worthless tank strap on you BC. I can tell no difference between my Mares BC and my two strap BP/W, as long as they are mounted correctly. Lower the tank and the BC shows an issue.
7. You really need to try different BC's. Some have great tank straps.
I would still suggest that there are BP/W people, that like all the tinkering and weighting, and there are people who just want to have fun diving. My only suggest is that if one gets a BC - make it a back inflate bc, not a vest type.
