Why do you get dry suit squeeze and not wet suit squeeze?

[Thalassamania]

It does matter, force is force. Define the force.

So, you want to learn the Force. . .Help you I can. My ally is the Force. And a powerful ally it is. It is what gives a Jedi his power. It is the energy field created by all living things. It surrounds us, penetrates us. It binds the universe together.

The Force is this and more. It cannot be fully described with words. You must feel the Force to understand it. To feel the force is what you will learn.

 

[ClayJar]

Hmm... does anyone want to donate a dive to science? You've got to have a shell suit, and you might get a little pinched. (I'd experiment on myself, but I can't be in the drysuit and pulling the wrinkles out at the same time. Tried that, and it didn't quite work.)

 

[Soggy]

So, you want to learn the Force. . .Help you I can. My ally is the Force. And a powerful ally it is. It is what gives a Jedi his power. It is the energy field created by all living things. It surrounds us, penetrates us. It binds the universe together.

The Force is this and more. It cannot be fully described with words. You must feel the Force to understand it. To feel the force is what you will learn.

The force is strong with Thalassamania. You must have high mitichlorian levels.

 

[ReefHound]

Boyle's law. I explained it above.

You didn't quantify the forces, we know the pressure increases, as your master has decreed the difference between the air pressure and water pressure is "Only a few psi different than ambient (if that)".

How is a suit pressing against your skin at a given PSI different than air or water pressing against your skin at a given PSI? Would you get squeeze in a chamber ride?

There seems to be two different lines of thought here. One group thinks squeeze is the higher pressure of the suit pushing on your skin. The other group thinks squeeze is the lower pressure of the air pulling on your skin.

 

[Soggy]

You didn't quantify the forces, we know the pressure increases, as your master has decreed the difference between the air pressure and water pressure is "Only a few psi different than ambient (if that)".

Who is my master?!

How is a suit pressing against your skin at a given PSI different than air or water pressing against your skin at a given PSI? Would you get squeeze in a chamber ride?

One more time. This is Boyle's law in action. You are inside a balloon. Fill a balloon with water. Increase the external pressure. The balloon stays the same size. Fill a balloon with air. Increase the external pressure. The balloon shrinks. You are inside the balloon. The balloon squeezes you because the volume of gas inside it is shrinking.

You would get squeeze in a chamber ride if you were in a drysuit, yes.

 

[H2Andy]

One more time. This is Boyle's law in action. You are inside a balloon. Fill a balloon with water. Increase the external pressure. The balloon stays the same size. Fill a balloon with air. Increase the external pressure. The balloon shrinks. You are inside the balloon. The balloon squeezes you because the volume of gas inside it is shrinking.

You would get squeeze in a chamber ride if you were in a drysuit, yes.

my keyboard gets it by now ...

 

[ClayJar]

Okay, we know squeeze happens because the air in the suit is compressible. That being a given, let's examine the mechanics of a squeezes.

To understand drysuit (or glove) squeeze, consider a ridge in the waterproof membrane of the suit. That ridge can be thought of as a flexible bag of air. When you increase the pressure (say, by descending), the volume of air in the ridge naturally should decrease proportionally.

For the early part of the squeeze, the "top" of the ridge (i.e. that portion farthest from the diver) can collapse in on itself. The ridge gradually collapses from an inverted V (or U, if you will) to more of an inverted Y shape. As the "open" end gets compressed smaller and smaller (closer to a T shape), more and more fabric is taken up by the pinched-together end of the Y. As you near T-ness, you can collect enough fabric in the closed portion of the ridges to the point that there is not enough open fabric left to comfortably encircle the diver -- you've basically "taken in" the fabric to make a much smaller suit, so you're squeezed by the suit constricting around you. (This is the classic "couldn't breathe" squeeze.)

Of course, since the material is not perfectly flexible, at the limit of its collapsibility, you can arrive at a condition where the "inside" (perhaps of one of those Y-fold ridges) is constrained from collapsing any more, which turns it into a small fixed volume (instead of the variable volume it had been prior to that point). As the volume is now fixed, the pressure is as well. The pressure against the inside of the tissue pressed against it, however, is (to a reasonable precision) the ambient pressure at that tissue's depth. In the interest of convenience, you can consider this a partial vacuum "pulling" on that tissue -- your skin, for example. (This yields the classic "drysuit hickey" result of a bad squeeze.)

So, basically, you start to feel the pinch squeeze because of the suit pressing in on you as it's taken in by the Y-ridges, and you get the big drysuit hickeys as you take it far enough that the suit has collapsed to the point it becomes a rigid body and creates small local areas of "partial vacuum". Both sides can be correct.

 

[rakkis]

It is not the diver that's getting squeezed. It is the air space surrounding the diver.

When the air decreases volume, it pulls on what's around it to take up that volume. i.e. the material and water outside and your skin and tissue inside.

The discomfort associated with squeeze is not pushing in. It is pulling out and applying shear forces on your skin.

In a fluid-filled wet suit, there is nothing outside your skin to shrink. Thus, your skin is not pulled and you don't feel discomfort.

 

[Thalassamania]

Of course, since the material is not perfectly flexible, at the limit of its collapsibility, you can arrive at a condition where the "inside" (perhaps of one of those Y-fold ridges) is constrained from collapsing any more, which turns it into a small fixed volume (instead of the variable volume it had been prior to that point). As the volume is now fixed, the pressure is as well. The pressure against the inside of the tissue pressed against it, however, is (to a reasonable precision) the ambient pressure at that tissue's depth. In the interest of convenience, you can consider this a partial vacuum "pulling" on that tissue -- your skin, for example. (This yields the classic "drysuit hickey" result of a bad squeeze.)

This is what I take to be "classic" suit squeeze.

 

[ReefHound]

When the air decreases volume, it pulls on what's around it to take up that volume. i.e. the material and water outside and your skin and tissue inside.

The discomfort associated with squeeze is not pushing in. It is pulling out and applying shear forces on your skin.

That makes perfect sense.