gas under pressure

[Sandtiger Steve]

Sorry these are stupid questions but i need some clarifcation about how gases work under pressure (been reading Cousteau). let me give a few examples:

1) let's say i'm 60 ft underwater and i'm breathing through a bamboo tube that goes vertically to an opening at the surface (a la James Bond). does that work? am i breating unpressurized gas while i'm under pressure? how would i understand the effects of that?

2) lets say in the same situation i'm breathing through a tube connected to an air compressor (like lobster divers in the third world, or like in Snuba). does some setting on the compressor have to relate to the depth of the hose? otherwise, won't you have a similar problem to #1? how does that work?

3) for skin diving (breath-hold), i assume you cannot get the bends -- can someone explain why that is?

thanks very much
SS

 

[gt3073b]

No stupid questions, only stupid answers like this one.

1) Think of an inflated balloon on the end of the bamboo pipe. The balloon keeps the air inside under greater pressure than the outside atmosphere (because the rubber is stretchy). The water around your body does the same thing. Since there is more pressure inside the balloon than at the end of the bamboo pipe, the air goes out of the balloon.

Now replace the balloon with a plastic bag. You can easily put air in the bag while you are out of water and the pressure is still equal. Put that bag in the sink (even just a few inches of water) and the water pressure squeezes the air out and up the pipe.

Your lungs can only pull against a couple feet of water. That is why a snorkel works but not a long pipe or hose.

2) The compressor or scuba tank feeds air under much greater pressure than the water around you. The regulator in your mouth has a diaphram in it that matches the pressure of the air output to the water around you. Therefore, the pressure inside and outside your body match, making breathing easy.

3) I don't think you can get the bends on a breath hold dive, but you can get something called Shallow Water Blackout. My understanding is that when you go to depth, the 02 in your blood is under pressure making a higher percentage of the 02 in the air you breath available. If you use most of the 02 and then surface, the pressure drops and the blood no longer has a high enough partial pressure of oxygen to supply your body causing you to black out. This is more like asphyxia.

Please do searches on this site for more complete or accurate answers. This is just an over-simplified explanation.

Bryan.

 

[vladimir]

Here's a good discussion on the topic:

http://www.scubaboard.com/showthread.php?t=178155&highlight=elephant+embolism

 

[Sandtiger Steve]

That's great -- thanks. so how much pressure is in a scuba tank -- ie, at what depth would the pressure of the compressed air equal the pressure of the water?

 

[h90]

1) physically not right but good for imagine: at 60 feet you have a high pressure, maybe like the hose when you give water for the garden.....
if you are at that deep, but trying to cheat the physics the bamboo tube is sucking with the same power the lung out of your body.
Or from the other side of view, the 3 bar outside try to press you thru the bamboo tube with high pressure.
Thats not really correct but good for imagine how it is.

2) now because what the compressor pumps in more than necessary just bubbles out in the water. If you would take the hose from the compressor in your mouth and close it/seal it, you would become a ballone, but so whats too much bubbles out in the water.

3) you get the bends from the nitrogen in your blood which builds up under the high pressure from the air in your lung. on a free dive you have only the nitrogen which is in your lung but no constant supply of new one, therefore not too much can move into your blood.

Hope it helps....

 

[loosebits]

About 6700'.. most tanks are rated to 3000 psi, air is 14.7 psi, every 33' of seawater exerts 14.7 psi of pressure.

 

[knotical]

3) for skin diving (breath-hold), i assume you cannot get the bends -- can someone explain why that is?

Although quite uncommon, you can get bends (DCS) from aggressive free-diving.
See for example this article: http://www.scuba-doc.com/breathhold.html
or about 3/4 of the way down this page: http://www.scubamed.com/divess.htm

 

[Sandtiger Steve]

About 6700'.. most tanks are rated to 3000 psi, air is 14.7 psi, every 33' of seawater exerts 14.7 psi of pressure.

ok-- that makes sense. but that's for a full tank right? so if you were down to, say, 10 psi on your tank (lets say it was nearly empty and you were doing decompression stops), you could be no deeper than 22 ft, or it would be like breathing through the bamboo tube -- is that right?
thanks

 

[muddiver]

Commercial divers use to experience, very long ago, a life ending event when their air supply was compromised called a catistrophic squeez. Like the bambo tube you mentioned, if a surface supplied diver's hose is cut or the air pressure in the hose drops because it is open to the surface, the air in the divers lungs will seek the path of least resistance to be able to expand and rise to the surface. The diver at depth gets crushed by the sourrounding water pressure that is filling the void left by the air escaping out of his lungs and up the supply hose. we have check valves in our breathing equipment now to prevent this.

You could probably get a breath at 22' off of 10 psi, but the loss of pressure in the tank would prevent another breath.

Also, the strength of the springs in the regulator on a SCUBA rig would probably keep one from getting any air out at 22' with less than 75 to 50 psi in the tank. Too much resistance in the regulator and hoses.

 

[loosebits]

ok-- that makes sense. but that's for a full tank right? so if you were down to, say, 10 psi on your tank (lets say it was nearly empty and you were doing decompression stops), you could be no deeper than 22 ft, or it would be like breathing through the bamboo tube -- is that right?
thanks

You'd be quite dead. Without getting into gas planning rules, typically you will surface with *at least* 500 psi. Most people consider gas below that to be last-ditch emergency reserve. Most regs will require the tank to have at say 80 - 120 psi in the tank over ambient pressure to work worth a damn. Also, pressure guages are usually only accurate to 50 or 100 psi (and I don't trust mine at all below 3 or 400).