Catastrophic Failure - How much weight to drop? Theoretical Discussion

[dumpsterDiver]

You are probably not going to have a Bc failure and run out of gas both at the same time. Recreational diving doesn't normally deal too well with independent, simultaneous failures. Right? So.......

Failure mod number 1: Complete loss of BC function

Response: Be able to ditch enough lead so that you can initiate an ascent by swimming up in a negatively bouyant condition. At some portion of the ascent, you will reach neutral buoyancy, which would allow you to pause your ascent, maybe rest a little and then the reaminder of your ascent would be floating up,,, i would do it in the spread eagle, (upside down skydiver position) which generates tons of drag, causes zero exertion and allows you to see the surface approaching.... Not sure exactly how much lead that is. This is the concept of a balanced rig... a rig you can swim up after ditching some lead..

You can estimate the required ditchable lead, by experimentation at depth.. empty the BC, remove the weight belt and start to remove weights until you feel like you are light enough to easily swim up... that is how much lead you would need to be able to ditch.. depends on your strength and swimming ability..If you are strong, you could ditch less lead and this would allow you to make a more controlled ascent.

Failure mod Number 2: Complete and instantaneous loss of scuba unit function, solo with no help at 100 feet. Presumably you would be neutral when this occurs...(in other words the BC is not compromised) .. Begin to swim up, which will make the diver increasingly buouyant, exhale gently, and as the ascent speed begiins to feel sufficient, stop kicking, rest and ride the bouyant ascent up. If it feels like you aren't going to make it, ditch some lead....If it is going too easy/too fast.. dump some air from the BC on the ascent.

I have had failure number 1 occur twice and number 2 never (although I have run very low on air on the bottom before).

Now if you want to be able to handle failure 1 and 2 at the same time, then you really should be considering using a redundant supply of air...like a pony bottle.

 

[rsingler]

Bingo, g1138! You seem to have the same opinion I do.

I concluded long ago (and wanted to revisit the issue now that I'm diving again), that weight belts that you can "drop in a hurry" are the wrong solution to an OOA problem!

If you have a first-stage catastrophic failure, you don't need to drop anything, but just make a controlled emergency ascent with the same flotation/weight neutrality that you've had all along, by venting your BC. Exactly right, g1138.

If you have a catastrophic BC failure, then you still have air, and you have all the time in the world (OK, not ALL the time in the world) to find your buddy, and get a lift to a depth where you're less negative.

And if someone says, "but what about BOTH failing!?" then I'll just suggest that they're in the wrong sport, or waaay too odds-sensitive.

What have I forgotten that makes immediately droppable weight systems so essential to be able to find in the dark with your eyes closed and upside down? And once you've dropped all that weight down deep, how do you not get bent in the last 33 feet?

Rob

---------- Post added February 17th, 2013 at 05:40 PM ----------

Sorry, dumpsterDiver! We were writing at the same time, and you answered my BC failure question. Although after your experiment, the amount you find to drop will be directly proportional to depth at which you can swim up the rest. But if it's more than a few pounds dropped, I'm still concerned that my final ascent rate will make bubbles, spreadeagled or trying to "swim down" or whatever.

I guess I'm not going to worry too much about droppable weight, once I get it distributed around my rig. A few pounds. And a pony.
It's not just little girls that should get a pony for their birthday!

Rob

 

[flots am]

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?

If you're talking about "me" personally, it wouldn't happen because I never dive with just a single tank and I certainly don't do deep cold dives in a wetsuit.

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?

However if you're talking in generalities, I'd drop "just enough". I wear a weight harness and it's not a big deal to pull up the flap and pull out 5 pounds and see if that helps.If not, I can pull out another 5 pounds. If necessary, I can pull out another 5 or ditch the whole thing.

Another aspect is that your hypothetical diver is just kicking Darwin in the balls yelling "Wake up! I'm here." With a thick wetsuit a negative tank, almost 30 pounds of lead, no redundant buoyancy, cold water, poor buddy skills and no redundant gas, there have been a bunch of bad decisions already. At the end of the day, if you do enough stuff wrong, you can get hurt or die.

Also because the dive just started, it should be possible to simply ditch all the weight (even all the equipment) and surface. There should be very little nitrogen buildup and surfacing should be pretty safe.

flots.

 

[MaxBottomtime]

Ditchable weight is one of those ideas in diving that had little thought before implementing it. A 160 pound diver in a 7mm suit should never need more than 15-16 pounds, especially with a heavy tank. If you are adding air to your BC on the way down, you have way more than you need. You should never have more than you could swim up at any point in a dive.

 

[rsingler]

Jeez, flots! I may be a brand new poster, but I've read a lot on SB in the last month. Your posts read the same in every thread!
No need to stir the pot here about "bad decisions."
What is it about "academic exercise" and "theoretical discussion" that you didn't get?
You gotta be kidding me: Darwin?

No, I just wanted to see an exchange about weight removal, buoyancy and ascent rates. And I got it. Thanks to all!

And that is the last time I will likely ever reply to flots on SB.

 

[toddster541]

The only time I remember considering dropping weight was on a wall dive when I got caught in a strong down-rip. If I hadn't been able to swim (perpendicular) out of it by the time I passed 100 feet, I was going to try to arrest myself against the rock wall with my dive tool and see about dropping some weight, if I still couldn't ascend. However, I did swim out of it.

 

[DevonDiver]

My idea of a worst-case scenario is probably a medical related issue. Any event that can immobilize the diver and necessitate an immediate ascent to the surface. In such an event, we can imagine the diver being incapable of any complex motor skills - depriving them of the ability to manipulate equipment or fin upwards.

In this scenario, assuming no buddy assistance, the only recourse may be buoyant (not controlled) emergency ascent. The diver should be able to dump sufficient weight to cause just enough positive buoyancy to take them to the surface. Once at the surface, they should be able to dump further weight to ensure ample surface flotation; ideally protecting their airway should they be rendered unconscious.

This scenario does lend itself to a notion of dividing ballast/dumpable weight into two tranches; that dumped to permit a gradual buoyant ascent...and that dumped to ensure positive surface flotation.

Defining an 'ideal' amount of weight to dump for ascent is an individual issue; depending upon the overall buoyancy factors affecting the diver (exposure protection, cylinder type/s etc). The first step would be to determine the overall necessary weighting - typically calculated by adjusting weight whilst maintaining a safety stop with a near-empty tank and no air in the BCD. Once an accurate amount of overall weighting is determined, it is possible to divide this total into two separately dumpable amounts.

The weight dumped at depth (for ascent) simply needs to permit positive buoyancy at any given stage of a dive. The diver is most negatively buoyant at the outset of a dive, when reaching bottom depth (max exposure suit buoyancy loss) plus maximum weight of gas in their cylinder/s. It would be relatively easy for a diver to conduct a separate weighting assessment at this stage of a dive, to determine how much weight needed to be dumped to create positive buoyancy. Simply reach bottom depth with full cylinder/s, empty the BCD and progressively remove weight in small increments until slight positive buoyancy is attained.

Obviously, at a later stage in the dive, dumping that same amount of weight would cause a greater buoyancy swing and faster ascent. Hardly ideal, but we are considering a worst-case scenario - and DCS risk is preferable to drowning. The only alternative would be to have more than two tranches of weight... to divide weight into multiple dumpable packages - where an appropriate amount/number of packages could be dumped depending on the phase of the dive - that approach seems self-defeating when considering a scenario in which the diver is significantly immobilised though...

 

[CT-Rich]

I know the question was about ditching weight at depth and I gave my answer too that. But I would like to interject a bit of reality to the discussion. most of the cases I have heard about where a diver intentionally dumped weight at depth was when they rolled into the water with their air off. in which case the more the merrier. Most of the other instances were either accidental, or they dumped weight on the surface in a panic situation... Just because you can do something doesn't mean you should. Getting bent is not the big worry (if you are making deco dives you should have a proper back-up). Barotrauma and an embolism are the more likely killers. If you feel like you are going to pass out, that would be really the only time I could see dumping lead as the smart choice. This is extremely rare that you would have much warning on this. Dumping weight should almost never be considered a solution to a problem and I would recommend against it. An uncontrolled ascent is asking for trouble, probably more than you are escaping. Should you know what to do? Yes, but mostly for the accidental dumps, not an intentional one.

the only dump you should take at depth is called a Warhammer....

 

[rjack321]

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.

Bent is just like any other sports injury, its not as bad as your phobia suggests... I doubt you'd have that much trouble swimming up ~25lbs. But ditching your whole belt wouldn't be that big a deal either. Just exhale the whole way.

 

[eelnoraa]

The number for LP95 isn't correct. It is -3lb empty, -10lb full. So you are carrying 7lb of gas at the beginning of the dive. That also means you should be over weighted by 7lb at 15ft at the beginning of the dive with full tank. Any less, you won't be able to hold a 15 ft safety stop at the end of the dive with nearly empty tank.

Now, you said your wetsuit is 28lb positive on surface and 3.5lb positive at 100ft. Now lets assume at 15ft, your wetsuit will be 25lb positive. At 100ft, you wetsuit goes down to +3.5lb. With full tank at 100ft, you -28.5lb. How much weight should you ditch if you have a total BC failure?? I really don't know. If you ditch too little, you still won't be able to swim up. If you ditch until you can swing up, you will have uncontrolled ascent at some point. The main issue here the change in buoyance of your wetsuit is too large. Your setup is unbalanced. You will need a working BC to compensate the change.