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Re: [Phys-L] under pressure



On 04/13/2014 10:36 AM, Anthony Lapinski wrote:
My chemistry colleague sent me this link -- good for fluids discussion.
They shake/open a can of soda in a pressurized (2.5 atm) module deep in
the ocean, and it doesn't "explode."

[ http://m.youtube.com/watch?v=EJiUWBiM8HE
]http://m.youtube.com/watch?v=EJiUWBiM8HE

I think normal (gauge) pressure in a soda can is 3-4 atm, depending on
temperature. So I guess the pressure outside the can, now 2.5 times more
that at sea level, is enough to make this demo work. I don't think their
brief explanations at the end are correct.

I am also wondering how thick the metal in these modules are, and is the
air inside pressurized so they have enough oxygen to breathe for a long
time?

OK, there's about 10 questions in there. Here are some answers,
in no particular order:

Scenario #1: Suppose you are inside a submarine, and plan to stay
inside. Then there is no advantage in pressurizing the air, and
several disadvantages, namely
-- Abnormal oxygen partial pressure is bad for you.
-- Abnormal nitrogen pressure is also bad for you.
-- High total pressure with normal oxygen and normal nitrogen partial
pressure requires /mixed gases/ involving some kind of inert gas,
which is expensive and complicated.

Pressurizing the workspace as a way of storing oxygen would be a
terrible idea. The usual practice is to store oxygen in liquid
form, or as a high-pressure gas in cylinders, and then use chemical
scrubbers to get rid of CO2. Nuclear submarines don't need to store
oxygen, because they can make all they want by electrolysis of water.

Note that pressure on the inside of a soda can under ordinary conditions
makes the thing /more/ rigid, and the same goes for an airliner pressurized
on the inside, or a hose carrying pressurized fluid.

In contrast, if the hose is pulling a vacuum, you need to worry that it
will collapse. Ditto for a submarine subjected to high pressure on the
outside. The walls need to be tremendously strong and rigid; otherwise
it will get crumpled and crushed.

Asking about "thickness" per se is not quite the right question. There
are lots of ways of engineering something to make it more rigid; simply
increasing the thickness is not usually optimal. An I-beam or a box
girder is vastly more rigid than a solid bar made with the same amount
of material. You can add /stiffening rings/ to a submarine. I've
added stiffening rings to vacuum hoses on many occasions.

Scenario #2: Suppose you are in a diving bell, or other "wet" undersea
module. The air pressure is in equilibrium with the water pressure at
the air/water interface. The advantage is that you can freely transfer
people and objects across the interface. Exotic mixed gases are used in
this situation.

Since there is not very much pressure differential, the walls do not
need to be too amazingly strong or rigid. There is /some/ pressure
differential, but it scales like the height of the module, since there
is air on the inside and water on the outside. This is not proportional
to the depth of deployment, just proportional to the floor-to-ceiling
height of the module. So the thing needs to be more robust than a motor
home, but not nearly as robust as a submarine.

=============

The problem with "flat" drinks has been known since almost the earliest
days of diving bells. Some folks wanted to celebrate some achievement
with champaign, and were disappointed.

I don't have any facts, but I wonder whether it is unhealthy to drink
the carbonated beverages under such conditions. The body has powerful
mechanisms for controlling blood CO2 (aka blood pH) ... but there are
limits to how much stress you want to put on the mechanisms.