The nature of water pressure?

[loosebits]

This has nothing to do with scuba diving except for the fact that it deals with water.

Air pressure is a simple concept. Air pressure is the cumulative force exerted on a surface from the impact of gas molecules. It has nothing to do with the "weight" of the air column above your head as we are often taught as children (if it were the weight of the column of air, then pressurized cabins in space ships would be impossible - weightlessness). One of the consequences of the nature of gas pressure is that when you heat up the gas, the pressure increases (the heated up molecules move faster and thus strike surfaces with more force).

Water pressure seemingly has nothing little to do with the force from molecular impacts. Water at 90 degrees exerts the same pressure as water at 40 degrees (my guess is that this is due to the way a liquid heats with rotational and vibrational motions dominating over translation of the molecules). The weight of the water column doesn't quite seem to make sense either... wouldn't pressure from the weight of the water above you head push you down to the bottom - inside an underwater cave, wouldn't you need to take in account the weight of the rock over your head? Is it simply a matter of the water trying to occupy the space you are in? If that is the case, why does it increase with depth without depending on your volume (the amount of space the water is trying to reclaim)? What would be the pressure at the center of a sphere of water 66 feet across in a weightless environment?

Anyone have any insights into the nature of water pressure - what causes it?

I degreed in chemistry way back when and we studied the **** out of the gas laws but we were never interested in the behavior of liquids in large quantites where pressure becomes evident.

 

[Don Burke]

Actually, the pressure of air does have to do with the weight of the column. Out in the atmosphere, the weight of the column above you is your air pressure.

If it wasn't, a barometric altimeter wouldn't work.

In an enclosed space, the air follows along with the Ideal Gas Law. I just finished a bottle of soda and when I screw the cap down on the "empty" bottle, I've entrapped air at 0psig or 14.7psia.

Water pressure is due to the weight of the column as well. A one square inch column of seawater weighs about .44 pounds per foot. That means 100fsw is 44psig.

This has nothing to do with scuba diving except for the fact that it deals with water.

Air pressure is a simple concept. Air pressure is the cumulative force exerted on a surface from the impact of gas molecules. It has nothing to do with the "weight" of the air column above your head as we are often taught as children (if it were the weight of the column of air, then pressurized cabins in space ships would be impossible - weightlessness). One of the consequences of the nature of gas pressure is that when you heat up the gas, the pressure increases (the heated up molecules move faster and thus strike surfaces with more force).

Water pressure seemingly has nothing little to do with the force from molecular impacts. Water at 90 degrees exerts the same pressure as water at 40 degrees (my guess is that this is due to the way a liquid heats with rotational and vibrational motions dominating over translation of the molecules). The weight of the water column doesn't quite seem to make sense either... wouldn't pressure from the weight of the water above you head push you down to the bottom - inside an underwater cave, wouldn't you need to take in account the weight of the rock over your head? Is it simply a matter of the water trying to occupy the space you are in? If that is the case, why does it increase with depth without depending on your volume (the amount of space the water is trying to reclaim)? What would be the pressure at the center of a sphere of water 66 feet across in a weightless environment?

Anyone have any insights into the nature of water pressure - what causes it?

I degreed in chemistry way back when and we studied the **** out of the gas laws but we were never interested in the behavior of liquids in large quantites where pressure becomes evident.

 

[ScubaRon]

I don't think there is a difference in the priciple between molecules in a liquid state or in a gaseous state. The pressure exerted on an object is the net difference between the force of the impact of all the molecules striking it. In an open system this boils indeed down to the weight of all the molecules above you.

In an overhead environment you also have to take into account the force that keeps the rock (or whatever) in place and this effectively eliminates the excess overhead weight.

 

[Walter]

Don is correct.

 

[Bob]

Actually, the pressure of air does have to do with the weight of the column. Out in the atmosphere, the weight of the column above you is your air pressure.

If it wasn't, a barometric altimeter wouldn't work.

In an enclosed space, the air follows along with the Ideal Gas Law. I just finished a bottle of soda and when I screw the cap down on the "empty" bottle, I've entrapped air at 0psig or 14.7psia.

Water pressure is due to the weight of the column as well. A one square inch column of seawater weighs about .44 pounds per foot. That means 100fsw is 44psig


Agreed, it's just simple physics. almtree: Bob

 

[donacheson]

... Air pressure is a simple concept. Air pressure is the cumulative force exerted on a surface from the impact of gas molecules. It has nothing to do with the "weight" of the air column above your head as we are often taught as children (if it were the weight of the column of air, then pressurized cabins in space ships would be impossible - weightlessness). One of the consequences of the nature of gas pressure is that when you heat up the gas, the pressure increases (the heated up molecules move faster and thus strike surfaces with more force).

Water pressure seemingly has nothing little to do with the force from molecular impacts. Water at 90 degrees exerts the same pressure as water at 40 degrees (my guess is that this is due to the way a liquid heats with rotational and vibrational motions dominating over translation of the molecules). The weight of the water column doesn't quite seem to make sense either... wouldn't pressure from the weight of the water above you head push you down to the bottom - inside an underwater cave, wouldn't you need to take in account the weight of the rock over your head? Is it simply a matter of the water trying to occupy the space you are in? If that is the case, why does it increase with depth without depending on your volume (the amount of space the water is trying to reclaim)? What would be the pressure at the center of a sphere of water 66 feet across in a weightless environment? ...

Without reviewing ancient history (when I studied kinetic theory od gases), I recall that the pressure exerted on the wall of a container by a gas is related to the kinetic energy of each molecule (a function of temperature alone) and the number of molecules striking the container walls per unit time and area.

However, from the larger view, both gases and liquids have mass and in a gravitational field have weight. The fluid (liquid or gas) in a column must support all the fluid above it (assuming "up" is the direction opposite to the force of gravity). Hence, pressure increases with depth in both the atmosphere and bodies of water. Since water is almost incompressible (it's density is approximately constant), pressure increases approximately linearly with depth. Air, on the other hand, is compressible and pressure decreases approximately exponentially with height.

Let's not get into the effect of temperature upon density, especially in water especially in water.

 

[coberry7]

It is possible to have a weightless environment in an air pressurized environment. Without adequate air pressure, your blood would boil. Astronauts blood does not, unless in the event of depressurization in a near zero pressure environment or less. 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.

Keep in mind, air is compressible, which is why it is compressed to 14.7 psi at sea level. Water is not compressible. Is that what you were getting at?

Colin Berry

 

[Hallmac]

Keep in mind, air is compressible, which is why it is compressed to 14.7 psi at sea level. Water is not compressible. Is that what you were getting at?

Colin Berry

Compressibility is the key here.

Hallmac

 

[coberry7]

Actually, the concepts of gravity, weight and mass are a lot more complicated, but suffice it to say that a pressurized environment can exist with weightlessness.

And a diver can experience greater AIR pressures without any increase in weight at depths lower then sea level, much the same as he can experience lower AIR pressures as he ascends above sea level. While water isn't compressing, it's pressure and weight is increasing as a diver descends, increasing the pressure of any given ambient air space. (Sinus passages, air coming out of a 2nd stage, bubbles, etc.) Will cause them to compress, as air is compressible.

Does that help? I can get into it further, but I'm not sure I need to.

Colin Berry

 

[coberry7]

Clarification: I said...

While water isn't compressing, it's pressure and weight is increasing as a diver descends, increasing the pressure of any given ambient air space. (Sinus passages, air coming out of a 2nd stage, bubbles, etc.)

I needed to say:...

I needed to say that the pressure of the water column increases the DENSITY, and lowers the VOLUME of any given air space at that depth. This is why you inject more air into your ears as you descend, and subequently vent this air as you ascend. Same is true for you wetsuit, BCD bladder, or any other airspace you took underwater.

The same is true for the lower atmosphere above sea level. Greater pressure compresses the air, increases it's density, and lowers the volume for any given amount of air.

This is true above sea level, too. I can't recall off the top of my head that equation, but it refers to pressure, density and volume. It is true in space, the bottom of the Marianas trench or the shore of my beloved Monterey Bay.



Sorry, been a while. No more beers for me tonight!!

Colin Berry