If (for our discussion) the most critical objective is avoidance of decompression sickness, how much weight should you drop in an emergency to be able to make it to the top in a controlled fashion, and probably not run out of exhalable lung volume on the way up? What should you drop?
Nothing. They call it a *Controlled* Emergency Swimming Ascent for a reason.
Lets make it interesting, just for arguments sake:
1. 160 lb guy with a 7mm full wetsuit plus a hooded vest, requiring a total of 28 lb of lead at the surface, diving Monterey Bay 55 degree water.
2. 95 cu ft steel tank with -5 lb buoyancy full, -1 lb buoyancy empty.
At the surface, he floats eyeball high with an empty tank. At 10 ft, he can do a fin pivot with an almost empty BC and variable lung volume.
But he has a catastrophic failure upon reaching the bottom at 99 ft/3 atm with a nearly full tank, and his buddy is nowhere to be seen. If you spare me the where was his buddy? and why not a drysuit?, then how much weight should he drop to probably make it to the surface alive?
But I won't spare you the question about the drysuit. After you've gotten too cold in a 5mm, you should be using a drysuit. Period.
At 99ft/3 ATA, his 28 lb buoyant wetsuit has been compressed to 1/8 its previous volume, or +3.5 lb, so hes 24.5 lb heavy. His tanks are almost full, so lets say theyre still -4 lb. Thats 28.5 lb that is being compensated for by the BC for neutral buoyancy at the bottom. Or am I missing something?
Why does he need to ballistically shoot to the surface when he's got a full tank of gas?
Normally this kind of situation either happens towards the end of a dive when someone goes OOA, or at some point when someone has a free-flow and drains their tank.
If he drops his weight belt, he can almost empty his BC and swim up his -4 lb tank without worrying too much about his BCs increasing buoyancy with all the air he had at the bottom, especially in the last 33 ft. Or maybe even doff the (now useless) BC and tank too! But oops! What about his wetsuit? By the time he reaches the surface, hes 28 lb light and moving fast.
Well, then, how about he just drops the BC and useless tank, and keeps his weight belt so hes neutral at the end? At the bottom, hes now 24.5lb heavy and cant swim up that much. Even if he drops some, with all that extra weight at the beginning of the ascent, hes working hard on one lungful of air. Bad. And he's now light at the end. Bad.
I've tried this with double-130s filled with 32% (-20# of gas) and as much drysuit squeeze as I could bear and I could kick off the bottom and get going fine. Without having redundant buoyancy doing a stop like that would kind of suck and I was working hard to tread water, but I could do it for awhile. If you let me ascend, though, then everything expands.
If he keeps his BC to have a neutral start with his weight belt, and bleeds air to a neutral end counting his expanding wetsuit, then why bother to have droppable weight in the first place? And what about BC failures?
Yes, tech divers don't use ditchable weight because uncontrollable ascents are contraindicated with tech diving. They do carry redundancy and are trained to avoid and solve problems underwater rather than reaching directly for the surface bailout.
Most BC failures are also recoverable (e.g. losing your LP inflator or dump valve) and you can still breathe off your regulator at the surface to get to the boat. If you're considering both catastrophic BC failure at the same time as an OOA, you've got a diver that needs to plan better or take up golfing. At that point, though, if you really must mitigate the problem the solution is a drysuit, and then a scooter.
For rec diving I'll have a drysuit if its cold, and if its warm enough for a 5mm wetsuit then I can swim it up. For deep tech there's usually a scooter involved (or for cave you can figure out some way to crawl out).
If he drops a lot, with an easy start (buoyancy-wise) hes moves fast at the beginning, but even faster at the end. Bad.
Give me your expertise: with a catastrophic failure at depth, how do you get to the top unbent, and (probably) alive? I dont want the answer to be: divers that go to 100 ft in coldish water have to use a drysuit--because this is an academic exercise.
Well, that's the best answer.
Should he carry two separate droppable weight systems, so he can keep some and drop some? How much to drop?
No. And None.
---------- Post added March 11th, 2013 at 02:04 AM ----------
My conclusions:
If it's a BC failure, I need help from my buddy (rec divers only - that's another discussion).
Most likely you don't. It may not float you comfortably on the surface, but it'll trap enough gas to get you neutral and you'll be able to keep a regulator in your mouth and get back on the boat.
This is why you need to practice gas management and never run out of gas. If you're going OOA and getting back on the boat with less than 500 psi a lot, then you're severely limiting your options if anything else goes wrong.
If it's a first stage failure, I'm already neutral and I just swim up, vent BC air, exhale and hope for the best.
If it's a medical emergency, I won't drop the victim's weight, but I'll pop a little burst of air into the BC and help him/swim him up.
Dropping the weight belt of another diver underwater, particularly an unconscious one, is a way to make absolutely certain that you kill them. They will embolise and there's no coming back. Wherever you got the idea that you should do that, you definitely need to adjust that.
Reaching down with my right hand to blindly drop all my weight is something I'm going to try to erase from my muscle memory, and leave for a conscious, slower decision for special cases when DCS isn't a concern, but getting air is. You know, the run-of-the-mill stupid mistakes that at least once happen to all of us, where you might be OOA and flots will call you stupid and bring Darwin down on you.
You should have been trained to only drop weights on the surface. Swim to the surface, orally inflate (either off your regulator or just use the atmosphere -- depending on sea conditions and your gas supply) and *then* drop your weights.
I just think I'm going to try to remove "drop my weight belt!" from my early decision tree. Thank you all for a great thread!
If you were properly instructed, and you paid attention, it should never have been there in the first place.
