Water in Wing

[The Chairman]

Where to begin? For starters glass isn't rigid. Glass is a unique material, but it is a poor example of rigid.

O rly? When's the last time you saw glass flow at room temperature? For our discussion, given the limited time involved, a glass is a perfect description of a rigid vessel. If all you want is to find a mistake in my reasoning, you've picked a poor one.

The fact remains that if I took two flat sheets of 16 gauge stainless say 4' in diameter and welded them together around the periphery I could very much inflate them, and could very much have them return to zero volume between them. Claims of "bladder suppleness" playing a part are simply false.

There is no such "spring" tension in any of your bladders. I have to ask: have you done this? If not, why not? SS sheeting would make a poor bladder choice, so why bring it up? Unlike your steel bladder, once a normal, actually in use bladder has been partially inflated, it will retain that shape unless other forces act on it. Since it appears you want to split hairs, by partially, I mean the bladder has the same pressure as the surrounding (ambient) pressure. I guarantee it won't act like your sheets of SS welded together because it has far more flexibility than steel. Suppleness describes that for me. Choose another word if you want, but either way, the vessel tends to retain it's shape until it has been acted on by other forces. If the vessel collapsed at the same rate the air left it, then there would be no room for water to enter. It can't, so water finds it's way in. No biggie.

Ah, no I did not. Not *once* did I claim that pressure differential was at play. And I specifically cited the density difference, repeatedly. Did you actually read my posts?

You mean, like here?

Fluids and gases *always* flow from areas of high pressure to lower pressure, the laws of physics demand it.

BTW, gravity has nothing to do with it either. The gravity exerted on the air and the water are constant. The only issue is density as demonstrated by Archimedes Principle.

 

[cool_hardware52]

Yes, of course. Has anyone said anything else?

Well Pete seems to think I did, even though I said the exact opposite over and over.

Read my post again. I wrote compressible. Compressible ≠ compressed, although the latter demands the former.

Ah, I did miss that, and I do agree. I'll add that "under pressure" is really not relevant.

To expand a bit:

If you open a vent at the high point of a flexible container that contains gas less dense than the surrounding water it will escape. Any volume above the vent won't.

Tobin

 

[BRT]

I'm sure you guys are right and water doesn't enter an empty BCD but somehow I get a lot more water in my BCD when I end a dive too light and try every which way to get the last of the air out. Must be osmosis or something. Or maybe diving with too much weight or a mask with a purge or my split fins. But I still get more water in trying to purge the last drop of air out.

 

[The Chairman]

But I still get more water in trying to purge the last drop of air out.

That sounds reasonable to me. There's still air in the voids that you're trying desperately to get out. Have you tried to pause your breathing on the exhale? Most people have a natural pause after they inhale. With a little practice, you can pause right after the exhale reducing your floatiness a bit. You'll probably use more air doing this, I know I do, but it's an option if you find yourself a bit floaty at the end.

 

[cool_hardware52]

O rly? When's the last time you saw glass flow at room temperature? For our discussion, given the limited time involved, a glass is a perfect description of a rigid vessel. If all you want is to find a mistake in my reasoning, you've picked a poor one.

Er, ah, well have you ever dropped a drinking glass on the floor and it bounced but didn't break? Ever pushed in the middle of a large window pane? Did it deflect? Was that because it's perfectly rigid? Please explain.

There is no such "spring" tension in any of your bladders.

Not true. Pure Urethane films have considerable elasticity. We've inflated *just the bladder* outside of a wing shell. At OPV pressures (~2.5 psi) they stretch quite a bit, but do not deform. Eventually they burst at high enough pressures. The wing shell limits the volume the bladder can achieve to well below the point where it stretches. Of course this entire discussion is a red herring that has zero bearing on the issue at hand.

I have to ask: have you done this? If not, why not? SS sheeting would make a poor bladder choice, so why bring it up?

One more time, you claim the "suppleness" of the bladder will impact how much water will enter a BC in normal use. Apparently you have some misconception that two flat pieces of material welded together can't return to zero volume. My example of stainless steel was used precisely because .062" stainless is just a tiny bit stiffer than the stiffest materials used for bladders, yet one could inflate my example stainless bladder to low pressures, well below the yield point of the stainless, and it would return to zero volume when vented.

You mean, like here?

Fluids and gases *always* flow from areas of high pressure to lower pressure, the laws of physics demand it.

Ah yes the out of context quote, just what I expected.

I guess I have to detail this step by step.

1) What is the subject of this thread?

A: How does water enter a BC

2) What popular myth was offered several times?

A: Leaving the dump open after the wing was empty leads to water in the BC.

3) What would be required for such a phenomena to occur, i.e. water to flow into a completely empty BC.

A: Pressure differential.

That's way I first said

Water doesn't get into a bc because the exhaust was held open too long, i.e. after the bladder was empty. There is no pressure differential to cause this.

Followed by

Fluids and gases *always* flow from areas of high pressure to lower pressure, the laws of physics demand it.

To explain that a pressure differential would be necessary for fluid movement into an empty BC.

Note the 2nd statement says nothing about the relative pressures inside and outside a bladder. It discusses fluid flows only. Still not sure how you thinks this contradicts my specific, clear statement immediately preceding it.

BTW, gravity has nothing to do with it either. The gravity exerted on the air and the water are constant. The only issue is density as demonstrated by Archimedes Principle.

Next I expect you to tell me that mass and weight are the same thing.

If I have a bottle 1/2 filled with Oil and 1/2 filled with water on the surface of the earth (or anywhere else there is gravity) the less dense oil will "float" on top of the more dense water. Why?

Because of the forces of gravity.

If I travel to zero gravity environment such as outer space can I expect the less dense oil to still float on the water? If not why?

Density is mass per unit volume, it's not a force.

https://en.wikipedia.org/wiki/Density

Without the *Force* of gravity you wouldn't have buoyancy...........

https://en.m.wikipedia.org/wiki/Buoyancy

Now if we wish to do "work", i.e. displace a mass some distance, gas out of a BC or water down a power inflator and corrugated hose we are going to need a force.

That's why I said

When these valves are opened this water runs down the corrugated hose as the gas flow out, the force at play is gravity.

Physics 101 kinda stuff.......



Tobin

---------- Post added October 18th, 2015 at 02:25 PM ----------

I'm sure you guys are right and water doesn't enter an empty BCD but somehow I get a lot more water in my BCD when I end a dive too light and try every which way to get the last of the air out. Must be osmosis or something. Or maybe diving with too much weight or a mask with a purge or my split fins. But I still get more water in trying to purge the last drop of air out.

It's largely a function of how many times you dump. A Square profile dive results in a bit of water. A busier profile, training dive with a bunch of ascents or repeated attempts to get the last cc of gas out results in more water.

Tobin

 

[The Chairman]

I'll stick with Archimedes principle and differences in density. You're all over the place with such a convoluted description.

For this experiment, I took a DSS wing and inflated it. Holding it by the inflator, I let the wing vent for 5 minutes. At the very beginning, I could feel air escaping on my cheek, but that soon stopped. then I folded it in quarters and attempted to compress the BC. As I expected, there was still a fair amount of air in the wing. Then I repeated the entire experiment, but let it sit on the couch so I could take pictures. It was about the same amount of air left in the wing. Here are the pics to prove it (sorry for the blur, but I was using only one hand):

Patiently allowing the wing to deflate naturally

The wing failed to completely deflate after five minutes of holding the deflator open.

I could have used any wing and the result would have been similar. This has been on 100 or so dives.

 

[rolhardt]

Why does a drysuit leak? It leaks because there is a breach somewhere in the envelope. Water will enter at any point of the suit, not just at the top due to gravity and there is no water trapped above a valve that only enters when the valve is held open and overhead. Also, the wing is mostly flat when empty or near empty. This could be equated to a suit squeeze which we counter by adding air to the suit. I contend that less air in a wing or suit increases the pressure differential between the inside of the wing or suit and the surrounding water. Therefore, any time an opening in the envelope occurs water is forced in.

 

[redacted]

I contend that less air in a wing or suit increases the pressure differential between the inside of the wing or suit and the surrounding water. Therefore, any time an opening in the envelope occurs water is forced in.

Art vs science?

 

[The Chairman]

Art vs science?

A squeeze happens when there are more voids than air to adequately fill them at that depth (pressure). It's essentially a vacuum and given the opportunity, it will suck in water. The deeper you go the more water it will suck in until it equalizes to ambient pressures.

 

[cool_hardware52]

I'll stick with Archimedes principle and differences in density. You're all over the place with such a convoluted description.

Pete had you stayed with Archimedes principle I would have said nothing more, but you felt the need to add that "gravity has nothing to do with it." That just laughable. Without gravity there would be no buoyancy. Sorry if the explanation of Buoyancy being entirely dependent on gravity was beyond you. I'm at a loss as to how to make it any simpler.

For this experiment, I took a DSS wing and inflated it. Holding it by the inflator, I let the wing vent for 5 minutes. At the very beginning, I could feel air escaping on my cheek, but that soon stopped. then I folded it in quarters and attempted to compress the BC. As I expected, there was still a fair amount of air in the wing. Then I repeated the entire experiment, but let it sit on the couch so I could take pictures. It was about the same amount of air left in the wing. Here are the pics to prove it (sorry for the blur, but I was using only one hand):

Pete, which is it? "Bladder Suppleness" or Density / Buoyancy that causes wings to vent, under water?

All you have proven with your er, ah, well, experiment is "Bladder Suppleness" plays no part.

Not sure why you want to disprove your own assertions, but have at.

BTW, who has ever claimed that bladders vent due to potential energy created when filling them with gas?

What this thread obviously needs is another red herrring.....

Tobin

---------- Post added October 18th, 2015 at 03:17 PM ----------

Why does a drysuit leak? It leaks because there is a breach somewhere in the envelope. Water will enter at any point of the suit, not just at the top due to gravity and there is no water trapped above a valve that only enters when the valve is held open and overhead. Also, the wing is mostly flat when empty or near empty. This could be equated to a suit squeeze which we counter by adding air to the suit. I contend that less air in a wing or suit increases the pressure differential between the inside of the wing or suit and the surrounding water. Therefore, any time an opening in the envelope occurs water is forced in.

Drysuits leak because there is (or can be) a pressure differential, small, but non zero between the inside and the outside.


Tobin