The partial pressure of gases below sea level?

[scilearner]

Hello everyone,

 

The partial pressure of the atmosphere 33 feat benath the surface is doubled the normal amount. (2x760 mmHg). I can understand how lower you go down higher the pressure but I don't understand how you can measure atmospheric pressure under the sea. I mean under the sea you got water around you not atmosphere. Are the gases in atmosphere dissolved in this water. My question is mainly related to divers under the sea. They get double the atmospheric pressure of nitrogen and nitrogen narcosis could occur. So how do you get double the N2 in the atmosphere when you are surrounded by the sea. Also why do these effects occur when divers are using an oxygen tank. The gases inside the oxygen tank is normal atmospheric pressure right. Thanks

[Sisyphus]

It's not "atmospheric pressure," it's water pressure. But that doesn't really change anything. The general term is ambient pressure, and it doesn't really matter what material is surrounding the pressurized object.

 

I have no diving experience, but I assume the gases in the oxygen tank must be at ambient pressure, not at 1 atmosphere, otherwise divers wouldn't be able to inflate their lungs.

[CaptainPanic]

First of all: there are several systems for breathing under water. Let's only discuss the most simple one: just normal air from a gas tank.

 

The pressure inside the air tanks is pressurized, typically over 100 bar (I'm guessing 200 bar(?) - I never went diving though). Then there is some gadget to regulate the pressure. And then the diver breathes the gas which is at the same pressure as the water.

 

I could proceed to explain the whole breathing apparatus to you - but Wikipedia did that for me already.

 

The important thing to remember is that the air in your lungs is at the same pressure as the ambient pressure (= pressure around you). At the surface, the air is 1 atm (atmospheric). Under 10 meters (33 ft) of water, the pressure is 2 bar - so, the air in your lungs will be 2 bar too. And that's why you get double the N2.

[scilearner]

Thanks for the replies both of you . It was indeed very helpful. Ok I understand why I don't understand this now. I want to clarify these points.

 

So the pressure inside the lungs is equal to the pressure outside the body. Is this because pressure outside pushes on you and lung increases the pressure by same amount to neutralize this. Please feel free to explain this more. So can lungs regulate pressure with out a tank. When you climb up a mountain does the lung regulate pressure to keep up with the low pressure outside. How does the lung do this and how does the lung neutralize the pressure acting all around your body.

 

Thanks a lot for the help!!

[Sisyphus]

The lung doesn't have to do anything for this to happen. If you take a balloon filled with air and push it under water, the pressure outside will be greater than the pressure inside, causing it to shrink until the pressures are equal. It's unequal pressures that have to be actively maintained with force, and your muscles simply can't support a very big differential between your lungs and ambient pressure. There's a reason snorkels aren't several feet long - you just can't force unpressurized air into your lungs against more than a small water pressure.

[CaptainPanic]

So the pressure inside the lungs is equal to the pressure outside the body. Is this because pressure outside pushes on you and lung increases the pressure by same amount to neutralize this.

 

It's the gas inside the lungs that needs to push equally hard - not the lungs themselves. But you're basically correct.

 

And the lungs, or the gas, don't have to do anything. It just happens. The gas simply gets compressed like the balloon example.

 

In a theoretical case where you would be under 10 meters of water, and you would somehow happen to have air in your lungs at 1 bar (not 2 bar)... then the sea would simply crush you until the pressure inside your lungs is 2 bar (and the volume is only half of what it was before).

Luckily, the breathing apparatus for scuba diving provides air at the right pressure... so thing is being compressed - you can just breathe normally - but you do breathe air at a higher pressure!

 

 

The lungs are really very simple: they're just an open connection with the air around you. Whatever the pressure there, it's the same in your lungs. On a mountain, the air pressure is low, and the pressure in your lungs will also be low. Under water, the pressure is high, and in your lungs it will be high.

It has to be like that, because if the pressure would not be the same the results would be painful. If you have more air in your lungs than outside (higher pressure inside than outside), then the air pushes so hard that you would just automatically breathe out. Your only alternative is to explode.

If the pressure in your lungs is lower than outside, then your chest would be crushed... or, air would rush in through your mouth.

Edited November 6, 2009 by CaptainPanic

[mzatanoskas]

My knowledge of the subject is from a generalised scuba/freediving background rather than science so apologies if not entirely accurate...

 

With diving the first problem caused by changing pressure is the compression and expansion.

 

As you go deeper underwater, all the water above you pushes down more and more on your body, in other words exerts pressure.

 

Your body is mostly water and water doesn't compress or expand very much, so no problem there.

 

However within your body are pockets of air (lungs, inner ear and sinuses) and of course air is a gas and gases do compress or expand a lot. You will also have pockets of air in your mask.

 

You must make sure these pockets of gas are kept at roughly the same pressure as the external environment or else there will be trouble.

 

If you don't "equalize" the pressure for these pockets of gas, as you dive deeper and deeper, the weight of the water pushes more and more on them and they compress and get smaller and smaller.

 

When you are scuba diving, you breathe air from a highly compressed source, something like 120 bar or so, and the Scuba system cleverly shifts that pressure down to match the ambient pressure.

 

When you take a deep breath of air from a Scuba system 10 metres under the water, you are breathing air at 2 bar pressure. You are essentially breathing twice the amount of air in one breath than you would at the surface. Because of this your lungs don't change in size at all.

 

A problem does arise for the pockets of air in your inner ear and sinuses. These pockets are surrounded by inflexible skull. They are actually joined to your nose and mouth cavity by very small tubes that are usually closed. You need to consciously force some extra air into these pockets by one of a number of different techniques in order to stop them from compressing too much and causing problems. You also need to force air into your mask. If you didn't the space between your mask would get smaller and smaller until the mask frame couldn't shrink anymore at which point your eyeballs would pop out of your skull into the mask!

 

This carries on as you go deeper, but of course the difference in pressure changes slower the deeper you go, so the most difficulty is in those first 10 metres or so.

 

Now when you want to go back up to the surface again, the opposite happens. If you are at 30 metres, you are breathing air at 4 bar. As you go up, this air expands as the pressure from the water pushing on you decreases. As for your lungs, no problem, you just breathe that air out. Usually the same applies to your inner ear and sinuses, the air just forces it's way out of the small tubes. Sometimes however if you have a cold or if you rise too quickly the tube gets blocked and you get a reverse ear/sinus squeeze. The air in the pockets expands but can't escape anywhere. This is damn painful but usually sorts itself out if you just take your time.

 

The real danger would be if you held your breath while ascending from a scuba dive. As the pressure from the water is taken away, the air expands and if you hold your breath it has nowhere to go. What can happen is that your airway can then be forced shut and your lung ruptures instead. This is not good at all, but very rare because you just need to keep breathing to stop it happening.

 

Freediving is when you just take a deep breath at the surface and dive on one breath.

 

In this case you still need to worry about equalizing your inner ear and sinuses.

 

Your lungs however are very flexible, like a balloon to a certain extent and so is your rib cage. As you dive down, the water pressure pushes on your lungs, the air inside them compresses and just get smaller and smaller. It's quite interesting to observe because you can see your diaphragm get pushed up into your rib cage when you get quite deep.

 

Once you get to around the 40 metre or so mark, however you start reaching the smallest size your lung can get. If you go past this depth (or even at shallower depths) you can get lung squeeze, where a bit of your lung rips and blood pours into your lung to make up the pressure difference. However us mammals also have something called the Mammalian Dive Reflex where blood shifts to the lungs and the lungs kind of turn inside out a bit to fill up that ever shrinking pocket of air. With highly trained and talented divers the reflex can allow humans to dive to 200+ metres on one breath.

 

On the way up when freediving, you also have to let the extra air out of your inner ear/sinuses/mask (usually happens automatically), but you don't have to worry about your lungs as they will just expand back to the same size as they were when you took your deep breath at the surface.

 

Check this picture out of a freediver ascending from a dive. The air bubbles are from the expanding air in the diver's mask as the diver rises. The diver had forced that air into his mask on the way down to equalize the pressure.

 

Freedivers don't have to worry about nitrogen narcosis or the bends either until very deep depths (around 100m or more). I'm guessing this is because although the pressure gets very high, the amount of nitrogen in the air in their lungs is no different to the surface. Also the surface area of the lung to absorb the nitrogen is going to be proportionally less as the lung gets smaller. I don't know for sure.

 

Well I sure went off on a bit of a tangent there. Sorry for the novel, hope I didn't just confuse the matter. And just because freediving is so amazing, here's a

.