# Trying to understand water pressure / air pressure

i have been researching deep dives. I understand that deep-divers have the same pressure inside of their bodies as there is outside. Yet i still cannot understand how they do it. Even if they have the same pressure in their lungs and i assume their blood there is a large area that is not covered. I mean if i push down with the force of 100 kg on my finger, should it matter if i have 100 kg of pressure inside the blood in my finger? It would probably help a bit but i still don't understand why it doesn't get squeezed. Do deep divers feel a squeezing sensation? Does it become like swimming in syrup at deep depths? Honestly i don't understand how they do it with 100 kg pushing down on them... Even if the force of 100 kg is pushing inside them how come they can even move their arms??

I also have another question. I get that as you add more and more water on top of you there will be more and more pressure, well, on top of you... But say for example under the arms of a diver. Is it the exact(i guess i mean to ask if this has been tested rigourously) same pressure there? After all the shoulder is taking the pressure from above. Maybe a better example would be a closed diving-bell. The pressure below it is exactly the same as on it's sides(?) And why would this be.

And finally... As i understand it the pressure inside a closed diving bell is the same as the water pressure. But gas is easier to "move around in" -- or objects can more easily do work - to say it in scientific(:D) than when in liquids. Is this true or am i deluded?

These are my questions. I realize perhaps i should make 3 separate posts but they are all aimed at understanding the same thing and are related to eachother so i hope it will be allowed to stand.

• When you blow up a party balloon how high do you think the pressure is inside? – Solar Mike Nov 26 '19 at 19:53
• Higher than the surrounding pressure? I don't get the downvote. What is wrong with my post? – CuriousGeorge Nov 27 '19 at 7:04

Our body is 60% water and this water permeates throughout the different tissues, keeping the outside and inside pressure at equilibrium. At the maximum depth of deep diving, 40 meters, the hydrostatic water pressure is 4atm, which is far below the pressure our tissues can handle. just walking exposes your feet and toes tissues to more than that pressure. and as you said people push and press and lift weights with their hand all the time. Even small daily tasks such as flexing your arm to pull open a car door expose some tissue around your joints like wrist and elbow to high pressure.

I am not getting into issues with nitrogen accumulation and rate of ascent because you said you already looked them up.

The feeling of water getting harder to swim in or move around, say its thickness, in water, is called kinematic viscosity, which can be obtained by dividing the dynamic viscosity of a fluid by its density. But in case of water, because it is essentially an incompressible material, its kinematic viscosity doesn't change with depth.

As to your question regarding the pressure of water above and below the shoulder of the swimmer, the water pressure is the same in all directions, only varies by the level of the water. water is a liquid and liquids assume the shape of their container and maintain the same pressure at any given level regardless of the direction they apply that pressure.

And yes normally the viscosity of gases in normal temperatures we experience daily is much less than the water so they can be pushed around easier or they can flow through a propeller much faster.

• I don't think it's the water inside of our bodies that are at the same pressure as the outside(in deep diving). I always assumed it was the oxygen or trimix or whatever the diver was breathing... But you raise another problem -- unconsciously ;-- there is no reason the water inside our bodies should be equal to the pressure of the water outside it... unless it somehow gets inside our bodies by osmosis through the skin(!) I didn't say anything about nitrogen accumulation or rate of ascent but that is fine we dont need to talk about that now. – CuriousGeorge Nov 27 '19 at 7:02
• I will look up kinematic viscosity and dynamic viscosity. For the question about water pressure i mostly wondered if this had been tested rigorously. For the question about gases i wondered if gases under water pressure, that is specifically under a closed diving bell would be easier to do work in since it is a gas -- even if it has the same pressure as the surrounding water. I hope this was clear. – CuriousGeorge Nov 27 '19 at 7:02
• Not sure how much pressure it is but 332 meters were the deepest freedive i could find. Maybe i have to talk to him to figure out how it feels :D – CuriousGeorge Nov 27 '19 at 7:09

Everything comes down to Newton's law. If you push on it, it moves. So if it's not rigid, and you squeeze on it, then it'll shrink until the pressure inside is equal to the pressure outside.

I understand that deep-divers have the same pressure inside of their bodies as there is outside. Yet i still cannot understand how they do it.

It's not like it's done consciously. Our tissues are flexible and so is our skin. So any force on our skin must be balanced lest we collapse or blow up. So if you squeeze on our outsides, then our skin flexes just enough that our insides get squeezed.

I don't know if divers feel the pressure or not.

But say for example under the arms of a diver. Is it the exact(i guess i mean to ask if this has been tested rigourously) same pressure there?

No. The water at lower depths is holding up the water on top of it, so the pressure is necessarily higher. There is, in fact, a pressure gradient of about 1/2 PSI per foot of depth. So if you're six feet tall and floating upright, your feet experience 3PSI more than your head.

This is why things float -- put a buoyant thing into water, and the bottom surfaces see more force pushing up than the top surfaces get pushing down, and the net result is a force pushing up.

And finally... As i understand it the pressure inside a closed diving bell is the same as the water pressure.

That depends on the "diving bell". A true diving bell is open at the bottom, and pressurized so that the air doesn't all get crammed into the top. But submarines and deep-sea submersibles are pressure vessels that keep their insides at 15PSI absolute through the strength of their hulls.

But diving bell or submarine, yes, it's easier to work when you can just breath instead of using breathing apparatus.

• I seem to be a master at being misunderstood. What i meant by a closed diving bell are the ones that don't introduce pressurized air inside them through additional means. That are simply bells. Not sure if the terminology allows completely closed vessels but who knows... it's early in the morning here and im too lazy to check :) – CuriousGeorge Nov 28 '19 at 7:00