Posted by Kendall Raine on August 01, 2001 at 13:18:03:
In Reply to: Buoyancy control in drysuit diving posted by Eins on August 01, 2001 at 11:56:32:
I think your question is "how does one use dual buoyancy devices simultaneously (BC and drysuit) simply, easily and safely?" If I've got that right, I think it's a really good question.
Just so we get semantics out of the way, "simply," "easily" and "safely" are loaded expressions. What is simple, easy and safe for me may be, or seem to be, the opposite to someone else. Nevertheless, my point of reference is one involving close to 1,000 drysuit dives many of which involved lengthy decompression. As such, precise buoyancy control for me is a matter of survival. I can't afford to screw up.
That said, I use just enough inflation gas in my drysuit to allow ease of mobility when swimming or working at depth. This is almost never enough to compensate for weight thus requiring I use some gas in my wing. As I change depth, I manage the gas in my drysuit so that it's always only enough to allow freedom of movement with the BC making up the residual for trim. This has the added benefit of keeping me cooler at depth which is what I want. Were I puffed up like the Michelin Man at depth, I wouldn't swim well. Also, the risk of an out of control ascent due to inversion is much greater if all my buoyancy is in my drysuit. For my type of diving, an out of control ascent would be fatal. For anyone, however, an out of control ascent could be a very serious problem
When I'm doing a hang, either deco or safety stop, I'll use the drysuit as the primary buoyancy device. This results in a net "inflation" of the drysuit due to reduced ambient pressure-not additional gas. I do this in order to increase loft in the underwear and make me warmer. Being warm on the hang increases my deco efficiency.
This technique works really well for me and was easy to master. Frankly, the idea of using the drysuit for the primary buoyancy device at depth has never appealed to me for several reasons:
1) You can't dump gas from a dry suit inverted-ankle dumps have got to be one of the industries most profoundly stupid ideas, 2) You can't swim efficiently when your suit isn't form fitting-just look at an inflated puffer fish-and 3) the rate and precision of gas dumping is less with a drysuit than a BC making micro changes tougher to do. This is important at depth when small buoyancy changes are important.
I know the little manual that comes with DUI drysuits says use your drysuit as your sole inflation device with your BC as a backup. While the technique I use may take slighlty more time to master, I think it superior for the reasons mentioned.