Water in Wing

[phoenix31tt]

How? The oral inflate / deflate in the typical power inflator is an oring sealed poppet. If the oring is off it's seat how can it allow gas to flow in one direction and stop water from flowing in the other?

I was referencing this comment

As gas exits from one orifice, it causes a venturi flow that will draw water in from any other.

that said if I dump from on orifice water would enter through another... I'm saying dump from one orifice allows water to enter through the SAME orifice...

I.e if you dump from butt dump water will enter there... Not if you dump from the butt dump water will enter through the corrugated...

We're probably saying the same thing


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[The Chairman]

It would be fun to secure a BC (with no diver in it) under water and fill it to the point where the OPV vents and see how much water enters vs simply venting it.

The much maligned BWODs allowed very little water to get in, if they were inflated sufficiently. There might be a couple of inches of water pressure in such a system which is still not a lot, but that's substantially more than most BCs which are designed to operate in ambient pressures. How much water gets let in is a function of density differential and bladder stiffness. Since an infinite differential and a perfect suppleness are not achievable, there will always be some water let in. Also, it should be pointed out that there is no 'empty' BC. As long as any void exists it will be filled with either air or water. Nature abhors a vacuum almost as much as I do. Once you start allowing air out under the surface, water will find it's way in.

I'm not sure about your BP/W, but I am pretty sure some of the older pull dump assemblies on normal BC's included a one way check valve - butterfly valve - whatever you call them - same as the exhaust valve on a regulator.. Sea quest had it i think, not sure of they still do? But this should stop the exchange of air and water, yes?

I don't believe this ever existed, but if you produce one I'll change my mind. Getting some water in your BC is a non-event. No lives or gear should be endangered by this happening. It's absolutely normal.

I'm saying dump from one orifice allows water to enter through the SAME orifice...

That is precisely what happens. Water enters as air leaves. There's a conservation of space that must be obeyed. That is mitigated by the other forces noted earlier.

 

[cool_hardware52]

Also, it should be pointed out that there is no 'empty' BC. As long as any void exists it will be filled with either air or water. Nature abhors a vacuum almost as much as I do. Once you start allowing air out under the surface, water will find it's way in.
That is precisely what happens. Water enters as air leaves. There's a conservation of space that must be obeyed. That is mitigated by the other forces noted earlier.

I disagree. Nearly all Wing bladders are made by welding two flat sheets of urethane together. It's easy to have zero volume between two flat sheets. I can have zero volume between two flat sheets of 16 gauge stainless if needed.

Might be a problem with convoluted bladders typical of jackets, but not most wings. Can you full evacuate a lift bag? I can.

Tobin

 

[The Chairman]

But then you have cavities where hoses and OPV are attached as well as the space in the corrugated hose. Your bladders are good, but they still have voids. Water gets into my DSS wing as easily as my Dive Rite wing. There's no advantage to either and small amounts of water are to be expected in any bladder. Just remember to drain them from time to time. What's so hard about that?

I disagree. Nearly all Wing bladders are made by welding two flat sheets of urethane together. It's easy to have zero volume between two flat sheets. I can have zero volume between two flat sheets of 16 gauge stainless if needed.

Might be a problem with convoluted bladders typical of jackets, but not most wings. Can you full evacuate a lift bag? I can.

Tobin

 

[cool_hardware52]

But then you have cavities where hoses and OPV are attached as well as the space in the corrugated hose. Your bladders are good, but they still have voids. Water gets into my DSS wing as easily as my Dive Rite wing. There's no advantage to either and small amounts of water are to be expected in any bladder. Just remember to drain them from time to time. What's so hard about that?

I've not once claimed that DSS wings are better or worse than any other wings WRT to water intrusion, not sure why you bring that up.

Water will enter any BC, I've described in detail how and why this happens.

You claimed bladder materials are what limit evacuation of the BC. It's not. With a "2D" bladder suppleness plays no part.

How much water gets let in is a function of density differential and bladder stiffness. Since an infinite differential and a perfect suppleness are not achievable, there will always be some water let in.

I mentioned the corrugated hose in post #10

What causes water to enter a BC is the corrugated hose. There is a bit of water sitting on top of the oral inflate valve / quick dump valve (if so equipped)

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

The idea that water enters a BC because of user error, i.e. holding the dump open after all the gas has left is pervasive, and largely false, like so many other myths in scuba. But they get repeated and repeated and repeated without challenge until they become "common knowledge"

Tobin

 

[The Chairman]

You claimed bladder materials are what limit evacuation of the BC. It's not. With a "2D" bladder suppleness plays no part.

Physics disagree with you. Once air is in a BC, the return of the bladder to an uninflated state is not automatic nor is it gauranteed. As air leaves the BC some water intrudes due to the forces I've outlined previously. A bladder is not rigid like a glass, but then it's not perfectly supple either. It's mostly supple but that still allows some water to enter as the air escapes. Don't believe me? Partially inflate a BC in air. Now open any orifice. It's probably going to retain the shape because there is no reason for the bladder to return to it's original form. The weight of the BC might cas a little gas to escape. Buoyancy (difference in density) is the real reason air leaves a BC.

I mentioned the corrugated hose in post #10

You actually blamed it for water coming in, which was kind of silly. An orifice is an orifice and the physics are largely the same. You also attributed evacuation of air in a BC to a pressure differential which is incorrect. They are at essentially the same ambient pressure. It's all about buoyancy. Air, is far less dense than water, ergo it want's to float. Put the nozzle at or below the level of the air in the bladder and nothing will happen. Oh, there might be a few inches of pressure developed inadvertently when you hold the deflator above your head, but even that's due to the difference in density between water and air. Sure, there might be some difference in cavitation as gas leaves the BC and that might cause one orifice to introduce more water than another. I bet that difference is pretty minimal.

The idea that water enters a BC because of user error, i.e. holding the dump open after all the gas has left is pervasive, and largely false, like so many other myths in scuba. But they get repeated and repeated and repeated without challenge until they become "common knowledge"

I don't believe I've ever made this assertion nor did the OP. The discussion was about a BC used during the normal course of diving. Water getting in any BC is a completely normal event. How much is a function of technique and how you mitigate the physics of air escaping from an amorphic vessel under water. In any event, the less you have to vent the less water can get into your BC. Using your lungs for precise buoyancy control is a good technique for any OC diver to master.

To those who don't understand what I mean by a "few inches" of pressure. 6 inches of water pressure produces less than one quarter of one psi. Typical water pressure in a house is about 40 psi. It's all about the difference in density or buoyancy.

 

[Storker]

Physics disagree with you. Once air is in a BC, the return of the bladder to an uninflated state is not automatic nor is it gauranteed.

You've got a wrong understanding of the physics. If you have a confined, compressible volume of gas under pressure (say, a wing bladder underwater), opening this volume to the surroundings (e.g. by opening a dump vave or inflator valve) will force the gas out of that volume. Unless gas is trapped, say, in the volume above the corrugated hose (i.e. at lower pressure than at the exit).

 

[cool_hardware52]

Physics disagree with you. Once air is in a BC, the return of the bladder to an uninflated state is not automatic nor is it gauranteed. As air leaves the BC some water intrudes due to the forces I've outlined previously. A bladder is not rigid like a glass, but then it's not perfectly supple either. It's mostly supple but that still allows some water to enter as the air escapes. Don't believe me? Partially inflate a BC in air. Now open any orifice. It's probably going to retain the shape because there is no reason for the bladder to return to it's original form. The weight of the BC might cas a little gas to escape. Buoyancy (difference in density) is the real reason air leaves a BC.

Where to begin? For starters glass isn't rigid. Glass is a unique material, but it is a poor example of rigid. Need I specify that the gas in the BC has to be at the exhaust valve in order for it to be successfully exhausted? Really? Do you teach something else to your scuba students? 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.

You also attributed evacuation of air in a BC to a pressure differential which is incorrect.

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?

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.


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

They don't of course for the very same reasons I've detailed, no pressure differential from inside the bag to outside the bag.

What people fail to grasp is that water can flow into the bladder at the same time gas is escaping. The gas isn't forced out at high pressure or flows, it only leaves because it's less dense than the water, the gas flows up and the water flows down at the same time.

It's gravity and the density differential that causes water to enter a BC that is being vented from a High point.

Please do let me know what part of this leads you to the conclusion I think there is a pressure differential between the inside of a partially inflated BC and the surrounding water.


Tobin

---------- Post added October 18th, 2015 at 12:31 PM ----------

You've got a wrong understanding of the physics. If you have a confined, compressible volume of gas under pressure (say, a wing bladder underwater), opening this volume to the surroundings (e.g. by opening a dump vave or inflator valve) will force the gas out of that volume. Unless gas is trapped, say, in the volume above the corrugated hose (i.e. at lower pressure than at the exit).

The gas in the wing is at the same pressure as the surrounding water. It maybe compressed relative to the surface, but it's not compressed relative to the surrounding water.


Tobin

 

[The Chairman]

This is simply an extension of Archimedes principle. An object is buoyed up by the mass it displaces less it's own mass. It's pretty common for people to mistake buoyancy for pressure, but that's why bubbles, which are at the same pressure as the water they are in, rise so quickly to the surface. Remember, salt water is 804 times denser than air. (air at sea level @ 1.275 kg/m³, salt water @ 1025 kg/m³) Also, it takes 2'3" to create a single psi pressure differential. So, no, I don't have a wrong understanding of the physics.

 

[Storker]

The gas in the wing is at the same pressure as the surrounding water.

Yes, of course. Has anyone said anything else?

It maybe compressed relative to the surface, but it's not compressed relative to the surrounding water.

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