Self-propagating negative bouyancy

[indiana*joe]

In an article in Scuba Diving mag, they mentioned as a problem with deep dives a concept I have heard before, but have yet to understand the physics of. Namely Self-propagating negative bouyancy at depth.

http://www.scubadiving.com/training/lessons/200307lessons.shtml

Lessons for Life
1 Deep diving requires proper training, planning, topside support and gear. Divers need a larger gas supply, high-performance regulators and high-capacity BCs.

2 Extra Weight is dangerous on a deep dive. The compression of wetsuits and BCs compounds negative buoyancy. At about 200 feet, negative buoyancy is self-propagating.

3 Rapid Descents aggravate nitrogen narcosis. The maximum safe descent rate is 70 feet per minute. Pauses allow the body to compensate for the increasing pressure.


Can anyone explain what is meant by the last sentence of item #2?

 

[Diver0001]

indiana*joe once bubbled...
In an article in Scuba Diving mag, they mentioned as a problem with deep dives a concept I have heard before, but have yet to understand the physics of. Namely Self-propagating negative bouyancy at depth.

http://www.scubadiving.com/training/lessons/200307lessons.shtml

Lessons for Life
1 Deep diving requires proper training, planning, topside support and gear. Divers need a larger gas supply, high-performance regulators and high-capacity BCs.

2 Extra Weight is dangerous on a deep dive. The compression of wetsuits and BCs compounds negative buoyancy. At about 200 feet, negative buoyancy is self-propagating.

3 Rapid Descents aggravate nitrogen narcosis. The maximum safe descent rate is 70 feet per minute. Pauses allow the body to compensate for the increasing pressure.


Can anyone explain what is meant by the last sentence of item #2?

It probably means that a typical inflator can't put air in a BCD fast enough to overcome the increasing compression of a sufficiently quick descent at that depth. I dno't know how fast you would need to be descending for that to happen but I would venture that it's very fast. I wouldn't lose any sleep over it.

R..

 

[CD_in_Chitown]

Consider this, a buddy of mine descending with us to 110fsw in blue water decided to race us down and doesn't start adding air until 100fsw or so. When he got to the bottom it looked like a bomb had gone off (mushroom cloud) because you can't wait that late to add air. That's rate of descent problems, I'm not clear what the 200fsw mark means in this context but the rate at which you cannot overcome negative buoyancy would have to be shallower than that I'd think.

Now on a hard-bottom dive in safe rec. depths that's not such an issue (you will stop at the bottom). I had another buddy descend on a wall in Honduras (swears he was adding air by 75fsw) who couldn't stop his descent until 153fsw, with no bottom in site. I wouldn't lose any sleep over it, but I'm aware that it does have pertinance to me.

Chris

 

[pdoege]

As you go deeper your wetsuit and BC compress and lose bouyancy, so you become more negative and go deeper, wash rinse repeat.

200 feet is sort of deep to be dive bombing anyway.

I did talk to a guy once who used a dual bladder wing and a drysuit. He claimed to be able to dive bomb down to 220' and then hit the brakes by inflating all three items at once.

Silly thing to do, but there you have it.

Peter

 

[indiana*joe]

Assuming pressure in the bc to be equal to ambient pressure, at 200fsw does it take any more volume (bc) to get to neutral than at 100fsw. Beyond 60fsw I would assume that even a 7mm is not going to change significantly in bouyancy and a drysuit should maintain shape (because of added air) in either situation. Or even assume skin (brrrrrrrrrr!) Everything else being equal - only a change in bc volume - does it take more volume to counter the weight of the object at depth or not? Do you have to displace the entire column above the object or just the volume of the object itself. (Must have been asleep in physics that day.....)

 

[divermasterB]

indiana*joe once bubbled...
Beyond 60fsw I would assume that even a 7mm is not going to change significantly in bouyancy and

A 7mm farmer john can add about 15 lbs of buoyancy over a naked diver (takes 15 lbs to sink just the suit) at the surface. Roughly you could say it takes 7 lbs to sink at 33, 5 lbs at 66, and 3.5 at 100 ft.

So, a diver diving to 100 ft, in a 7mm and not adding air to the bc will need to compensate for an 11 lb shift in buoyancy over that 100 ft.

All of these numbers are of course approximate.

a drysuit should maintain shape (because of added air) in either situation.

That is assuming adding air, plus, the affects I am talking about are even more pronounced in a drysuit, if the diver does not add air.

Or even assume skin (brrrrrrrrrr!) Everything else being equal - only a change in bc volume - does it take more volume to counter the weight of the object at depth or not? Do you have to displace the entire column above the object or just the volume of the object itself. (Must have been asleep in physics that day.....)

The weight of an object in water is it's weight on land minus it's buoyancy (determined by calculating Volume (ft3) * 64 lbs/ft3).

To make that neutral you will need a volume of air to displace a weight of water equal to the neutral weight of the object. So we know sea water is 64 lbs/ft3 you can calculate it...

 

[Diver0001]

indiana*joe once bubbled...
Assuming pressure in the bc to be equal to ambient pressure, at 200fsw does it take any more volume (bc) to get to neutral than at 100fsw. Beyond 60fsw I would assume that even a 7mm is not going to change significantly in bouyancy and a drysuit should maintain shape (because of added air) in either situation. Or even assume skin (brrrrrrrrrr!) Everything else being equal - only a change in bc volume - does it take more volume to counter the weight of the object at depth or not? Do you have to displace the entire column above the object or just the volume of the object itself. (Must have been asleep in physics that day.....)

Well, in a way yes. If you have a kg of negative buoyancy (equal to one litre of water more or less) at the surface then you have to add 1 litre of air to your BCD to counter it. At 200 ft that's about what..... 7 ata or so .... so even though you have to add 1 litre of air to your bcd, it's compressed so you're acutally needing to pump 7 litres of air out of your tank and into your bcd.

Is this what you're after?

R..

 

[DeepTechScuba]

Diving deeper than 100 ft probably is not wise without technical gear, in most divers' opinions.

Diving deeper than 170 ft is definitely not wise without technical gear, in my opinion.

Wetsuits are definitely NOT technical gear. Diving deep with a wetsuit on is russian roulette, if not suicide. If you really need an exposure suit where you are going on a really deep dive, then you should be in a drysuit.

Self-perpetuating negative buoyancy, coined by the author cited, sounds like he means that no matter what you do from a given ponit on, you will be negatively buoyant. The issue would be (1) being able to quickly inflate your B/C to stop your descent, or (2) having enough B/C lift capacity to make you positively buoyant when the suit has been squeezed by ambient pressure to zero buoyancy, or (3) having enough air or other gas left in your tank(s) to be able inflate your B/C enough to stop your descent.

First step, check the lift rating on your B/C. It needs to equal or exceed all the negative buoyant weight on your diving rig. The negative weight consists of the lead you are wearing, plus the underwater weight of your regs, valves, manifold, lights, gauges, and other gear, as well as the negative buoyancy of the compressed air in your tanks. A fully rigged technical diver with 4 scuba tanks on has about 15 lbs of additional negative buoyancy at the beginning of a dive from the diving gasses alone.

Step two, check the inflation speed of your LP power inflator. These are rated to different rates of inflation, and only technical B/Cs like OMS deliver enogh air quickly enough to perform quickly at deep depths. (I cannot speak for Halcyon LP power inflators since I do not own one. I am sure JJ thought of this, so I trust his gear is safe at deep depths. But I do know that JJ does not like double-inflators and double bladders, because they are an addtional failure point.)

If you dive too deep than you were trained to dive, you will discover yourself in a perpetuating negative buoyancy situation that you cannot do anything about, if you go too deep.

Some tech divers like to rocket down during their descent in order to make the most of their bottom time. This is a trick best practiced with a bottom underneath you somewhere. If there is no bottom, such as when you are diving a wall, then that is NOT a good time to practice this trick.

A 75 to 100 lb OMS B/C with high performance LP powerinflators (notice I said inflatorS ... S ... as in TWO of them) and twin air bladders inside the wing should allow sufficiently fast inflation to stop any reasonable descent in mid-water.

When I descend, I am normally descending an anchor line or a down-line, and by equalizing my drysuit for suit squeeze I can slow down my descent enough to be able to put on the brakes with my B/C as I get within 10 or 20 ft of my target. But my technical buddies have usually already gotten down there, whether by dive bombing or whatnot, and they are already waiting for me.

Some technical instructors teach you to be able to inflate both your drysuit and your B/C with one hand.

You have to do the math before you go down. And you have to have the right equipment on too.

Or else Murphy's Law and Darwin's principles of Natural Selection will combine to make sure that you won't live to see the sunrise.

 

[Don Burke]

indiana*joe once bubbled... In an article in Scuba Diving mag, they mentioned as a problem with deep dives a concept I have heard before, but have yet to understand the physics of. Namely Self-propagating negative bouyancy at depth.

http://www.scubadiving.com/training/lessons/200307lessons.shtml

Lessons for Life
1 Deep diving requires proper training, planning, topside support and gear. Divers need a larger gas supply, high-performance regulators and high-capacity BCs.

2 Extra Weight is dangerous on a deep dive. The compression of wetsuits and BCs compounds negative buoyancy. At about 200 feet, negative buoyancy is self-propagating.

3 Rapid Descents aggravate nitrogen narcosis. The maximum safe descent rate is 70 feet per minute. Pauses allow the body to compensate for the increasing pressure.


Can anyone explain what is meant by the last sentence of item #2?

I think the writer is trying to say that the deeper you go, the more things compress. Every object put in the ocean does this to some extent. If you don't keep adding gas to the BC on the way down, you'll accelerate until you slam into the bottom, go below where the IP of your regulator can overcome sea pressure, ox tox, or get so narked you forget how to operate your inflator.

I don't know what is so magical about 200 feet. About half of the buoyancy loss is in the top 33 feet.

At 200 feet, you are dealing with about 88psig of sea pressure or 7atm, which will take quite a bit of the buoyancy out of anything made of neoprene. The volume of gas (as expressed in SCF)required to make substantial changes in what lift the BC is providing increases with sea pressure, so it would be best to be adjusting your BC all the way down.

I disagree with the high capacity BC statement.

Your BC needs to overcome the gas in your tanks, neoprene compression and the full in-water weight of any stage/deco bottles you are carrying.

If you are wearing double 131s, that's about 16 pounds. Stage bottles will only add about two pounds each. I'll call it 20 pounds so far.

If you are wearing a wetsuit that is 20 pounds positive at the surface, it will be a little more than 10 pounds positive at 33 feet and a little more than 5 pounds positive at 100 feet (foamed neoprene doesn't compress quite as much as a free bubble). That means it only has five pounds to lose below 100 feet, so "larger BC for deep" doesn't wash. If it needs to be big, it needs to be big shallow too.

Since you weight for a shallow depth, say 10 feet, the wetsuit will be about 16 pounds positive, so the change is actually from 16 pounds at 10 feet to about 3 pounds at 200 feet for a swing of 13 pounds.

20 plus 13 comes to 33. Allowing for safety factor, a 55 pound wing will do it for this dive even if your stages are quite a bit heavier than the 2 pounds I used.

Incidentally, if you held a gun to my head and told me to make this dive, I'd tell you to go ahead and shoot.

 

[Scuba_Vixen]

That in a dual bladder BC, you can inflate both bladders for more bouyancy! That's Absolutely False. Either bladder can inflate to fill the entire space within the wing. There are 2 of them for redundancy only, incase one fails, or the reg to that inflator has to be valved off. If you inflate them both, you'll likely rip the wing apart. OK next issue..

You're power inflator inflates your wing at the same rate (in effective volume per time) no matter what depth..... That's what the regulator is for..it supplies air at the ambient pressure, plus the IP (usually 135# to 150#) This means if you're 10# negative at 30' or 10# negative at 300', your bc will correct in the same time at either depth. If you're at 200' and need 7 times the air, you get it because the reg is giving you 7 times the surface pressure, plus the ip. OK, next issue...

The whole story is an excercise in sensationalism, speculation, and scare tactic. Any modern regulator in reasonable condition will work at 200'. He couldn't have equalised if he was descending as fast as the story tells, and would have stopped the descent because of the pain early on. Here they are, going down like cannon balls, pain like you could only imagine, and they keep on going, ... gimme a break.....You can make a controled descent to 200' and spend 4 minutes of NDL and work back up a wall maintaining a no stop dive, and still swim around the reef on top the wall for a while, all on 1 Al 80. That self propagating negative bouyancy thing while it may sound impressive and profound, is poetic license for the story, but otherwise pure crap. If there were anything to it, I couldn't be posting this. If wetsuits are suicide for deep diving, then hummingbirds and bumblebees can't fly, and I'm posting from beyond.

I'm not suggesting they weren't doing something stupid, or that it was ok to do, it's just that No One is that effing stupid. I mean Darwin Award candidates here......Which is what makes me think the story is either fiction to scare you out of doing dumb *shinola*, or an actual accident that's been turned into a full blown "urban legend".

Sorry for getting into such a rant, I just believe divers need clear facts and true information, and they can think for themselves. If you can't dazzle me with brilliance, don't try to befuddle me with B___ l S___t ......



Darlene