Question about pressure, submarines and deco.

[Scubakevdm]

Great discussion! This is what this board is best for! Hopefully even the most novice layperson can understand this topic now. Buccaneer, please do go back and read the 1st 3 chapters of the divers encyclopedia. These are very important concepts that you want to pass on to new divers/students correctly. One caveat, the regulator doesn't change the pressure per se (such as a computer controlled pressure valve), but allows the ambient water pressure to act on the compressed air leaving the tank (not directly but through a piston or diaphragm) thereby allowing you to breath air at the ambient pressure of your particular depth. The 2nd stage is just a demand delivery device that allows you to receive the ambient air from the 1st stage. Y'all can just slap me for nitpicking.

Actually, the regulator does change the pressure to around 140 psi over ambient. Gas must overcome both ambient water pressure and spring tension in the first stage. The second stage does further reduce pressure to just over ambient through the relationship of force applied by another spring and the area of the oriface. This is not the case in the ****le rotary regulator, because they simply do not exist.

Hey why won't it write "W-A-N-K-L-E"?

 

[Seuss]

> where i am not getting the connection is that the air inside obviously compresses at depth, because it becomes more dense in porportion to your depth.

Nope... forget everything that you think you know and start from the beginning.

There are two types of containers:
1) compressable
2) noncompressable

A compressable container is something like a balloon, or a plastic water bottle, or our lungs. A compressable container will get smaller as the pressure around the container gets larger. Because the container is getting smaller, the gas inside the container has less volume for the same number of molecules, hence the pressure inside the container increases.

A noncompressable container is something like a scuba tank, or a submarine. The walls of a noncompressable container do not flex or shrink as the pressure outside of the container increases. Because the volume of the container doesn't change, the pressure inside the container remains constant. (assuming constant temperature)

When we breath from a scuba tank underwater, the regulator(s) reduce the pressure in the tank to the pressure of the water. When the air enters our lungs, it goes from a noncompressable container into a compressable one. At depth, it takes more volume of air from the noncompressable container (the scuba tank) to fill a compressable container (our lungs) than it would at the surface.

Ack... I just noticed there are four more pages of replies... sorry if I am repeating what others have already said.

 

[whitedragon13]

The air in the tank never "compresses at depth." It just takes the first stage more air to make up for the ambient pressure and then add 140 psi on top. I think you might not only need to reread your gas laws, but check out how regulators work.

http://entertainment.howstuffworks.com/scuba.htm

 

[RPanick]

Actually, airplanes can be pressurized to one ATA. They do it for special medical conditions, like when someone gets bent. They charge you for it though, and it isn't cheap. I don't know if they fly lower to compensate for the preasure difference in order to reduce the stress on the plane. But I suspect that the structure of the airplane probably probably can handle it, the stresses of flying at 600 MPH or through a storm are probably much more than the difference in cabin pressure.

An earlier post was correct about taking engine power to keep the plane compressed. There may also be a bit of a safety factor at work too. If you do pop something like a window, there is a lot less air to move with lower preasure which reduces the tendancy to be sucked out the window or have as much stuff flying around.

Check out the myth busters for a good one on that.

Bob.