Drysuit squeeze

[Web Monkey]

Anybody know why adding air to a drysuit makes the "suit squeeze" go away?

When the suit has air in it, the water pressure on the suit is exactly the same as the air pressure in the drysuit and the air pressure on the diver is the same as the suit pressure and the water pressure.

When the suit has no air in it, the water pressure certainly doesn't change, although now it's transmitted directly to the diver instead of to the air in the suit, then the diver.

So if the pressure on the diver is the same, why does it feel different?

Terry

 

[Wreck]

The principles involved are similar to vacuum packing. When you break that vacuum pack seal, the air goes in and the surface tension on the enclosure is relaxed back into something closer to it's original form.

Think of it this way: a diver does not compress anywhere near as much as the air in the drysuit. The human body is mostly water and water does not really compress for most practical purposes. So what's happening is the diver's body may compress ever so slightly, if at all, but the air in the drysuit is really going to compress down between the outward force of the body and the drysuit surface. This is what causes the squeeze. Adding more air relieves the tension of the drysuit material, allowing it to relax back into it's ordinary state.

This isn't a very coherent explanation, but it might point you in the right direction.

 

[vjongene]

Anybody know why adding air to a drysuit makes the "suit squeeze" go away?

When the suit has air in it, the water pressure on the suit is exactly the same as the air pressure in the drysuit and the air pressure on the diver is the same as the suit pressure and the water pressure.

You are confusing pressure and volume. For the suit not to squeeze or balloon, the *volume* of air in the suit has to remain constant. Boyle's Law states that the product of pressure and volume is a constant if the amount of gas doesn't change. So if the pressure increases (you are going down), the volume decreases (the suit starts to squeeze). If you want to keep the volume constant, you have to add air proportionately to the increase in pressure. Conversely, when you are coming up air has to be vented out of the suit to keep you from becoming a surface-bound missile. This is why your purge valve should be open during ascent.

HTH,

Victor J.

 

[Web Monkey]

I understand that the volume changes according to pressure, my question was why at a constant pressure (depth) does adding air to a drysuit make it stop squeezing?

Terry

You are confusing pressure and volume. For the suit not to squeeze or balloon, the *volume* of air in the suit has to remain constant. Boyle's Law states that the product of pressure and volume is a constant if the amount of gas doesn't change. So if the pressure increases (you are going down), the volume decreases (the suit starts to squeeze). If you want to keep the volume constant, you have to add air proportionately to the increase in pressure. Conversely, when you are coming up air has to be vented out of the suit to keep you from becoming a surface-bound missile. This is why your purge valve should be open during ascent.

HTH,

Victor J.

 

[gj62]

I understand that the volume changes according to pressure, my question was why at a constant pressure (depth) does adding air to a drysuit make it stop squeezing?

Terry

Put your drysuit on. Now, stick a vacuum under the seal and suck the air out. You've created drysuit squeeze on the surface. How are you going to fix this? Add air to the suit. What is happening is when you descend to a depth, the air in the suit compresses, but your body does not. This causes the squeeze (same as mask squeeze, which is why your nose is inside the mask). Adding air relieves this squeeze. The external pressure is the same, but now there is enough air inside the suit to equalize the pressure surrounding the suit. It is purely for comfort - we don't like to feel the suit "glued" to our skin - the pressure on our bodies remains constant at the same depth.

Now, if you are saying that remaining at a constant depth your suit will begin to squeeze again, then you've got a leak...

 

[Schmanch]

Squeeze Doesn't Go Away,

I believe what you are referring to is the feeling of the squeeze, which is not attributed directly to the water pressure on the suit so much as the suit pushing on your body.

In other words, when you at depth, and you feel the drysuit squeezing you, what you are feeling is the pressure of the water directly against your body via the suit. By adding gas to the suit, the pressure in the suit is still roughly equal to the ambient water temperature, but at this point you are essentialy floating in a gas bubble. You no longer feel the water "directly against your body even thought the net change in pressure was zero.

Hope that helps

Schmanch

 

[Schmanch]

Ooops

Temperature should read pressure.

Schmanch

 

[Rick Murchison]

If the drysuit were a perfect fit against your skin and there were no air spaces, then your logic would prevail and there would be no squeeze. But that's not the case. Your underwear, creases in the suit, any place that holds air on the surface is going to shrink as you descend as the air loses volume due to pressure. At the point where the available air volume is less than the minimum space the drysuit/underwear can shrink to, any further descent will cause a relative vacuum in those spaces, and your skin will be pulled on to try to fill the space - and if you don't add some gas to relieve the vacuum, you'll end up with drysuit hickeys all over the place.
Rick

 

[Web Monkey]

That would do it! Thanks!

It bothers me when it looks like the laws of physics aren't working properly

I never thought about it until the other day when I went down under-weighted quite a bit and couldn't add enough air to the suit to fix the squeeze.

Then I started wondering why, since there's no problem with "wetsuit squeeze" . . .

Thanks!

Terry

If the drysuit were a perfect fit against your skin and there were no air spaces, then your logic would prevail and there would be no squeeze. But that's not the case. Your underwear, creases in the suit, any place that holds air on the surface is going to shrink as you descend as the air loses volume due to pressure. At the point where the available air volume is less than the minimum space the drysuit/underwear can shrink to, any further descent will cause a relative vacuum in those spaces, and your skin will be pulled on to try to fill the space - and if you don't add some gas to relieve the vacuum, you'll end up with drysuit hickeys all over the place.
Rick

 

[gfisher4792]

...since there's no problem with "wetsuit squeeze" . . .

Terry

If you don't think there's a problem with wetsuit squeeze, you should see me trying to squeeze into one