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Why does pressure in an enclosed container go up under the sea?

  1. Jul 1, 2012 #1
    Ok stupid question I know but I'm probably just thinking about it all wrong. If I have a container that can't be compressed in any way filled with air and I bring it to the bottom of the sea why would the air pressure inside increase? The way I'm thinking is that it is separated from any outside pressure and therefore the pressure shouldn't change, I know it does but I don't understand how if this container can't be compressed.

    I know if I brought the same container up a mountain there would be less pressure but still don't know why. Someone was trying to tell me the weight of the atmosphere presses down on the container but like I said if it can't be compressed how does the outside atmosphere effect the inside pressure?

    Like I said I am probably thinking of it all wrong, I'm thinking of it as a force pushing on something. Like if I took that same container and put a few boulders on it the pressure inside wouldn't change so why would it change with the weight of the atmosphere or water on top?

    I was talking about this with a physics student and I guess I just didn't get the answer and I'm now curious.
    Thanks.
     
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  3. Jul 1, 2012 #2

    phinds

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    If the container is totally sealed and completely incompressible, why would you think the inside pressure would change because the outside pressure changes?
     
  4. Jul 1, 2012 #3

    tiny-tim

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    Welcome to PF!

    Hi B0mbtrack! Welcome to PF! :smile:
    no, if it "resists high pressure", then the air inside will stay at atmospheric pressure …

    that's how a bathysphere works!
     
  5. Jul 1, 2012 #4
    Hey guys, thanks for the responses. I never heard of a bathysphere before :) So now if I add a human into the equation and drop him to the bottom of the sea in the same container I could raise him back up without needing to worry about differences in pressure then? My example is just assuming the container can go down without being compressed at all.

    I always thought the person would die due to pressure changes.
     
    Last edited: Jul 1, 2012
  6. Jul 1, 2012 #5
    One last question. Unfortunately my example is from some movie, and maybe this is why I'm getting confused.

    I forget the movie, maybe the abyss or the sphere. Someone was trying to escape a submarine that was pretty much at the bottom of the sea. He went in his escape pod and raced to the surface as quickly as possible. On the way the pressure inside the pod changed so much so that blood started coming out of his ears, eyes, and he died. What would have caused pressure difference like that inside a container? I'm thinking the pod walls simply being compressed at first then expanding as it went up but I didnt think there would be that much of a difference in pressure. Anyway, thanks again.
     
  7. Jul 1, 2012 #6

    phinds

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    It was a MOVIE dude ... do you really think they care about real physics?
     
  8. Jul 1, 2012 #7

    HallsofIvy

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    Well, how do you think the people in the submarine survived the trip down to begin with? I would recommend that you not get your science from movies!

    This kind of thing can happen to people who were down in "caissons", working on the bottom of a river, lake, or (not too deep) sea. That's because, in order to be able to work on the bottom, the caisson must be open at the bottom and, to keep the water at that depth from coming in, must be under pressure. A person can exist under pressure as long as the air in his lungs, sinuses, etc. are all at that pressure. When such a person comes up, he must come up slowly, taking time for the pressure within his body to lower as he comes up. If he comes up to quickly, the pressure inside his body is higher than the pressure outside and can cause blood vessel ruptures, "bends", and death. The same kind of people who go deep using aqualungs but NOT people in full "diver suits" (the kind with the big round helmet) and inside submarines. They stay at or close to sea level pressure all the time.
     
  9. Jul 1, 2012 #8
    Thanks again guys, I know not to get my science from movies but I also thought that is actually what would happen and could not understand why. Now I know why I couldn't understand it :) because it wouldn't happen. Thanks again, that's been bugging me.
     
  10. Jul 1, 2012 #9
    Yeah, that's Hollywood science, not reality. Kind of ridiculous, actually. I'm a scuba diver and I know a lot of the effects of rapid pressure change, and the whole pressure change was just a dumb excuse for that (apparently uninformed) director to make a horrifying scene. I highly, very, very much doubt blood would ever come out from any pressure change.

    People have been exposed to vacuum before, though. In one case a pilot ejected at ~50,000 feet, and another was a space suit test failure. Both times they ended up okay. The pilot account says he was bruised all over, and the space suit failure account made no mention of such to my memory.

    For some real information on rapid decompression, look up "decompression sickness", commonly known as DCS.

    /

    The metal container you described - that is a nuclear attack submarine, or a spacecraft. Both can experience rapid and drastic external pressure changes with no effect on the interior.

    Pressure is directly proportional to density, if temperature and gas composition are kept the same. i.e, hold the temperature and assuming you keep the same air and don't flood the room with a denser or less dense gas, then pressure will change directly proportionally to density.

    There is no reason the pressure would change in a container higher up or lower down. Like you said, the walls might expand or contract slightly under different stresses (contract under pressure, or inflate like a balloon as you ascend), but for a rigid material this will be mostly negligable.

    (One picture I'm a bit sorry I can't find is where some submariner hung a clothesline across the width of a submarine (some hallway area was wide enough I guess). The clothesline was tight at the surface, but in another picture at some deep running depth, it had some slack because the walls had compressed.)

    Once again, the bathysphere thing, it's a U.S. Navy procedure on submarines:
    Here's a clip on the inside: (begins around 2:20 )

    And as for ascent to lower than 1 atm pressure, that's been done plenty of times, also
    (Astronauts wear spacesuits during launch for safety reasons in case of rapid depressurization, normally though, the cabin pressure stays the same. For awhile the shuttle astronauts didn't wear a pressure suit on launch, and they were fine, but after Challenger broke apart, they returned to using pressure suits on launch in case of emergency, as it might have saved the lives of the Challenger astronauts had they been wearing them.)

    One note: your ears pop in an airliner as you climb because they don't seal the cabin until 8,000 feet. But once they do, then the pressure inside stays the same. (They do this so there isn't as much stress on the aircraft's hull at cruising altitude, giving it a higher service ceiling which means faster and more fuel-efficient flying) If they didn't seal the cabin at all, then everyone inside would die, since there's serious health issues with the low pressures starting as low as 12,000 feet and being rather severe around 25,000 feet (hence the emergency oxygen masks).

    /

    Where'd you get the idea the pressure would change in a hard container?

    In the scuba classes I took, they taught it would, which is wrong. They did it to explain how each breath of air takes more air at deeper depths. But the air in the scuba tank doesn't change pressure at any depth. The scuba tank holds a fixed volume at a fixed pressure, and thus has a fixed mass of air.
    There's two valves on a set of scuba gear; one reduces the pressure down to 150 psi (10.2 atm), and the second reduces it to ambient pressure. The reason each breath takes more air at deeper depths, is that the outside pressure is higher, and each breath fills the same volume - your lungs, throat, mouth, etc. But at a higher pressure, the same volume holds more mass, so each breath takes more mass out of the scuba tank.

    PADI thinks people will get confused, though, so they teach that the air in the tank compresses, when in reality it doesn't. It really bothers me that they do that... I got very confused trusting the instructor when they said that. I don't know how they convince the instructors that pressure can change in a fixed volume without changing the mass or temperature, since it's obviously impossible...

    (On a side note, I have a half-sister who is becoming an instructor for pilot's licenses. They teach the Bernoulli effect causes lift, but that's pretty much wrong. Once again, it's simplified/"dumbed down". But she tried to explain the real truth to students in the instructor examination course, and she lost points for it for making it "too complicated".)
     
    Last edited by a moderator: Sep 25, 2014
  11. Jul 1, 2012 #10
    Wow a lot of good info there thanks. I took a scuba course too where they taught that. It was padi. I haven't finished my dives yet as I'm waiting for the water to warm up but cant wait. Im on long island and took the class during the winter in a heated indoor pool. Im probably going to do my four dives in september. I was confused about it when they taught it for the same reason I've been confused this whole time but just figured it was some science I didn't understand. I was also curious about the airliner but you answered that for me too. Thanks again for taking the time to answer that. I'll have to check out those videos.
     
  12. Jul 1, 2012 #11
    You're welcome :)

    The video's aren't very important, they're just of a submarine doing an emergency surface and later the Apollo 11 launch - just examples of closed vehicles going through rapid pressure change.

    Part of me is outraged that they teach the courses incorrectly to dumb it down, but the other part understands some people wouldn't quiet get it, and that could be dangerous. So mixed feelings on that. But I'm pretty sure coming up with a simplified, but still technically accurate explanation shouldn't be too hard. I just went into all the details in the explanations here, I trust you could get it since you're asking about it in the first place. People who think and wonder tend to get smart.
     
  13. Jul 2, 2012 #12

    russ_watters

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    In The Abyss, the submersible drilling rig is open at the bottom and pressurized...
     
  14. Jul 2, 2012 #13

    tiny-tim

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    Hi MattRob! :smile:
    So if you're in, say, a bathysphere or caisson where the air pressure is 1.2 atmospheres, then each breath takes 20% more oxygen, irrespective of whether you're breathing it direct or through a scuba apparatus? and the same is true if you're using scuba in clear water where the pressure is 1.2 atmospheres?

    ie the scuba makes no difference?

    as a matter of interest, does this mean that each breath lasts 20% longer because there's 20% more, or does the increased pressure cause a quicker absorption of the oxygen, so it only lasts say 10% longer?
    (assuming it's air)​
     
  15. Jul 2, 2012 #14

    D H

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    Pressure is pressure. It makes no difference.

    Sidebar: Bathyspheres (which are an old submersible technology) were designed to have the internal pressure at 1 atmosphere, even at depth. Submersibles have gone to the Challenger Deep, where the external pressure is about 1100 atmospheres. Internal pressure: 1 atmosphere.


    We breath for two reasons, to take in oxygen and eliminate carbon dioxide. Air doesn't last as long at depth in recreational diving equipment (no rebreather) because you need to breath at more or less the surface rate to avoid CO2 toxicity. The exhaled gas contains a lot of unused oxygen.
     
  16. Jul 4, 2012 #15
    Redo the experiment this time put a window in the container and use a balloon not a gauge. It’s the outside pressure that changed not inside pressure.
     
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