The nature of water pressure?

[loosebits]

Hahahahahahah... you guys are funny.

Water isn't pressurized (of consequence) at any depth. It transfers pressure but it isn't pressurized. If you enclosed water in a rigid container at 100' it wouldn't be any more pressurized than the water you enclosed at 10'.

Now if you included a little bubble of gas while getting your 100' water that gas would be pressurized. If that little bubble of gas happened to be in the bourdon tube of an SPG then the SPG would maintain its reading of 100' until the gas was allowed to expand and fill a larger space, thus changing the shape of the bourdon tube.

Now that makes sense! The SPG is reading the pressure of a gas being compressed by the weight of the water. There really is no such thing as water pressure, just some force being transduced through the water and acting on a compressable gas (in this case, gravity). That makes perfect sence and answers why I can't feel the water pressure on my leg but I can feel the pressure exerted by a tight wetsuit.

Thanks UP, that answers my question.

 

[Don Burke]

Is Don and Big-T saying that water pressure can be stored????? This is nonsence, if it weren't you would be warned not to drink any *high pressure* water at depth because your bladder could rupture on the way up (leading to the 2nd rule of diving, always pee continuously).

There is no significant change in volume when water changes pressure. In fact, the only change is due to disolved gasses within the water.

If I fill a rigid container at depth (regardless of shape), a pressure gauage inside the container will always read ambiant pressure!!!

Not true. If the container is rigid, the pressure won't be transferred.

Can't we think of people as containers of fluid? If my fluids are at 1 atm at the surface and are still 1 atm at depth, I would be crushed.

So you agree they are pressurized. Since you aren't crushed and there is no rigid container around you, something has to have felt those stresses.

Numerical density (and temperature) being the direct cause of air pressure (regardless of how that density was *formed* be it by gravity or an air compresser) is fundemental to the gas laws.

No matter how many times you repeat this, it still won't be correct.

Knowing the numerical density and temperature of a gas, I can actually calculate the pressure of the gas (the ideal gas law, p = n/V * RT, n/V being number of molecules per volume - numerical density, R being a costant and T being the temperature absolute).

That's true, but it still doesn't explain the nature of the vessel containing the gas.

All of the other gas laws can be simply derived from the ideal gas law (also know as the equation of state for ideal gasses).

True enough, however when you try to apply gas law to liquid, it doesn't work since the nature of liquids is to not expand to fit the container.

Don, you are confusing filling a container of water at depth and a container of air at depth. Water brought up in a rigid container from 5000 feet will not be under pressure. How do they bring up specimin bottles from extreme depth?

They use containers that vent off any expansion.

Do theu have to put in extremely strong containers?

See above.

When they open the lid, what happends?

See above.

Does all of that high pressure water escape.

See above.

Make sure that when you a are pumping water out of a well that you don't get the high pressure water from the bottom of the well

When pumping water out of a well, the water is either drawn by a pump pulling a partial vacuum on the water, allowing ambient air pressure to force the water up the pipe, or it is pumped by a submerged pump at the bottom of the well, or a combination of several pumps. The configuration depends mainly on the amount of lift needed.

I'm done arguing the whole bringing up water from depth, if anyone believes the pressure inside the container is constant and doesn't always reflect ambient pressure, I won't be able to convince them without the doing a test - take a soft walled container (such as a balloon) to 33 feet, let some water in it (no air), tie it off and then come to the surface, has the balloon expanded to twice its size? No, it hasn't therefor the water pressure can't be stored (try it with air, you will find that the balloon has expanded).

You can't seem to make up your mind if you want to use a rigid container or a flexible one.

A better test would be to fill the balloon with water at the surface and then run it down to 150 feet or so to see the compression of the disolved gasses within the water.

 

[Don Burke]

Hahahahahahah... you guys are funny.

Water isn't pressurized (of consequence) at any depth. It transfers pressure but it isn't pressurized. If you enclosed water in a rigid container at 100' it wouldn't be any more pressurized than the water you enclosed at 10'.

Now if you included a little bubble of gas while getting your 100' water that gas would be pressurized. If that little bubble of gas happened to be in the bourdon tube of an SPG then the SPG would maintain its reading of 100' until the gas was allowed to expand and fill a larger space, thus changing the shape of the bourdon tube.

I've got to disagree with your wording there. The water will indeed be pressurized, just not compressed to any significant extent.

Submarine NR-1 is a good platform to demonstrate that seawater does indeed compress. While most submarines have a negative pressure/ buoyancy coeficient, the NR-1 has a positive one. As the ship goes deeper, the hull compresses more slowly than the seawater due to the dissolved gasses. This is in contrast to more conventional submarines that compress faster than the seawater. As I recall, my last ship lost about 800 pounds of buoyancy per hundred feet of depth. It was close enough to linear that we just used that number over the operating range.

Gosh, Pug, even solids can hold residual stresses.

 

[donacheson]

... Simply put, astronauts exist in a pressurized medium outside of most of the effects of gravity. Gravity is what gives them weight, not air pressure from an air column, or from a pressurized capsule. ...

Astronauts do NOt exist in an environment outside most of the effcts of gravity. Instead, in orbit the force of gravity (weight) upon their bodies and their spacecraft is compensated by an equal and oppositely directed force, cenrifugal force. Likewise, a neutral diver's weight is compensated by and equal and opposite force, buoyancy.

 

[donacheson]

I've got to disagree with your wording there. The water will indeed be pressurized, just not compressed to any significant extent.

...

Gosh, Pug, even solids can hold residual stresses.

That's right. A physicist would argue that the work done to store energy in a compressible fluid is the product of the force required to compress it times the distance over which that force acts. Since a gas is very compressible, a lot of energy can be stored in it - hence the explosive potential of compressed gases. On the other hand, liquids are (almost) incompressible. While a lot of force can be exerted on a liquid, it doesn't compress much, so the work done and energy stored is very small.

Of course solids can hold residual stresses. That's why springs can store energy. That's also why tempered glass shatters so violently.

 

[Uncle Pug]

I've got to disagree with your wording there. The water will indeed be pressurized, just not compressed to any significant extent.

That's what makes you guys so funny.

Hahahahahahah... you guys are funny.

Water isn't pressurized (of consequence) at any depth.

 

[Don Burke]

That's what makes you guys so funny.

Ya gotta grab your thills any place you can..

 

[jonnythan]

Water does indeed compress under pressure. It's so small it's almost immesurable.

If you fill a coke bottle with water at depth and take it to the surface, it will expand slightly. Since no container is 100% absolutely rigid, you won't notice the difference.

 

[Uncle Pug]

Hahahaha

 

[WarmWaterDiver]

I've been away on a work assignment for a while, and so away from this board, but I was reminded of something while reading through what all's been posted on this thread since the last time I saw it.

Loosebits, air is pushing up underneath you. There is a general buoyancy of air term for a lumped mass at sea level as I recall, that was about 0.0011 correction factor. I used to work in the arena of independant inspection of marine cargo custody transfer back before the fall of the old Soviet Union. The Easten Bloc countries were the only ones I encountered that would create bills of lading for marine bulk liquid cargos with the measurements being "tonnes in vacuuo" - or metric tons in vacuum. If the buyer wasn't very astute, they would get around 0.11% less "tonnes" than in air at sea level - and using the fluid specific gravity, that much less volume in their receipt than they would otherwise think. A small number, for sure - until its multiplied by many "tonnes" of cargo over a long period of time - but this is really peeling the economic onion to a very thin skin.

In practice, if you had a vacuum chamber with a set of scales, that you could weigh yourself on, at sea level, you would weigh about 0.11% less than on your scales outside the vacuum chamber (depending on how well you approximate a lumped mass). Maybe some kooky diet plan will emerge from this now . . . or some porouct to hawk on infomercials - "Don't be a Lumped Mass - just purchase The Vacuuo Scales and Diet Plan!"

Hope that helps out!