What exactly is squeeze?

[elmer fudd]

So I was doing a little bit of reading about squeeze and one thing I found interesting is that much of what we consider squeeze is actually suction. For example mask squeeze. It's not pressure against your face and eyes that you are feeling, but suction as the air pressure inside the mask no longer matches the outside pressure. In effect your face and eyes are being sucked into the mask.

In regards to drysuits, part of the squeeze sensation is your skin being sucked into the folds of the suit.

 

[DevonDiver]

My understanding of pressure is that you are exposed to 1 atm for every 10m of depth and I could be wrong, but I think that the amount of air in your suit should have zero impact on that. That air volume is inversely proportional to depth is understood.

Yes, there is an extra 1 atm of pressure applied for every 10m further depth.

Thus, at 10m depth the pressure is 2 atm (doubled) and consequently, the air volume of a drysuit is reduced to 1/2. At 20m the pressure is 3 atm and consequently the air volume is reduced to 1/3. 30m equals 4 atm and 1/4 volume.

To my way of thinking, cold is a good reason to inflate a suit, but squeeze brought on by an ill fitting suit isn't. If you could design it out or at least minimize it that would be a very worthwhile goal.

Air inside a drysuit provides insultation. If the suit were allowed to reduce in volume upon descent (not equalising) then it would decrease the insulation provided by the suit. However, the issue of squeeze would always be the primary reason to add air to the suit on descent.

It really can be painful...

Any flexible container will reduce in volume as external pressure is increased. Boyle's Law cannot be circumvented.

The only way to "design out" the inevitablity of squeeze in a drysuit, would be to make the suit inflexible.

There are some 'hard' pressure suits utilised in scientific and/or commercial diving - but the expense, complexity and support required to use those suits would make them a ludicrous option for recreational diving.

 

[elmer fudd]

Any flexible container will reduce in volume as external pressure is increased. Boyle's Law cannot be circumvented.

The only way to "design out" the inevitablity of squeeze in a drysuit, would be to make the suit inflexible.

I don't think you're right about this part. Gasses are compressible, liquids and solids are not, and even if the air volume inside the suit is compressed, it doesn't necessarily mean you will be squeezed by it. Air at depth is always compressed relative to it's volume at the surface after all, but we don't always experience a squeeze.

Looking at the picture you posted the bruises actually appear to correspond to what would be folds in the suit. My guess is if you eliminate the folds you eliminate much of the squeeze factor as well.

 

[Tropicalwolf]

This is exactly what people are talking about when they sell used wetsuits, i.e. "No Compression Damage".

 

[DevonDiver]

I don't think you're right about this part. Gasses are compressible, liquids and solids are not, and even if the air volume inside the suit is compressed, it doesn't necessarily mean you will be squeezed by it. Air at depth is always compressed relative to it's volume at the surface after all, but we don't always experience a squeeze.

I don't mean to appear arrogant, but please check my profile/qualifications/experience. I am right.

Having said that... you have every right to go drysuit diving without bothering to equalise your suit on descent. Please be my guest....

Boyle's Law is a given certainty. Any air space within an unequalised drysuit (flexible container) will reduce in volume upon descent.

It is not called a Gas Law because it only happens occasionally....

As the volume of that air space, surrounding the body, diminishes it will eventiually cause a squeeze to be felt. The onset of this squeeze is affected by the material composition of the suit, but eventually the volume will be sufficiently reduced with any suit to cause squeeze pain and damage.

Looking at the picture you posted the bruises actually appear to correspond to what would be folds in the suit. My guess is if you eliminate the folds you eliminate much of the squeeze factor as well.

No drysuit is a 'perfect' skin-tight fit. The POINT of a drysuit is to allow an insulating layer of air to exist between the suit and the diver. Therefore, the suit is ALWAYS bigger than the diver. There is always more suit material surface, than skin surface underneath it.... thus when compressed, the suit must fold in places - especially in areas of joints (shoulders, elbows, knees and hips).

As that (bigger) suit is compressed tightly against the skin by pressure, it will crease and fold. That will cause the pinching most commonly noticed during squeezes. Thicker drysuit material (i.e. neoprene) will delay this effect, but will not prevent it.

Also, any undergarments worn below the drysuit will be squeezed into/against the flesh - causing the same 'pinching' pain and damage.

In addition, you seem preoccupied by the 'pinch' factor of a squeeze. It isn't just a case of the material pinching you. There is also a 'suction' effect on the skin, as the compression tries to suck your body to fill the air space. That can cause bruising in the same way as a 'hickey' type kiss will cause bruising by sucking on the skin.

To understand this (if you've never experienced a 'hickey'...just seal your mouth over your arm and suck on it. The reduced air pressure in your mouth will draw the skin of your arm inward to your mouth. It will also draw blood to the surface of the skin, as capillaries burst.

Now...consider the huge difference in pressure between what you can achieve by sucking with your mouth (max 10psi)...and the pressure that is exerted by atmospheric pressure at depth (14.7 lbs per square inch PER atm).

 

[cloudflint]

^^ what he said

If you descend without adding any air to the suit then due to Boyle's law the volume of the suit is going to decrease, crushing you and causing a suction which will damage your skin. The only way to prevent this is to add more air in order to keep a constant volume as you descend. The only way to prevent having to do this would be to get all the air out of the dry-suit, which would effectively shrink wrap you and make you unable to move, you would also be freezing since the dry suit relies on the layer of air for insulation.

Also, here's a video showing what 11 atmospheres of pressure can do to a diver if your suit isnt equalised: http://www.youtube.com/watch?v=LEY3fN4N3D8 obviously we dont dive those helmets so much any-more but its a good example of what the pressure can do.

 

[DevonDiver]

Great link. I loved that episode of mythbusters... it certainly illustrates Boyle's Law. Kinda makes me feel sick to imagine that happening with a real diver!

Also.... that shows exactly why I would never get interested in vintage diving equipment! lol Although...theoretically...this could happen with your regular scuba mask!

 

[cloudflint]

Great link. I loved that episode of mythbusters... it certainly illustrates Boyle's Law. Kinda makes me feel sick to imagine that happening with a real diver!

Also.... that shows exactly why I would never get interested in vintage diving equipment! lol Although...theoretically...this could happen with your regular scuba mask!

Yeah if you went down without equalising your scuba mask a similar thing could happen , you could suck your eyeballs right out your sockets :depressed:.

I think the reason the suit in that video got so crushed is because it was running off surface supply so when they shut off the air and opened the end all the air was forced out the suit, the end result being that you had the same amount of air in the suit as you would have at the surface, it was just compressed down to ten times smaller than normal. The fact that with that instance there is a big helmet full of air kinda exasperates the problem. So basically if you put on one of those suits and just jumped down to 100 meters it would cause an identical effect.

 

[TSandM]

Elmer, you do have a point -- as I said, if you could buy a full neoprene drysuit that fit like a second skin, you'd have very little chance of squeeze. But the suit would be cold at depth. And ANY place where the suit gapped away from the body, that space would reduce at depth. It might not cause "squeeze" but it would probably restrict mobility.

Laminate and compressed neoprene suits have little or no ability to stretch. Mobility for the diver is gained through the ability of the suit to move with respect to the diver's body. When a suit is severely compressed against the diver, much mobility is lost -- even to the point where you can't reach your inflator or dump valves. The reason for extra fabric is to allow the diver to move -- your "perfectly cut" dry suit would not be safe because of inadequate mobility.

 

[Kingpatzer]

Where this does get a bit more interesting is how much squeeze could be eliminated by a better undergarment design? If one wore a thick neoprene wetsuit under the drysuit to minimize and normalize the air space's direct contact with the skin, then put over that a cover with a loft-able material, the result might well be a more comfortable diving experience underwater. However, I can't imagine that producing a more comfortable experience prior to getting into the water.