Water in BCD

[Steve_C]

Got blown out in Key largo, and wanted to dive so went over to Jules and buddied up with an instructor who doing some practice with a new OW student of his so that Jules would let me in. The women had buoyancy issues. She got down with no problem but then would soon float up. He kept adding more and more weight.

Totally bogus. If you can get down, then you do not need more weight. Adding more weight was just doing the obvious. Making her add more air to the BCD and becoming even less stable.

Perfect example why any toning down of the basic physics is not a good idea in MHO.

 

[oldschoolto]

Oh, it gets crazier!!! Just ask about split-fins, BP/Ws, smoking on dive boats, cave diving, dive etiquette, wearing gloves, using reef hooks, 3mm vs 5mm, DIR, etc, etc, etc.

You forgot Pee in a wetsuit.....
Jim....

 

[TSandM]

Well, I think for the most part, diving is a physical skill, and the basic rules are pretty simple. Don't go too deep or stay too long; watch your gas, and control where you are in the water column. A deep intellectual understanding of the physics and physiology involved isn't necessary to do most recreational diving. I DO believe that one should not teach a subject one doesn't understand, but the agencies that certify instructors obviously put a much higher value on the ability to go through the physical process than on the theory, as they probably should.

One of the things that REALLY pleased me when I got involved with GUE was that the instructional materials were written for educated adults. They may not always be well proofread or edited, but they aren't aimed at fourth graders. However, fourth graders CAN get certified (not by GUE, though) so the agencies that permit this have to write their manuals so that children can read and understand them, which seriously limits the depth into which they can go on the science. I wish they would set a higher standard for instructors, though.

 

[TMHeimer]

Side note-- If you are diving with a tank that has a little water in it you are heavier because unlike a BCD, a tank is not a flexible container. Correct?

 

[Crass3000]

Side note-- If you are diving with a tank that has a little water in it you are heavier because unlike a BCD, a tank is not a flexible container. Correct?

I don't think so. The water in the tank is basically the same density given it's (for the matter of this point) not compressible so there should be no difference. I could be wrong but I think it's the same deal.

 

[dfx]

Side note-- If you are diving with a tank that has a little water in it you are heavier because unlike a BCD, a tank is not a flexible container. Correct?

Correct. One cubic foot of water weighs 62 pounds. One cubic foot of air at 3000 psi weighs about 16 pounds if my math is correct. With less pressure in the tank, the difference is even more.

 

[eponym]

After my last dive, I was emptying water from and I made some sort of joke to an instructor who had just finished a class. His words to me were, don't worry, everyone gets a lot of water in their BCD and I often add some on purpose if I need more weight. Forget the point that I wondered why someone with an instructor rating would need to do something like this to correct a weight issue, my real question is that I thought that the weight of water, in water was ambient weight. The water in my BCD at depth should not actually add weight below the surface . . . would it?

Years ago I might have insisted that the instructor had to be joking.

But I've since heard an instructor tell open water diver students, in course after course, that the air they breathe in gets compressed in their lungs as it travels to smaller and smaller regions, and sudddenly, Blam! it turns into a liquid.

I kid you not. I think of this as the John Madden physiology lecture.

 

[dfx]

Nope, not kdding, I actuallly challenged him on it and he told me how wrong I was. Funny though. Hope he isn't teaching his students to add water when they can't stay down. Ok, Maybe not so funny.

Of course you could fill your BCD with fresh water for a salt water dive, which is gonna make you lighter

 

[Panama Jones]

One cubic foot of air at 3000 psi weighs about 16 pounds if my math is correct.

Maybe I spoke too soon when I said I think logically. But here goes. If 1 cu ft of air at 3000 psi weighs 16 lbs, then why does an empty tank only gain about 5 lbs when you add 80 cu ft at 3000 psi? Or am I in a different context?

 

[eponym]

Panama Jones -

One cubic foot of dry air at one atmosphere weighs about .08 pounds.

77.4 cubic feet of one-atmosphere air (what's compressed into your "AL80" tank when it's full) weighs about 6.2 pounds.

The five-pound swing is a good measure for breathing a 3000psi AL80 down to 500psi.




As to what a cubic foot of 300psi air weighs, you are probably correct. Do you know what the cubic footage of the interior of your tank is? One cubic foot is about 28 litres, and an AL80 has about an 11-litre volume.

16 pounds * 11 / 28 = 6.2 pounds