Based on
a recent thread regarding a balanced rig, a question arose as to just how much buoyancy is lost when a drysuit floods. It just so happens that I wanted to directly measure my trilaminate drysuit buoyancy while wearing my Fourth Element Xerotherm+Arctic undergarments (together) before I take a trip to Lake Michigan. (Knowing this makes it easy to be correctly weighted with various combinations of gear I use, whether that's a single AL80, single LP85 steel, or double versions of either; AL or steel backplate or sidemount, etc.) Mission accomplished: wearing just the suit (lofted with my normal amount of air, exhaust wide open), I needed 31 lb of lead to be neutral.
For any dive, I want to be weighted such that I am neutral with an empty wing & about 1 lb of air in the tank (about 500 psi in an AL80). (If I need to use that reserve air, I can breath shallow and still remain neutral.) For a cold-water, single-tank dive, I would use my LP85 steel tank. Because the steel tank, stainless backplate, lights, etc. partially offset the positive suit buoyancy, I only need an additional 16 lbs of lead to be neutral at the end of the dive. However, if I am neutral at the end, I will be about -7 lb at the beginning of the dive (the weight of the non-reserve gas in my tank), so I put a bit of air in my wing to compensate. The positive wing and suit buoyancy exactly balance all the negative equipment/lead/gas, by design. This is what I consider "properly weighted" (though I concede that it's useful to be a couple lbs heavier in some situations).
I had a bit of time left... so I opened the zipper underwater and completely flooded the suit. I mean COMPLETELY, making sure no air was left. I then removed the belt... and stayed right there on the bottom. With a fully flooded suit, I had the same buoyancy that I have in a swimsuit alone -- basically neutral. However, that means before dropping that weight belt, I was negatively buoyant by 31 lbs and could not have kicked to the surface. If I were on the above dive wearing this suit and both suit and wing were to completely fail, I would be at -38 lb (suit + gas).
So can I get out of this pickle? Dropping my 16 lbs of lead would be a good start (net of -22 lbs now). I carry a 6 ft DSMB that conveniently develops about 22 lb of lift (taking me to neutral). What if I lost the DSMB? (I would actually carry 2 for such a dive.) Honestly though, I would try to salvage some buoyancy out of the wing and/or suit by putting the damaged portion lower in the water. I expect I could become positively buoyant without too much trouble. If not? Well, that's one reason I like to dive with a solid/capable buddy. Their fully-functioning wing is only supporting 7 lbs of gas weight, leaving plenty to help drag my unlucky butt to the surface.
My initial desire for this post was to simply report that I lost the entirety of my suit buoyancy when it was completely flooded. However, this being the Basic Forum, I decided to illustrate some of the thinking I feel a drysuit diver should undertake. (A similar issue is faced by wetsuit divers, so don't ignore all this -- you'll lose buoyancy due to suit compression at depth.) What are the ramifications of your equipment choices? Can you deal with a wing failure? Can you deal with a total suit failure? What options -- plural -- do you have if they both somehow fail? Do you even need ditchable weight at all? Is it safe to ditch ALL your lead (without inducing an uncontrolled/runaway ascent)? Play the 'what if' game from the comfort of your chair.
If you haven't seen it, I strongly urge you to check out the
Optimal Buoyancy Computer. That is a tool available for download that was created specifically to explore this topic, and my heartfelt thanks to
@rsingler for that. It's certainly useful to help pin down the amount of lead you need, but having insight into how much of that can/should be ditchable and additional backup buoyancy required may well be critical someday.
