The Naked Scientists

The Naked Scientists Forum

Author Topic: Does water brought up from the depths decompress?  (Read 3264 times)

Offline Doug1943

  • First timers
  • *
  • Posts: 2
    • View Profile
It takes a lot of force to compress water (as with all liquids) but it can be done.

Water at a depth of several thousand meters is compressed about 1% ... i.e. it is 99% of its uncompressed volume.

Suppose we lowered a sturdy metal container several thousand meters below sea level, allowed it to fill with water, and then (via some clever remote device) screwed a lid into place on it.

Presumably a pressure gauge inside the device would not change its reading as we brought the device back up to the surface, provided we kept the lid sealed.

And presumably  when we did unseal the lid, the 1% expansion of the water would produce [manifest? ]  a very large unbalanced force.

Is this right?

« Last Edit: 23/12/2013 09:22:14 by chris »


 

Offline Ethos_

  • Neilep Level Member
  • ******
  • Posts: 1277
  • Thanked: 14 times
    • View Profile
Re: When water decompresses
« Reply #1 on: 20/12/2013 03:06:23 »
It takes a lot of force to compress water (as with all liquids) but it can be done.

Water at a depth of several thousand meters is compressed about 1% ... i.e. it is 99% of its uncompressed volume.

Suppose we lowered a sturdy metal container several thousand meters below sea level, allowed it to fill with water, and then (via some clever remote device) screwed a lid into place on it.

Presumably a pressure gauge inside the device would not change its reading as we brought the device back up to the surface, provided we kept the lid sealed.

And presumably  when we did unseal the lid, the 1% expansion of the water would produce [manifest? ]  a very large unbalanced force.

Is this right?
That would be accurate but the energy you could expect to gain from this would not be equal to the energy it took to lower and raise the vessel to the surface again. In short,,...............a waste of time and energy.
« Last Edit: 20/12/2013 04:02:19 by Ethos_ »
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: When water decompresses
« Reply #2 on: 20/12/2013 03:49:50 »
Quote from: Doug1943
And presumably  when we did unseal the lid, the 1% expansion of the water would produce [manifest? ]  a very large unbalanced force.
On what do you think this force would be exerted on?
 

Offline Phractality

  • Hero Member
  • *****
  • Posts: 523
  • Thanked: 1 times
    • View Profile
Re: When water decompresses
« Reply #3 on: 20/12/2013 04:35:02 »
The problem is much more complex than what has been suggested.

Screws aren't going to hold a lid on a box with that much pressure difference. It's better to have a metal sphere with a small tapered hole to equalize the pressure before sealing it with a tapered plug. The hole and plug should be larger on the inside, so the pressure seals the water in.

As the pressure outside the sphere eases, the pressure inside will stretch the metal sphere. The walls will get thinner and the volume, both inside and out, will increase. As the inside volume increases, the pressure inside will ease. How much the inside pressure eases depends on the compressive modulus of the metal, as well as the thickness of the sphere. Strength of metal determines whether it will rupture before reaching the surface.

The compressive modulus of water is about 2.2 x 10^9 Pa near the surface, but it increases at greater depths.

Steel's elastic modulus in tension is about 3 x 10^7 Pa, but I assume that also will vary at such high pressures.

Looking at those numbers, I suspect only a small fraction of the pressure will remain at the surface.

The only way your box will hold that much pressure will be to make it out of unobtainium.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: When water decompresses
« Reply #4 on: 20/12/2013 05:52:47 »
Welding tanks typically take about 2200 PSI, or about the pressure of water at 1.5 km, but I think some tanks may go higher.  SCUBA tanks may go up to 3,000 PSI. 

It would be easy enough to submerge a large tank full of your favorite gas (oxygen, nitrogen, etc, or just air).  Compress it into a smaller tank.  Then bring it back up to the surface. 

You could run a compressed air engine, or compressed air car with the resulting pressurized gas.

Of course, going with a gas, you would be fighting buoyancy to submerge it, and perhaps a lack of buoyancy to bring it back to the surface.
 

Offline Doug1943

  • First timers
  • *
  • Posts: 2
    • View Profile
Re: When water decompresses
« Reply #5 on: 20/12/2013 12:32:30 »
Everyone: thank you for taking the time to reply.

This is a thought experiment, not a practical proposal. I'm trying to understand the nature of pressure in fluids, starting with liquids. I would like to get a simple, but true, mental model, which I can teach to kids, based on the idea of how small particles have to act, under the influence of forces.

So let's have a  sphere of unobtanium, lower to 5000 meters, fill it with water, seal it with an unbreakable seal, bring it back to the surface and open the seal.

Would we see something like happened when they were testing the first bathysphere, about eighty years ago, having lowered it (unoccupied) down a long way, and then pulled it back up:
 

"It was apparent that something was very wrong," Will wrote, "and as the bathysphere swung clear I saw a needle of water shooting across the face of the port window. Weighing much more than she should have, she came over the side and was lowered to the deck. Looking through one of the good windows I could see that she was almost full of water. There were curious ripples on the top of the water, and I knew that the space above was filled with air, but such air as no human being could tolerate for a moment. Unceasingly the thin stream of water and air drove obliquely across the outer face of the quartz. I began to unscrew the giant wingbolt in the center of the door and after the first few turns, a strange high singing came forth, then a fine mist, steam -like in consistency, shot out, a needle of steam, then another and another. This warned me that I should have sensed when I looked through the window that the contents of the bathysphere were under terrific pressure. I cleared the deck in front of the door of everyone, staff and crew.  I cleared the deck in front of the door of everyone, staff and crew. One motion picture camera was placed on the upper deck and a second one close to, but well to one side of the bathysphere. Carefully, little by little, two of us turned the brass handles, soaked with the spray, and I listened as the high, musical tone of impatient confined elements gradually descended the scale, a quarter tone or less at each slight turn. Realizing what might happen; we leaned back as far as possible from the line of fire. Suddenly without the slightest warning, the bolt was torn from our hands and the mass of heavy metal shot across the deck like a shell from a gun. The trajectory was almost straight and the brass bolt hurtled into the steel winch thirty feet across the deck and sheared a half-inch notch gouged out by the harder metal. This was followed by a solid cylinder of water, which slackened after a while to a cataract, pouring out of the hole in the door, some air mingled with the water looking like hot steam. Instead of compressed air shooting through ice-cold water. If I had been in the way, I would have been decapitated. " (Above from: Half Mile Down by William Beebe, Published by Duell Sloan Pearce, New York, 1951.)  "  [More at: newbielink:https://sites.google.com/site/cwilliambeebe/Home/bathysphere [nonactive]]

I assume that the dramatic effects here were mainly from compressed air. But other people who know a lot more than I do, say that the expansion of 1600 litres of water (my quick, very rough calculation of the volume of that bathysphere) to 1616 litres, i.e. a 1% expansion, would also produce the effect described above. This doesn't seem obvious to me, but it is true that it would have taken a lot of force to compress water even by that 1%, so perhaps it is so.

PMB: You ask on what do I think this force would be exerted? A good question. Let me see if I can think it through.

At first, on the bottom of the ocean, there are indeed great forces, due to the weight of the water above, but they are all balanced... water inside the bathysphere presses on the inner walls, water on the outside presses back, everyone is happy.

We (remotely) close the entry port. Nothing changes.

We begin to pull the thing up, and the pressure outside drops, but not the pressure inside, so an unbalanced force begins to push outward on the unobtanium sphere, which, being unobtanium, only expands by a very teeny teeny little bit, thus not alleviating the internal pressure much at all. (Good old unobtanium!)

Now we're at the surface. We open the hatch -- standing out of the way.

Whoosh!!!!  Water shoots out of the entry port (and of course the bathysphere recoils in the opposite direction). Air molecules are collided with, and kinetic energy is transferred.

Have I left anything out?

« Last Edit: 20/12/2013 12:38:07 by Doug1943 »
 

Offline Phractality

  • Hero Member
  • *****
  • Posts: 523
  • Thanked: 1 times
    • View Profile
Re: When water decompresses
« Reply #6 on: 20/12/2013 18:04:14 »
The pressure required to compress water by 1% would compress the air inside the sphere by perhaps 99.99% (wild guess). I wonder if the air could even be compressed to the point of turning to liquid air. Allowing some water to escape gradually, in a stream that could slice the ship's hull in two, would allow the little bubble of air to expand. After all the water (above the small opening) has been vented, the stream would turn from water to air, or perhaps steam, with a deafening high-pitched whistle, until the pressure inside the sphere is equalized with the outside.
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4707
  • Thanked: 153 times
  • life is too short to drink instant coffee
    • View Profile
Re: When water decompresses
« Reply #7 on: 21/12/2013 00:34:19 »
I wonder if the air could even be compressed to the point of turning to liquid air.

Only if the temperature was below -140 C, at which point the water would be well solid.
 

Offline Phractality

  • Hero Member
  • *****
  • Posts: 523
  • Thanked: 1 times
    • View Profile
Re: When water decompresses
« Reply #8 on: 21/12/2013 03:36:24 »
Only if the temperature was below -140 C, at which point the water would be well solid.

I'm not sure if that's right or wrong. To know for sure, you would have to specify the temperature and pressure and locate them on a chart such as the one on Wikipedia's Tripple Point article.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8659
  • Thanked: 42 times
    • View Profile
Re: When water decompresses
« Reply #9 on: 21/12/2013 10:20:13 »
Only if the temperature was below -140 C, at which point the water would be well solid.

I'm not sure if that's right or wrong. Wikipedia's Tripple Point article.

I am.
He's right
http://en.wikipedia.org/wiki/Critical_point_(thermodynamics)
« Last Edit: 21/12/2013 20:28:24 by CliffordK »
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: When water decompresses
« Reply #10 on: 21/12/2013 20:46:29 »
Apparently one forms Red Oxygen at about 10 GPa.

However, the maximum depth of the Mariana Trench is about 10.9 km, and gives a pressure of about 0.1 GPa, and not high enough pressure to form red oxygen.

One could certainly choose other gases that would liquefy at pressures found at ocean depths.

The critical pressure/temperature for CO2 is: 304.25 K (31.1C), and 7.39 MPa, or the equivalent depth of about 0.73 km.  CO2, however, may not be a good choice as the temperature quickly drops until it turns to a solid when the pressure is decreased, thus slowing the evaporation/sublimation rate.
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8659
  • Thanked: 42 times
    • View Profile
Re: When water decompresses
« Reply #11 on: 21/12/2013 20:51:28 »
The big issue with CO2 would be solubility. (That might be a problem with any gas come to think of it.)
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
Re: When water decompresses
« Reply #12 on: 21/12/2013 22:05:40 »
The big issue with CO2 would be solubility. (That might be a problem with any gas come to think of it.)
You could, of course, put your gas into an inner tube, or similar rubber bladder so that it would be isolated from the water.  However, solubility may not be a problem.

With CO2, you could make a fizzy soda. 
Oxygen or Nitrogen might be similar with making an Oxygen or Nitrogen rich liquid, which would still be expelled at pressure.

Acetylene, of course, is routinely dissolved in acetone for storage and transportation.
 

The Naked Scientists Forum

Re: When water decompresses
« Reply #12 on: 21/12/2013 22:05:40 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums