Self-propagating negative bouyancy

[Marvintpa]

Earlier in this thread someone put forth the ideas that:
1) inflating both bladders in a wing at the same time would not increase buoyancy and was therefore erroneous to the discussion
2) ear pain caused by a rapid descent would cause the dive to be aborted, and this entire issue was just journalistic sensationalism

To respond:

1) A diver wearing doubles who has correctly configured them to operate from separate posts will manage to fully inflate FASTER than someone inflating only one bladder (the final amount of buoyancy will be the same, but the speed to attain it will be different) and the issue in question was arresting descent in a hurry, which this would address.

2) Free divers in the "unlimited" class ride a weighted sled to 500' at speeds greater than it's possible to descend with scuba gear, which would seem to suggest that clearing ears is not an issue (sure, some freedivers purposefully flood their siunuses to avoid some of these issues, but not all)

 

[Scuba_Vixen]

is indeed possible, and would be faster ...by what, 3 or 4 seconds to max inflation!.. A whopping 10 feet max at a 120' /min plus descent rate. .. And at a risk of damaging the bc and failure..only room for 1 fully inflated bladdder in there, how do you know when both are half inflated and it's time to stop. The notion of inflating both simultaneously for any legitamate reason is just Nuts. No agency that condones redundant bladders also condones simultaneous inflation, nor do the manufacturers.


"2) Free divers in the "unlimited" class ride a weighted sled to 500' at speeds greater than it's possible to descend with scuba gear, which would seem to suggest that clearing ears is not an issue (sure, some freedivers purposefully flood their siunuses to avoid some of these issues, but not all) "

Gimme a break, that's apples to oranges if ever there was such a comparison, the Darwin candidates in the story weren't even as wise or experienced as the average AOW diver. I bet there's not a diver here that doesn't have to clear going down, and experience increasing pain as they descend if they fail to do so.

The story just isn't good journalism, it's a tabloid scare feature, and has no place in a dive publication.


Darlene

 

[japan-diver]

Since one body was never recovered we don't know exactly what happened and the writer tried to propose a possible scenario. I have certitifed many students who could equalize quite easily and could drop very quickly. Also freedive quite a bit - though not to 400 ft - and can drop at over 120ft/min and have chased a few divers over edges and down at faster rates while divemastering.

A new diver such as these decending rapidly and being narced may not even notice pain in the ears, so ear pain stopping the dive isn't a valid theory.

We had a similar accident here a few years ago that resulted in a couple dead and couple bent while new divers tried to find the bottom of one of the dead walls around here. Divers plummenting a couple aborted but two continued down and bodies were found at over 150ft and 200ft.

The story I think was presented to make a point and that I think it did, I don't find the facts that out of porportion to what may have actually happened.

 

[BrendanC]

Dual bladders, dual inflators, dual posts. . .it always seemed way too complicated to me. I've used a wet suit and doubles and single wing and been fine. Of course, the doubles have to be aluminum 80's; there's no way you'd get me in a set of big steel doubles with a wetsuit on.

I've seen some pretty interesting failures with BC's. One of my favorites was the first stage failure that led to a BC autofilling. With a single bladder and single inflator, the solution was simple: dump gas out of the lower dump while shutting down the reg that failed. Add another bladder, inflator, and butt dump, and you've got a heck of problem on your hands.

I like the idea of having my buoyancy in one, easy to adjust location. If my wing fails, I have my weight distributed so I can drop some lead and swim on up.

B.

 

[indiana*joe]

I have been amused at reading all of the sentiment expressed. I was looking for the facts. I have not read anyone stating that an object is any less bouyant at depth (barring compressed air spaces) so, the only factors is compression. So, divermasterB appeared to be focused on what was actually happening. The suit compresses, giving a bouyancy swing of 15+ lbs. The other "facts" are that the air in the BC compresses at the diver descends so, in theory, a "weak" inflator assembly "could" find itself in a situation where the air being delivered to the BC (at ambient pressure as one post made it plain) is actually being compressed as fast or faster than the next volume of air can fill the space caused by the compression. That sounds too confusing.

A. 1 ml of air is placed in the BC at ambient pressure creating 1ml of volume.
B. Diver descends 10-20 ft
C. Air delivered in A. is compressed to space X
D. Repeat cycle....

Does air delivered to the BC at 100fsw at ambient pressure compress to 1/2 its vollume at 133fsw? Does the little chart we learned about atmospheres reset as we are starting over each time with air at ambient pressure? If that is the case the above situation becomes more real. Otherwise, the air would not show any noticable decrease in volume over the 33' from 100 to 133.

So, the only 2 factors possible seem to be compression of the air in the BC and compression of any protective gear (BC or Dry - Drysuits are tough to inflate orally!)

Finally, to all who says that they would never attempt such a dive or say it would never happen to them, are only touting fate. There are such things as accidents, problems created by the situation, equipment failures, buddy problems, ad infinitum.....

I don't believe that anyone who finds themselves in a bad situation planned it. I hope to look at the reasons why things happen differently underwater than they do up here on land so if I happen to find myself there, I might have a better chance to react in a sensible and informed way.

Dive safe kids!

 

[joe_at_subtidal]

The science is:
If we have a volume of 40 cuft at the surface, we'll have 10 cuft at 99' (4 ATAs) and 8 cuft at 132' (5 ata's), so the compression from is less than half.

JJT

 

[Scuba_Vixen]

thrown in for good measure, Everyone basically concured that the concept of "self perpetuating negative bouyancy" is a total crock.

The "weak inflator" thing holds no logic, Any inflator fills the bc as fast at any depth as fast as it does at the surface. If it doesn't seem fast enough, you can do it orally, doing it orally at 200' is no harder than orally at the surface. The rate of inflation is NOT depth dependant.



"I like the idea of having my buoyancy in one, easy to adjust location. If my wing fails, I have my weight distributed so I can drop some lead and swim on up."

OK, so you drop some weight and start swimming up...Just about the point you get to your deco stops, your suit starts uncompressing and you're facing that polaris missle ascent. I'll have to pass on that skill.

If you have a blow-by on an inflator (bad 1st stage/stuck inflator/whatever), just disconnect the hose to it, With a redundant bladder, reach back for the redundant inflator (it's on the other reg, (dump the filled bladder) and do a safe, normal, in-control ascent on the redundant bladder. It may be a bit inconvenient, but it beats the snot out of a trip to the chamber.



Safe diving,


Darlene

 

[DeepTechScuba]

There are lots of plusses to a redundant B/C inflator system. Problem is, you do not see a lot of them available in anything but tech gear. Or special orders.

We have gone through the popular evolution of the redundant second stage, call it what you will: Air2, octo, or full service additional 2nd stage.

We have not yet gone through the evolution to a completely redundant B/C inflator system with a separate air bladder for it.

Zeagle makes one, called The Big Bertha. Its lift capacity is 85 lbs. It is interchangable and fits onto any Zeagle B/C system.

I love mine!

 

[Dryglove]

DeepTechScuba once bubbled...

Zeagle makes one, called The Big Bertha. Its lift capacity is 85 lbs. It is interchangable and fits onto any Zeagle B/C system.

Why do you need a bladder with 85lbs of lift. I suppose you dive this with a single tank too.

 

[ew1usnr]

****Anti-flame preliminary disclaimer: I am not an expert and am not challenging anyone's expertise. The situation I am discussing applies to open water, non-technical diving. The concept discussed may not be in accordance with the philosophies of some or all dive instruction organizations."*****

Scuba_Vixen once bubbled...
If you have a blow-by on an inflator (bad 1st stage/stuck inflator/whatever), just disconnect the hose to it.

My inflator hose connects and disconnects very easy when there is no pressure in it. When it is pressurized its harder to get off. If the inflator is stuck in the fill position, and I'm going up while trying to yank the dump valve while also trying to get the disconnect off at the same time, its not fun, and its not especially simple.

Scuba_Vixen once bubbled...
Any inflator fills the bc as fast at any depth as fast as it does at the surface. If it doesn't seem fast enough, you can do it orally, doing it orally at 200' is no harder than orally at the surface.

Exactly. When I was certified we used manual inflators. Shortly after that I tried my first auto-inflator and thought that it was terrific. I've used them (for the most part) ever since. Now, at least where I live, you can't go on a dive boat without one. But, auto-inflators aren't perfect. I've seen mine and other people's screw up on a number of instances. They require maintenance, and even so, they can still malfunction. I am beginning to wonder if the risk that auto-inflators represent (through run-away inflation ) outweighs their convenience. If the inflator plug is capped, the inflator can be used manually, the inflator hose can be done away with, and the risk of inflator malfuntion is eliminated. If I want to go on a boat dive I can always put the inflator plug back in. My question is: Should the auto-inflator be regarded as an optional convenience item, or as a necessary safety item whose benefits far exceed its added liabilities?