Buoyancy Issues

[PerroneFord]

LOL!!!

Any wonder why I am apprehensive about diving with you! Wanna dive a U-Boat?

 

[Thalassamania]

LOL!!!

Any wonder why I am apprehensive about diving with you! Wanna dive a U-Boat?

Yeah, let's get wrecked. I'll even wear that suit.

 

[ClayJar]

I have a friend at work who is about to start his OW classes. Apparently, the photos, videos, and stories I've had are all too much for him. He and his fiancee are anxiously awaiting the class.

Anyway, I've been giving him little tidbits of knowledge to give him a head start. One of those tidbits is a little physics demonstration I whipped up. I made two Cartesian divers out of bottles containing small plastic tubes capped at one end and weighted with paperclips. One has two small paperclips and a small bubble, and the other has four large paperclips and a much larger bubble. Both bubbles are sized so neutral buoyancy only requires gripping the bottles with moderate pressure.

First, I handed over the large bubble diver, and I had him do his best to hold the diver at a line mid-water. After he had it down, I then swapped him over to the small bubble diver and had him repeat the excercise. Then I had him tell me which one was easier to control (by far, the small bubble diver), and I asked if he could figure out why. Together (hehe), we arrived at the conclusion that the larger bubble changed volume in response to changes in his grip pressure much more than the smaller bubble, as it was, well, larger.

I then explained that if you're overweighted, like the large bubble diver was, you, too, will have a much larger volume of air in your BC than you'd need if you were properly weighted, like the small bubble diver. That extra air will expand and contract more with changes in pressure, i.e. depth, and will make it that much harder to keep yourself stable in the water.

Anyway, it was an interesting little physics lesson for him, and he now knows that he doesn't want to be overweighted and *why*.

 

[TSandM]

What a COOL idea for a demonstration! Ever thought about teaching?

 

[RJP]

LOL!!!

Any wonder why I am apprehensive about diving with you! Wanna dive a U-Boat?

But wait, aren't most U-Boats plastered to the bottom?

 

[Thalassamania]

… That extra air will expand and contract more with changes in pressure, i.e. depth, and will make it that much harder to keep yourself stable in the water.

Anyway, it was an interesting little physics lesson for him, and he now knows that he doesn't want to be overweighted and *why*.

Yes! I need to make a pair tonight. Anyone know how to make the divers horizontal?

 

[ClayJar]

What a COOL idea for a demonstration! Ever thought about teaching?

Actually, quite a bit. In fact, one of my diving goals is to become an instructor, as I simply enjoy helping people reach that "Aha!" moment where they not only realize what I'm teaching them, but they realize why they want to learn it.

(I've seen far too many people who have no clue that their diving is so much harder than it should be. Trim and buoyancy are such fundamentals of diving, not emphasizing them is teaching people how to *survive* underwater, not teaching them how to *dive*.)

Yes! I need to make a pair tonight. Anyone know how to make the divers horizontal?

Well, it's all static balancing... If you used a slightly curved tube (shaped like a very flat rainbow) with a dense weight in the center (under the rainbow), it may be possible to get a stable configuration that appears to be horizontal. Or you could just fake it by having a vertical tube inside a plastic horizontal diver.

 

[MikeFerrara]

Buoyancy control is difficult for students to learn and thus it is difficult for instructors to teach. It takes time and care; often more time than is available in a short class and more care than many instructors are willing to give. PADI’s solution to this has been to require instructors to teach a completely useless and counterproductive “skill” the fin pivot. The only good thing about a fin pivot is that it is easier to teach than real buoyancy control. The PADI program has you learn this “skill” that you will never use and then learn real buoyancy control in a later class, the so called “Peak Performance Buoyancy” class. I fell that the fin pivot just develops bad habits in divers and puts them in a frame of kinesthetic sense that they drop back into whenever they are stressed or confused. I seen many new divers, in mid water, try and do a new skill or get stressed by something and revert to a knees down, head up attitude, loose all sense of buoyancy control and start to sink. If what we call the Law of Primacy in the Ed biz, what you learn first is often what you learn best. It’s too late for you to do anything about this.

What you describe is often the result of the way these skills are commonly taught. Right here in this thread we've had posters (instructors?) recommend that those having trouble doing fin pivots weight their ankles. Why encourage a diver who is already trimmed to get out of trim? BTW, a diver trimmed for horizontal can do a fin pivot but it isn't easy. You simple keep your fins on the bottom by arching your back, though you are never really neutral rather just using body position to change orientation.

After being taught to be trimmed head up and not taught what needs to change to fix it, divers end up in head up trim when they get out trying to really dive. In order to move forward without also ascending, they need to be neutral. If they stop kiking they will go vertical and sink, unless the add air to the bc. This is something we see in the water ever day. Besides, since we go through life head up, it's natural to want to assume that position...and it works great for walking. It takes practice for divers to get away from wanting to be vertical.

This is the only place that I disagree with you and Lynne. There are times when I have to hover upright, at various angles and even upside down (e.g., collecting delicate jellies). I do not dive like a deep submarine staying level all the time, my body moves in the direction that my head leads and for me, true trim and balance is the ability to be able to hold most any conceivable attitude in that water, not just the horizontal one that you can trim your rig to stabilize at.

Yes. If (and using popular terms) our center of buoyancy and center of gravity are in the exact same place, we can assume and maintain any position without effort because those apposing forces are always in line. It's not always easy to get the mechanical aspect this perfect because things change throughout the dive that effect those centers...our tank gets more buoyant as it empties and our suit compresses as we go deeper. So, we get as close as we can and compensate for the changes and lack of perfect alignment with body position...position of the legs, arms, head, shoulders, the arch in the back, ect.

IME divers have a far easier time when these principles are explained, demonstrated and them practiced.

 

[SparticleBrane]

...I still don't know what the heck a fin pivot is. Whatever it is, it sounds useless.

 

[verybaddiver]

I think the biggest problem we have Mike, is half the instructors out there can't do this stuff.
The PADI ppb video has dangling gauges and large consoles (which is lol)
The PADI ow has all skills performed on the bottom of the pool
(I'm padi trained I dont know other agencies materials)

But i'm thankful that PADI got me introduced to scuba, for a nominal fee, whereas a more expensive course would (probably) of kept me away.


Fin pivot

Is exactly what it says on the box. You pivot on the tips of your fins, using breath control to allow your upper body to raise and fall with each breath. Useful in a very basic level, but buoyancy needs to be taught mid water, and this is a poor excuse for not bothering to teach what's needed