Catastrophic Failure - How much weight to drop? Theoretical Discussion

[rsingler]

I wanted to start a discussion regarding the “ultimate emergency.” We all talk about redundancy and buddies and the pros/cons of various secondary air arrangements, but we have droppable weight systems for a reason: if all else fails, we need a controlled emergency ascent.
I’m a physician and retired USAF hyperbaric officer, and at my age I think I just might rather drown/die than get seriously bent. I don’t want to start a flame war here on “life vs. death”, but just set the parameters of the discussion. I’m an old diver with hundreds of dives 15-25 years ago on vintage equipment who is just now getting back into the fun. So...a fair amount of warm water experience, but nothing in a decade.
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?
Let’s make it interesting, just for argument’s 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?

At 99ft/3 ATA, his 28 lb buoyant wetsuit has been compressed to 1/8 it’s previous volume, or +3.5 lb, so he’s 24.5 lb heavy. His tanks are almost full, so let’s say they’re still -4 lb. That’s 28.5 lb that is being compensated for by the BC for neutral buoyancy at the bottom. Or am I missing something?
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 BC’s 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, he’s 28 lb light and moving fast.
Well, then, how about he just drops the BC and useless tank, and keeps his weight belt so he’s neutral at the end? At the bottom, he’s now 24.5lb heavy and can’t swim up that much. Even if he drops some, with all that extra weight at the beginning of the ascent, he’s working hard on one lungful of air. Bad. And he's now light at the end. Bad.

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?

If he drops a lot, with an easy start (buoyancy-wise) he’s 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 don’t 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.
Should he carry two separate droppable weight systems, so he can keep some and drop some? How much to drop?

Rob Singler
Napa, CA

 

[dumpsterDiver]

The answer (for me anyway) would be to carry a 6 cu-ft pony bottle. They probably did not make them when you dove last. For $250, you can have a complete, redundant system that would allow you to make a quick ascent and not drop lead and be much, much safer than blow and go. If the pony bottle is too much money or trouble, then just throw the belt and go for it Doc!

Suit compression is not identical to gas compression, so your numbers on suit compression are a little more drastic than actual.

 

[g1138]

Split the weight up between the belt and BC and create a more balanced system. Anyone fit to dive, should be fit enough to swim up 10lbs of negative buoyancy. And keep in mind that will decrease as the 7mm suit re-expands.

If for some reason your regulator fails, you have no buoyancy control, no air, but at 99ft, you should still probably have the ability to control your descent. That means having air in your BC, so you're not completely -28.5lbs at the bottom. It means adding air as you descend.
If you have to plan your dive such that you need to speed to the bottom because of your available air, you should probably reconsider your plan and get a bigger tank, or plan for a less aggressive, shallower profile.

I don't think planning for a reg failure and BC puncture is a realistic problem you may face.
The point of dive planning, training, and such is to mitigate risks. Preventative gear maintenance, rinsing, gear checks, the likes. If you keep trying to plan for the end-all-of-all catastrophic multiple equipment failures, you might as well not drive to the dive site. Because you know, your seatbelt retention system could fail, your air bags could fail, your accelerator will stick to the floor, and your hydraulics (power steering, brakes, and all) will fail.

So to your original problem. Reg failure, you should already have some air in your BC, even if you just reached the bottom. If it happens after descent you should be neutral already or close to it. So it wouldn't be that hard to swim up. Putting 16-20lbs on your belt and dropping that will assist you. The rest of the weight will be on your BC, so that's 8-14lbs heavy at 99ft. Easy to swim up.

Let's now say BC failure. Your BC pops and you vent all your air. You have a working reg, so drop your 16-20lbs belt if you need to and swim up.
If you're still not comfortable with dropping that much weight, put less on your belt and more on your BC. Mix it up with Weight integrated dump pockets. Set it so you feel comfortable.
And if all else fails and you're rocketing to the surface, face the surface, go horizontal, spread eagle, scull, and flare to control your ascent.

 

[fisheater]

In a situation like that (deep, steel tank, very thick wetsuit), the diver should have some weight on a belt and some on the scuba unit, not only for partial ditching at depth in case of total BC failure, but also to be able to ditch and don underwater safely.

 

[CT-Rich]

I feel much more comfortable with weight spread out on the belt and BCD. A partial ditch with the option of dumping more once on ther surface is the best solution. A complete dump on the bottom is going to make a missile the last thirty feet when your suit re-expands. Flairing would slow your ascent, some but you are in serious cluster **** mode at the time. I'll go with DD that a pony would be a best choice for this situation.

 

[danvolker]

Also....from the bottom after ditching weight....you could stop at 30 feet for a couple of minutes....then try to stop at 20, swimming downward during....then gradually let yourself be pulled up by the suit...the last 10 to 20 feet should not be such a problem then.....and much better than staying on the bottom

 

[Dsix36]

Yes, I read the OP's post so I know this is not the answer he wants to hear.

You can possibly get "unbent", but you can not get "undead"

I would rather be bent and alive than unbent and dead.

 

[rsingler]

All good comments, thanks! Agree that a pony or doubles is a good way to avoid getting here in the first place.
And I'm seeing replies that concur with splitting up the weight. Not sure I could swim down against having dumped more than 15 lb, but hey--this is a theoretical post.
And I'll maybe accept the limited compressibility of the wetsuit, so we're not talking all 24lb of air compensation when the exercise first begins, but I'm not sure about that.

I'll also accept that I won't likely have BOTH a BC failure and a first-stage O-ring blowout. Since there's air already going in the BC on the way to the bottom, I'm neutral whenever the problem starts (sorry I wasn't clear, g1138).

That said, if what we're really talking about is how much weight to dump in an emergency, I'm still waiting for a consensus. No way will I dump 18 lb unless it's "dead or bent", and you've heard my thoughts on that already.
So I'm thinking of a separate 6lb portion to drop (pockets or belt, your choice). That way, with any air in my BC at all, I'm probably not swimming up more than 15 pounds at the beginning, and probably MUCH less. And at 30 feet, I'm probably still a little negative (less and less negative as my tanks empty before the failure, of course), so maybe I can keep my ascent reasonable.

So there -- I've committed myself to a 6lb drop. Somebody shoot me down.
All in fun, of course.

Rob

 

[Glenn08]

First question = why would a 160 lb diver with a 7mm wet suit need 28 lbs of lead ? Seems like a lot to me. Second question = what is the 'catastrophic failure' ? Burst BC bladder ? Since you mention 'exhalable lung volume' you seem to suggest a regulator failure. The factor you don't seem to mention is what will happen if you don't make it to the surface. Drop all of your weight. Worst case = your rescuers should easily find your body floating at the surface.

 

[g1138]

If you're neutral wherever the problem starts then you're not negative at all. Forget about the wetsuit compression, your weights (dumpable or not), you're neutral so there's no reason to dump lead unless your legs are broken. Just one kick up and you're already starting your easy ascent. All that's left is to dump air on the way to the surface.

The consensus on what amount to dump is dependable on your risk VS reward factor for each individual. We could all try to plot our personal preferences for you and then give you a percentage, but the truth is no one can be bothered or has ever been bothered to do that.
So......I don't think you're going to get a consensual answer.