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mask squeeze?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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J Campbell

Well-Known Member
Sep 17, 2001
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I'm not understanding mask squeeze. It seems to me that as you dive deeper the water pressure will press the mask closer to your face and this compress the air within. So how does this "suck your eyeballs out" as some have described - or how does it suck blood into your eye vessels? It seems to me that a pressure action would happen - not a sucking action. What am I missing here?
 
Mask volume is constrained by the material the mask is made of ie. it does not compress in direct proportion to the pressure around it. But your eyeballs are more malleable than your mask effectively get sucked in to try to equalise the pressure


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I'm still not understanding this - where does the "suck" come from? When you dive there is PRESSURE on your mask, it is being pushed against your face, not pulled away from your face. If anything, it seems to me that this would cause your eyeballs to be pushed further into your head.
I'm not doubting the effect, I just need an understandable explanation. I have seen guys with the bloodshot eyes.
 
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The pressure is on all sides of the air space in your mask and your mask is not completely flexible. The pressure increases on the outside of your mask and the volume of the airspace inside your mask decreases. If the mask was completely elastic it would just get closer and closer to your face however the mask is not completely elastic so it gets to the point where the mask itself cannot deform to cope with the decrease in volume and it is your eyes / blood vessels are the next point to give I.e. Your eyes/blood vessels in them start to fill the space.

Sorry for the poor attempt at explanation!!


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If you want an extreme (and rather disturbing) illustration, check out the mythbusters clip about what happens to a hard hat diver if the check valve in his air supply line fails (i.e. the pressure inside the helmet suddenly drops to 1atm) . I don't remember off hand what depth they used for this test, but I want to say it was less than 200'.

Pressure at depth is serious business.
 
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The key is differential pressure. The pressure outside the mask builds much faster than the pressure inside (as go-hard said, due to the inflexibility of the mask). Whether you care to think of this as your eyeballs getting sucked out of your head by the 'vacuum' inside your mask, or getting pushed out by the water pressure on the rest of your body, the effect is the same.
 
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But isn't your body (other than air filled space) incompressable? Where is the air filled space behind your eyes that can push your eyes out?

And yes, that video demonstrates the effect very nicely...
 
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I would say that the rubber part of the mask, being flexible, starts to reach it's limit of flexibility. As external pressure increases, the air INSIDE compresses, ie: takes up less space. As this increases, the flesh inside the mask starts to move into the space because the skirt and the hard lenses can no longer take up the empty space. Not that there is a compression against the face anymore. So in that way, there is a suction created that the eyes and skin are affected by.

Your head is incompressible except for the sinuses so that's a moot point.
It sucks. Literally haha
 
It's precisely because your body is incompressible that this happens. Forget for a moment that you have a body and only picture the air-filled spaces that you are carrying down to the bottom of the ocean with you--sinuses, lungs, mask, stomach, intestines, etc. Every one of these air spaces is surrounded by mix of incompressible fluids (water, blood, various bodily fluids, etc.), rigid structures (ribcage, the frame of the mask, the bones of your face, etc.), and semi-rigid tissues (cartilage, skin, etc.).

At sea level, the average person's blood pressure of 120/80 works out to about 1.16 bar absolute. At 60m, this same person has a blood pressure of 7.16 bar absolute. The reason they don't explode in a giant spray of gore is that the surrounding water is pushing in on their entire body with a force of 6 bar. Since blood and most body tissues are incompressible, the net result is that their blood pressure is still 1.16 bar, so the force against the walls of their blood vessels remains unchanged.

The exceptions are where you have blood vessels passing close to air spaces that, for whatever reason, are at a lower relative pressure. This is usually because these air spaces are surrounded by some kind of rigid structure that prevents the air space from being compressed enough to balance the force of the water pressing in from the outside, such as your mask.

So, imagine your blood is pushing against the walls of your blood vessels with a force 7.16 bar (thanks to the water pushing on your body), and the air space inside your mask is only pushing back with a force of 6.16 bar (since the mask can't flex enough to fully compress the air inside). The difference of 1 bar is enough to rupture blood vessels, and generally make a mess of your face.
 
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Honestly, the effect is much closer to a hickey than a squeeze, but it's impossible to sound cool when you say "Man, I set a PB last weekend with FRC, but I couldn't equalize and wound up with a serious mask hickey."

I'll stick with calling it a squeeze, thanks.
 
OK! I got it now! I see the light! Psimian - thanks for your detailed explanation.
 
Oh, I kinda like mask hickey... Got me thinkin now - someone needs to make a mask that looks like one of those face suckers from Alien
 
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