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Question Mask volume and "compressibility ratio"? Why its important?

mil84

New Member
Apr 9, 2019
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Bratislava
So far I always read, the lower volume the better and there is nothing else to care about. I understand basic logic - the smaller volume, the easier to equalize.

But then I read some threads on this forum, and some users often mention things like "compressibility ratio".

I even found a post here: http://apneaddict.blogspot.com/p/masks.html - where guy computes compressibility ratio when testing Micromask & using some magical math comes to conslusion that "compressibility ratio of 2.8 would suggest no need for equalization before 18m."

To be honest, I absolutely dont get it.

How does he compute that?
And most important, why it matters if mask is easily compressible or not?

Using my commons sense - when water is pressing on your mask, it compress air inside and thats why you need to put extra air in. So best would be if it mask was hard rock solid - so it cant be pressed => you dont need to put any air inside as u go down.

Or am I wrong? Can somebody explain me this? Thanks :)
 

Nathan Vinski

Well-Known Member
Apr 19, 2015
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Math.

18m = 2.8 bar of pressure = 2.8 compressibility ratio.

If you equalised the mask at 10m you could make 46m without needing to equalise. 2bar x 2.8 = 5.6bar

The more compressible the better. No matter what, the air inside will shrink/be compressed inversely proportional to the pressure. The more compressible a mask, the deeper you can dive without needing to add air. a rigid mask would quickly create suction causing injury to your face and eyes
 
OP
OP
mil84

mil84

New Member
Apr 9, 2019
5
0
1
36
Bratislava
18m = 2.8 bar of pressure = 2.8 compressibility ratio.

If you equalised the mask at 10m you could make 46m without needing to equalise. 2bar x 2.8 = 5.6bar

Thanks now I finally get it.

The more compressible the better. No matter what, the air inside will shrink/be compressed inversely proportional to the pressure. The more compressible a mask, the deeper you can dive without needing to add air. a rigid mask would quickly create suction causing injury to your face and eyes

Not sure I understand. Teoretically would not be best mask which cant be compressed? For example from iron. This way it cant be compressed at all, air inside is not compressed, and you dont need to equalize. So why is more compressible better? It just makes mask squeeze earlier, compress air inside earlier, and you need to equalize earlier.
 
OP
OP
mil84

mil84

New Member
Apr 9, 2019
5
0
1
36
Bratislava
Sorry I forgot to format it and now cant edit. Posting again:

18m = 2.8 bar of pressure = 2.8 compressibility ratio. If you equalised the mask at 10m you could make 46m without needing to equalise. 2bar x 2.8 = 5.6bar
Thanks now I finally get it.

The more compressible the better. No matter what, the air inside will shrink/be compressed inversely proportional to the pressure. The more compressible a mask, the deeper you can dive without needing to add air. a rigid mask would quickly create suction causing injury to your face and eyes
Not sure I understand. Teoretically would not be best mask which cant be compressed? For example from iron. This way it cant be compressed at all, air inside is not compressed, and you dont need to equalize. So why is more compressible better? It just makes mask squeeze earlier, compress air inside earlier, and you need to equalize earlier.
 

NoFair

Active Member
Aug 18, 2014
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Sorry I forgot to format it and now cant edit. Posting again:



Thanks now I finally get it.



Not sure I understand. Teoretically would not be best mask which cant be compressed? For example from iron. This way it cant be compressed at all, air inside is not compressed, and you dont need to equalize. So why is more compressible better? It just makes mask squeeze earlier, compress air inside earlier, and you need to equalize earlier.
If the mask doesn't flex something else will try to (your face in this case) ;) If your face was made of iron you'd be fine
 
OP
OP
mil84

mil84

New Member
Apr 9, 2019
5
0
1
36
Bratislava
Ok thats a good point. But still - why its more flexible better? It just compress air more. And thats worse. Or I am missing some elementary fact to understand :))
 

Nathan Vinski

Well-Known Member
Apr 19, 2015
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Elementary fact: the air doesn't shrink because the mask is compressible, the mask compresses because the air shrinks.

If you had an iron mask for example, the air would shrink, but the mask wouldn't compress. This would suck your face and eyeballs into the rigid mask.

A soft mask = face and eyeballs stay where they are supposed to be while the mask gets sucked onto your face

In both cases you need to equalise the mask. The softer the mask, the less frequently you must equalise it as it can compress more beforr sucking your face and eyes.
 

HLanger1955

Well-Known Member
May 17, 2013
100
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Italy
When I got it correctly the point is that your body compresses at depth. If your mask doen't compress and you don't compensate, you'll get a relative underpressure in the mask volume, which sucks on face and eyes...Besides that the masks frame presses heavily on the face when not compensated.
 

Leander

Active Member
Oct 17, 2017
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Between here and nowhere
The mask pressure issue is because the face isn't rigid and will try to equalize the pressure inside the mask to the pressure outside? In case of a rigid mask ofcourse.

But imagine a steel sphere, perfectly round and hollow inside. Inside it has 1 liter of air at 1 bar. When you increase the pressure surrounding the sphere the steel shell will resist the relative low pressure inside and thus will resist collapse. Inside the volume is still the same and so is the absolute pressure, right?

The thing I don't get -and what the OP probably also wonders- is why your eyeballs feel like they're going to pop out of the skull (which they are actually doing) if you try to dive deep with pool goggles. But on the other hand you got the Badjau people, diving deep with wooden googles that cannot be equalized underwater. Yet they all seem to have their eyes intact afters years of diving. The only difference in goggle designis the material, the stiff wood vs the flexible silicon.
 
OP
OP
mil84

mil84

New Member
Apr 9, 2019
5
0
1
36
Bratislava
The thing I don't get -and what the OP probably also wonders- is why your eyeballs feel like they're going to pop out of the skull (which they are actually doing) if you try to dive deep with pool goggles.
Yes. I mean if you have mask which cant be compressed, also air inside should not compress right? Air inside is separated from water hermetically, so it doesnt know how deep I am. I dont quite understand what somebody mentioned "mask compress because air inside compress". Why its not opposite? I think air compress because mask is pushed against face by higher water pressure and lowers its volume => compress air.

But then again, try to push mask against your eyes on dry land, and same in 10m depth, its different. Your eyes dont wanna pop out of your head on dry land, but they want in depth. And I dont understand why, its exactly same to me. Also solid, woode googles which cant be compressed should therefore not cause any problems imho. But they do.

So I am still confused, but its not like its important for my diving. Its just theoretical question, asked out of curiousity, I just wanted to undertand it :)
 

HLanger1955

Well-Known Member
May 17, 2013
100
14
58
Italy
Yes. I mean if you have mask which cant be compressed, also air inside should not compress right? Air inside is separated from water hermetically, so it doesnt know how deep I am. I dont quite understand what somebody mentioned "mask compress because air inside compress". Why its not opposite? I think air compress because mask is pushed against face by higher water pressure and lowers its volume => compress air.

But then again, try to push mask against your eyes on dry land, and same in 10m depth, its different. Your eyes dont wanna pop out of your head on dry land, but they want in depth. And I dont understand why, its exactly same to me. Also solid, woode googles which cant be compressed should therefore not cause any problems imho. But they do.

So I am still confused, but its not like its important for my diving. Its just theoretical question, asked out of curiousity, I just wanted to undertand it :)
At depth your body is compressed, too. That way the inner pressure inside your body increases. If the pressure in the mask volume remained unchanged there would be a pressure difference between inside your body and mask -> that diffference sucks on your eyes....
 
Mar 22, 2009
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www.MakoSpearguns.com
I think the idea is that if the skirt of a mask is soft enough and fits in a particular way, then the skirt will compress and the faceplate will move closer to your face. If the mask itself displaces less volume at depth, then it will take less air from the diver to keep the mask more or less equalized.

The idea is that pressure and volume are inversely related. so a mask that is compressed has a reduced volume, and consequently the pressure inside has increased (realtive to a completely rigid mask). If the pressure increases from mask compression, then it will take less air from the lungs,

This deformation of the mask is limited before it may begin to leak and or become uncomfortable or the pressure inside the mask becomes too low relative to the external pressure. When this happens, a partial vacuum is forming in the mask and skin, and eyes and even blood is pulled toward the inside of the mask because of the pressure differential. If a diver allow the differential to become too large, then blood vessels in the whites of the eyes will begin to burst.
 

Nathan Vinski

Well-Known Member
Apr 19, 2015
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Canada
If you're having trouble understanding why the gass will compress regardless of the rigidity of the container it's in..

Read about Boyles law.

The basic principle is that the volume of a gas is inversely proportional to the absolute pressure.

This means that at 10m (2 bar pressure), the original gas volume WANTS to be 1/2.

In a compressible container, the container will deform to half its original volume.

In an incompressible container, the gas will produce a vacuume. This is what causes injury when not properly equalising the mask.

The idea that a rigid container can protect the gas from the pressure is false. It just increases the strength of the vacuume + the chance of having barotrauma in diving.

To reiterate. A mask squeeze DOES NOT occure while the mask is compressing. It occures when the mask STOPS compressing. If it never compresses, you will squeeze almost immediately If you don't equalise.
 
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