Drysuit for Primary Buoyancy (was Re: Wi

["David J. Hicks" <DaveHicks@xxxxxxxxxxxxxx>]

Like Robin, I was also taught that the drysuit should be used for 
buoyancy during the dive and the BCD only on the surface. The idea as I 
understand it is to have no air in the BCD at all during the dive so 
you only have to control one source of buoyancy (the suit) and don't 
have to use the buoyancy controls of the BCD at all. This works in 
practice for me for all dives down to about 45m - deeper than that and 
I do have to use my BCD as well to achieve the required buoyancy.

I guess the theory is that as you MUST have air in the drysuit to 
prevent squeeze, you are going to have to control its buoyancy by 
filling and venting as you descend and ascend anyway. If you have air 
in the BCD as well, then you will have to control its buoyancy too and 
you will have two things to cope with instead of one. The most striking 
illustration would be a controlled buoyant lift scenario. If the 
casualty has only been using his/her suit for buoyancy during the dive 
then the rescuer can ignore the BCD during the lift. If not, then the 
rescuer will have to try to control both - more difficult with only two 
hands and so more likely to result in loss of buoyancy control and too 
fast an ascent.

I suppose that with an auto-dumping valve on the drysuit you might in 
theory be able to inflate the suit for comfort at the deepest point and 
then forget about it and only use the BCD during the dive, but I have 
my doubts. I am pretty sure that in practice on the ascent you will 
have to manually control the increasing buoyancy from the expanding air 
in the suit.

I will reserve judgement and try it on a dive, but so far as most of my 
diving goes at the moment I am not convinced by the reasons given by 
John for using the BCD as the primary source of buoyancy during the 
dive.

> 1) Lots of air rolling around in your suit will unbalance you.  It
> may invert you and send you to the surface.  The air moves around
> when you do, ie. banking, rolling etc all cause it to move.  This 
> is not a problem with the wing. 

I have never found this to be a problem. Some people might say I'm an 
unbalanced individual, but I have never felt it to be a problem on a 
dive ;-). The one uncontrolled ascent I have had was due to a direct 
feed free-flow which I couldn't disconnect in time (better equipment 
maintenance and practice required on my part - lesson learnt). I 
generally don't have "lots of air" in my suit. If I feel I have too 
much (generally below 40-45m) THEN I use the BCD as a secondary source 
of buoyancy.
 
> 2) You can dump air faster from the wing than from the suit. (Unless 
> you pull your neck seal open - but then that's not very comfortable
> now is it?)
 
I don't know about this one - you can dump hands-free from a drysuit, 
just by lifting your wrist or raising your shoulder. This must be 
quicker than finding the dump valve on a BCD. I grant you that 
marginally more air will generally come out of a BCD more quickly than 
a suit once you have started dumping, but I have never found a drysuit 
to be inadequate in this respect.

> 3) The wing can directly support the weight of your tanks by wrapping
> around them.  This transfers the weight from your back.

Not sure of the physics on this one - watching the thread. All I can 
say is that I don't find the weight of my cylinders to be a problem. In 
fact my set-up is very comfy.
 
> 4) An inflated suit causes more drag in the water than an inflated
> wing. More drag means more effort in finning which means more
> gas consumed which means shorter dives and an increased inert gas
>loading. 

Again, not sure about this one. We can never be terribly streamlined as 
divers, so I find it hard to believe that it is going to make a 
noticeable difference, but I'll keep watching the thread and try it on 
a dive.
 
It seems to me that, as often is the case, it isn't simply a question 
of "good diving practice" vs bad diving practice - it is more about 
certain practice being more or less appropriate in certain conditions. 
For me the clincher at the moment is the controlled buoyant lift 
scenario - I have tried to cope with a dry-suit and an inflated 
BCD at the same time on an exercise and I didn't have enough hands. If 
someone can answer that one for me, and if I find using my wings as 
primary buoyancy to be better than the suit when I try it on the kind of 
dives that I do - then I'll be convinced!

Dave
BS-AC 0953, Manta Divers, SW London