Anybody know about SCUDA systems??

[JT2]

I was in my LDS Saturday, when I overheard a discussion between one of the shop Divemasters and a customer. The customer asked him if they sold any SCUDA (self contained underwater drinking apparatus) systems, and the Divemaster said no because they were dangerous. When the customer asked why they were dangerous the Divemaster told him that as he decended the increased pressure would force the fluid out of the bag and in through the drink tube at greater and greater force the deeper he went and whenever he clicked the release at any depth below about 20 feet the water would shoot in so hard that it would cause probable choking and therefor they are dangerous. It has always been my understanding that you can't compress water, so if I am right about this, then how can what the DM said possibly be true? I am not wanting to go out and buy one or anything, and I am not asking this question to discuss the validity of having one, personally I think it would just be something else in the way, I am simply curious about the physics part of it.

 

[jonnythan]

Whoever told you that was dead wrong. I have no direct experience with SCUDA, but it's totally untrue.

Imagine if you squeeze a water bottle.. water will shoot out one end, right? Well, that's beacuse there's a large *difference* in pressure between the bottle and the opening. Underwater, there's no pressure difference at all (except the difference in pressure between the top and bottom of the water resevoir, which is close to nothing).

You won't have your throat blown out or anything.

 

[Jeblis]

Ahh no. The water pressure increase is offset by the fact the you are breathing air at ambient pressure (so your lungs don't collapse). As you go down, the force on the water in the bag increases, but the pressure of the air in you mouth increases also.
Your regulator is designed to deliver air at ambient pressure, which is equal to the pressure of the water at a given depth.

 

[JT2]

Jeblis, how can the pressure inside the bag increase as you go down when there is no gas in it, it is full of water, and I don't think you can compress water. Also, are you saying that air pressure builds up in my mouth and lungs as I decend? I don't understand how this is possble if you are not ever holding your breath......Think about it, if you fill a balloon with water, and get all of the air out of it, it will stay the same size no matter how far you decend, and there wll be no pressure change inside the balloon unless there is something that I am missing here.

 

[Bob3]

If you use a darker beer or ale, you get more foaming problems. Just use a lighter pilsner & you'll be OK.
:mean:

 

[jonnythan]

Originally posted by JT2
Jeblis, how can the pressure inside the bag increase as you go down when there is no gas in it, it is full of water, and I don't think you can compress water. Also, are you saying that air pressure builds up in my mouth and lungs as I decend? I don't understand how this is possble if you are not ever holding your breath......Think about it, if you fill a balloon with water, and get all of the air out of it, it will stay the same size no matter how far you decend, and there wll be no pressure change inside the balloon unless there is something that I am missing here.

The pressure on everything increases with increasing depth. Fill one balloon with air and the other with water, and take both down to depth. The balloon with air gets smaller as the pressure increases, and the balloon with water does not get smaller.

They're both experiencing the same pressure change, the only difference is that air compresses and water does not (well, not much anyway). The water in the balloon doesn't experience any force making it want to rush out of the balloon or anything, and the air in the other balloon is just smaller. The pressure all around the balloon is pretty much equal.

The same thing occurs to your body. The air in your lungs compresses because of the water pressure surrounding your body. Everything in your body experiences the same pressure. Think of it like this... imagine your heart did not experience a change in water pressure as you descended. When at 99 or so feet, somewhere between the heart and the water, there must be something holding back 3 atmospheres of pressure difference.

A submarine is a different example. Submarines have strong, rigid bodies. They keep the air inside constant as they ascend and descend.. the steel outside of the submarine actually holds back the pressure difference between 1 atmosphere inside and, say, 10 atmospheres outside. This is why a submarine will implode if you take it deep enough.. the skin of the boat can only take so much pressure difference before it gives way. If the skin of the boat were made of, say, trilaminate instead of steel, the boat would shrink a great deal as it descended and the people inside would experience pressure changes.

To prove that air pressure builds up in your mouth and lungs as you descend, hold your breath at 30 feet then start ascending. You'll believe it's true when your lungs rip open. Your body and lungs are pressurized.. since the pressure in your body equals the pressure outside the body, you don't feel it. Your body is just like that balloon filled with water, with an air filled balloon inside (your lungs). If you held your breath at the surface (closed off the air balloon), your lungs would shrink as you descend. That's why the regulator keeps pumping pressurized air into them... to keep them the same volume at depth.

Air and water experience the same physics, but air compresses, so you can see the effect.

 

[g2]

Originally posted by jonnythan

To prove that air pressure builds up in your mouth and lungs as you descend, hold your breath at 30 feet then start ascending. You'll believe it's true when your lungs rip open.

Please don't do that.

Carbonated beverages work pretty good, because the fizz tends to keep seawater out of the bottle as you're drinking it. Still, you have to open the container with your teeth, otherwise it will squirt out into the ocean.

Champagne works great; the only problem is that it goes flat when you get deep enough. (That's a whole 'nother story!)

g2

 

[Bob3]

...it goes flat when you get deep enough...

I'd read that description when folks popped a bottle inside a caisson at the Brooklyn Bridge.

Wondering about that, I asked Bob Barth, the only Navy diver who had been on all 3 Sealab projects and asked him the same question.
Here's his reply:

"Don't know of any studies on "fizz loss" being conducted but as I recall they always had fizz, only difference was the cans would inhale instead of exhale when you
opened them."

We concluded a possible explanation could be that the CO2 went into solution & then reacted with the tongue/taste buds.
The folks drinking the champagne probably had the cheap stuff.

Bob's since retired from the Navy but currently works at the Navy Experimental Diving Unit in Panama City.
His book "SEA DWELLERS" recounts the Navy's early experiments with saturation diving and the SEALAB program.

 

[Jeblis]

It will not shoot out into you mouth at high speed.

You, the bag, your foot, your tank all experience the same amount of increase in pressure as you descend. The air in your lungs compresses rather easily. If you held your breath (the balloon analogy) the balloon and your lungs would get smaller, but you keep breathing as you descend. As you descend your regulator provideds air which is at a higher pressure (this is why your air consumption increases the farther down you go) Thus your ballons (lungs) do not compress.

As for water compressing is does so very little, but it is a bag with an open end and acts like a hydrolic system. It won't compress but it will move to offset the force applied to the bag (pressure from the surrounding water, you squeezing it, etc) This will cause the water to exit the bag through the tube if and only if the force applied to the bag is greater than the air pressure in your mouth. If you have continued to breath you got air at ambient pressure (the pressure of the water)

Pressure on bag = pressure in your mouth; Thus no movement of water

If you squeeze the bag (or suck), the pressure on the water in the bag is now greater than that in your mouth. Hence you get a drink. This difference in pressure is no greater than you would experience at the surface while taking a drink because the velocity of the water is related only to the difference in pressure you apply while sucking or squeezing the bag.

 

[DiveRite-SW]

Lemme see if I get this straight here. A bag full of liquid is taken underwater and then an opening is made. Which results in the liquid "Shooting Out" of the bag. Did I get that right?

So that explains why, when I cut myself diving the other day (I mean "I poked a hole in a liquid-filled flexible-side container") a little bit of blood oozed out (I mean " the liquid in the container shot out under high pressure") until it stopped by itself (I mean "until there was nothing left"). Gee, maybe that's why I feel so light-headed today.

If there is any gas (air, CO2, whatever) in the container, it will be affected by the changing pressure, but the liquid will not 'change pressure' as it is still in equilibrium with external forces due to the flexible walls. A solid-wall container (scuba tank, my head, etc) will react differently, however. There IS a pressure-differential in that case.

Personally, between that story and a few others you're related about yer LDS, I think you should stay as far away from that diveshop as possible. We'd kinda like to have your flexible liquid-filled (with whatever you choose to fill it with) pressurized container around for a little while longer!