Hey everyone, thanks for all your great responses. It was my first dive in 23 years and so we didn't go past one bar. It was just a reorientation dive.
About my initial buoyancy, I was positively buoyant at the surface, at the start of the dive, without any air in my BC. I weigh about 70kgs and had a one-piece 7mm wetsuit and a regular sized steel tank (not a tiny fatboy).
When I tried to descend, I would get to approximately 10' and start to float back up. So, my buddy have me a few more pounds, which brought my weight-belt up to 20 pounds, and when I breathed out I was able to descend below the surface. However, I was still a bit positive underwater (I believe air in my hood and in parts of my wetsuit contributed to this) and I had to swim to stay at a constant depth.
Once I got below 20' and had managed to squeeze out much of the excess air in my hood, I was actually a bit negatively buoyant, and could glide along okay but would come to rest on the bottom if I stopped (about 25'-30'). However, it was mostly as I was moving over some larger rocks, up towards the 15'-20' mark, that I experienced the destabilization of my horizontal trim. I didn't have much more than a few squirts in the BC at the time (umm, maybe a quarter full at a rough guess).
I think that TSandM has hit the shifting air/buoyancy issue I was having pretty much on the head:
The only way I can imagine that type of air movement affecting neutral buoyancy is if, for example, you tilted head down by having your feet move UP -- then the bottom of the wing would be higher than it was before, and the gas would expand a little and you'd get a little positive.
I believe this was accentuated by force of the movement, as I remember coming up over the rock from lower down and then trimmed to a horizontal position. I was then looking down into cracks between the rocks/boulders and this tilted my ass up a bit and the air bubble went from front to back, or from head to ass, and caused the instability. This seemed to be how the disturbance occurred mostly during the dive... when altering trim in this way.
Perhaps being so close to the surface also accentuated the issue.
I can't help but think that air would have a tendency to become trapped in one side, or the other, of a U shaped wing. The comments about the doughnut wings being pinched by the tank against the divers body makes sense. I guess it would depend on a few variables as to how much the pinch occurred. Perhaps an STA would provide more of a gap here by offsetting the tank from the backplate and body (single tank diving, of course).
The problem I have, and likely others new to diving, is that I am not in a position to try a few different BCs out. Our LDS' don't stock BP/W's and I know of no one in Canberra that uses one (in fact I don't know anyone that dives, here). So, I have to figure out which design and brand will best suit my particular sense of quality, design and technical appreciation... and not have aspects that I will find lacking or frustrating down the track.
There is one additional thought that I am having about weighting the rig. Many BCs and harnesses seem to have weights integrated into the side straps. While this seems the best for emergency dumping, I can't help but think that it is not an ideal location for trim. Certainly, having the weights forward of your back mounted wing would create a counter-lever effect and tend to pull you onto your face when vertical at the surface. A behaviour which has been experienced by many.
It would make more sense to me to have the weights actually strapped to the back of the tank or the backplate itself, situated right next to the wing. It seems to be similar to the problem of sprung weight in a car's wheels and drive system, which contacts the road. The less unsprung weight the better. In the case of the diver, the less weight attached to the diver themselves the better. Okay, it's not quite the same but the analogy is sound, I think.
I hope this isn't too long winded. All your previous responses have provided a lot of clarification as to general buoyancy behaviour and wing types and their behaviours.
