Water pressure affects air pressure

  • #1
In the movie The Abyss an oil rig crew works and lives at the bottom of the ocean in a deep sea structure. There is a compartment where there is an opening in the floor allowing the crew to lower a submersible craft without going through any kind of air locks etc.

We all should know from high school science class that when you take a small glass and turn it upside down and lower it into a tank, the trapped air in the glass will prevent the water from going in. So...

For the sake of easy math, let's say that this structure in the movie is at 6,600ft deep. I know that water pressure increases one atmosphere for every 33ft of depth. At 6,600 that would be 200 atmospheres or roughly 3,000lbs psi.

This water pressure would push up on the air, so my question is: How much would this increase the air pressure in the room?
 

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  • #3
sophiecentaur
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Summary:: How much does water pressure affect air pressure.

There is a compartment where there is an opening in the floor allowing the crew to lower a submersible craft without going through any kind of air locks etc.
Humans would not survive long periods at excessive pressures. Decompression sickness is only one of the problems and all sorts of gas mixtures have been tried for saturation diving. Bone necrosis is another result of long periods at high pressure. The limit of 'unprotected' diving is only about 500m.
So, to operate at the depths claimed in the film, there would have to be some form of air lock to allow the divers to live at around sea level pressures. The air lock could either be directly between living quarters and submersible craft or some arrangement that would allow the 'dock' to be at atmospheric pressure and then the craft would go through some pumped chamber to take it to the pressure at the operating depth. That sounds to me like a very expensive way of doing things - cheap for making the film though.
 
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  • #4
I've read about how humans can survive in air pressures of 70 atmospheres as long as they don't try to breathe, and that at that level even oxygen becomes toxic. I realize that the movie people took some liberties with reality, but if the water pressure is 3000psi, is the air pressure in the room the same? In other words does the water pressure compress the air in the room to same to equalize the forces?
 
  • #6
Baluncore
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The air in the diving bell will be at the same hydrostatic pressure as the water. At 200 atmospheres the air would be 200 times as dense as at sea level. It would need be an almost pure helium mix with a dash of oxygen, scrubbed of all CO2.

Breathing would be difficult with such dense air. Voices would certainly sound different. At what depth will the high pitched helium voice sound normal again? What would it sound like in He at 200 bar?
 
  • #7
sophiecentaur
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Surviving is very dependent on exposure time. I read, years ago that surgeons and others, operating under excess air pressure (extreme aseptic surgery) will suffer slightly. But escapes from submarine accidents have been survived from enormous depths because they ascend so rapidly from the ‘escape tower’. Of course it’s necessary to be breathing out all the time.
 
  • #8
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Breathing would be difficult with such dense air. Voices would certainly sound different. At what depth will the high pitched helium voice sound normal again? What would it sound like in He at 200 bar?

Generally speaking increasing pressure doesn't change the speed of sound in that gas so it wouldn't affect the Helium sound normally. Though at 200 atmosphere I wouldn't be surprised if other affects show up.

https://en.wikipedia.org/wiki/Speed_of_sound#:~:text=At a constant temperature, the,two contributions cancel out exactly.
 
  • #9
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the movie took some serious liberties with the laws of physics.

My suggestions: the film-makers decided to (a) not film the bits of of the script that would have made it physically possible because of budgetary constraints / so we could talk about them to give them more publicity
(b) accidentally skipped the dialogue where they talked about how nice it was to have forcefields keeping the water from turning them into pancakes when they opened the doors.
(c) cut them to shorten the film.


Fortunately, water isn't very compressible. The wikipedia article on propertities of water says:
The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.

Therefore, as long as the waterlock ('airlock') doors/walls were made strong enough to not deform significantly, they would only need to pump 1.8% of the volume of the 'airlock', from 1bar to 400bar, which isn't really a lot of work (even a cheap domestic pressure washer works at >100bar (1500psi) and pumps 200+litres/hour). Compare this to pushing the water out with compressed air, when as well as survivability problems you'd need to pump 400 times the volume of air.

E.g. I don't remember the film, but if, e.g. the opening of the pool to the room was 10m x 10m, and the pressure-lock was 3m x 3m x 5m, then they'd need to pump 0.81 m3 of water per cycle, and that's going to represent a water-rise of 8.1mm in the pool room.(about a third of an inch).
Thus they could just use a small pump and have it running almost continuously to empty the pool-room.
 
  • #10
Baluncore
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The density of the atmosphere at Earth's surface is 1.2 kg/m3;
At 200 atm depth that will become 200 * 1.2 = 240 kg/m3;
That is close to 1/4 the density of the water.
Some fish will be able to fly in air of that density.
The sharks will not be confined to the water.
 

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