Drysuit and squeeze question
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infieldg
Contributor
Your body will always experience the current water pressure, the difference is friction - fabric has almost infinitely more friction than air. Try this this standing in your garage in a drysuit;
- Have someone walk around you applying clamps to all the slack material.
- Try to move - impossible despite not having the extra pressure pressing the fabric against your body.
- Inflate the drysuit to expand the clamps (assume they don't fall over and keep applying pressure)
- Try to move - no problem as the air gap eliminates friction.
Theacquadude
New
This topic has been discussed 17 years ago and nobody seems to have compiled a sounding explanation in a comment. One thing I’m sure is that there are a lot of physics misconceptions here and it’s definitely not a straight forward topic.
First misconception I see is the straight comparison to a vacuum. When you vacuum seal a bag you’re removing air molecules from it, that’s what vacuum is. Different from what happens to a dry suit here, where the amount of air that is trapped at surface still remains the same under water no matter what pressure you’re at, which is totally different from a vacuum, there’s no vacuum there, the number of air molecules is the same!!
Second misconception is saying that “the pressure is too much”. The only thing exerting pressure in this situation is the water and atmosphere, hence, there is no difference between pressure in a dry suit/wetsuit/naked. If you’re under 5 bar of pressure your body will feel 5 bar independent of what suit you wear. (Check ReefRound’s comments on the OG post, he’s right!).
Third misconception is comparing the suit to a balloon and saying that if the volume of the balloon at, let say 5 bar, decreases by 5, the volume of the suit with a person inside will also try to decrease by 5 and squeeze the person. I keep reading this bs and it’s clearly physically wrong!!
I’ll explain this 3rd one in drawings. In the first drawing there’s a 25L dry suit filled with air and no body inside (like a balloon). It’s correct that its volume will be lowered to 5L under 5bar of pressure (P1*V1=P2*V2).
The thing here is that, when you have a body inside, let’s say a 20L body. The volume of air inside is now 5L in this case (Vsuit = Vbody + Vair).
And when you dive to 5 bar this air volume will compress until 1L, V2=5L/(5bar*1bar)=1L only!
So you still have 1L of air left between your body and the suit which should be comfortable enough.
Ps: And the ambient (water) pressure is 5 bar, just as the air inside the suit is 5 bar and your body fluids are also equalized to 5 bar, all in equilibrium (equalized). It’s definitely not an unbearable pressure!
————————————————
But still dry suit squeeze happens, and it’s a real thing, why?
I think the best explanation by far has to do with the material of the suit, rakkis and ClayJar commented that in the OG thread. I’ll try to make a summary here:
When you go down, pieces of the fabric touches your skin creating air pockets, it’s these air pockets that have their volume reduced UNTIL the fabric becomes so packed that it becomes rigid, then the only way of reducing the volume is pulling your skin into the air pockets, hence the bruises. This effect of the fabric becoming rigid is what makes you unable to move your arms and legs and also your belly/chest muscles to breath. Just as infieldg exampled here on top!
See how it’s not an “extreme pressure” exerted by the suit? The pressure is still 5 bar for everything in here.
TLDR:
I - suit touches your skin leaving air pockets
II - under pressure, air pocket reduces in volume until the fabric becomes rigid (due to compression)
III - since suit can’t reduce in volume anymore it pulls up your skin into the air pockets, leaving bruises.
I just wanted to compile all the correct answers and debunk the physically wrong explanations here. Hope this helps!
First misconception I see is the straight comparison to a vacuum. When you vacuum seal a bag you’re removing air molecules from it, that’s what vacuum is. Different from what happens to a dry suit here, where the amount of air that is trapped at surface still remains the same under water no matter what pressure you’re at, which is totally different from a vacuum, there’s no vacuum there, the number of air molecules is the same!!
Second misconception is saying that “the pressure is too much”. The only thing exerting pressure in this situation is the water and atmosphere, hence, there is no difference between pressure in a dry suit/wetsuit/naked. If you’re under 5 bar of pressure your body will feel 5 bar independent of what suit you wear. (Check ReefRound’s comments on the OG post, he’s right!).
Third misconception is comparing the suit to a balloon and saying that if the volume of the balloon at, let say 5 bar, decreases by 5, the volume of the suit with a person inside will also try to decrease by 5 and squeeze the person. I keep reading this bs and it’s clearly physically wrong!!
I’ll explain this 3rd one in drawings. In the first drawing there’s a 25L dry suit filled with air and no body inside (like a balloon). It’s correct that its volume will be lowered to 5L under 5bar of pressure (P1*V1=P2*V2).
The thing here is that, when you have a body inside, let’s say a 20L body. The volume of air inside is now 5L in this case (Vsuit = Vbody + Vair).
And when you dive to 5 bar this air volume will compress until 1L, V2=5L/(5bar*1bar)=1L only!
So you still have 1L of air left between your body and the suit which should be comfortable enough.
Ps: And the ambient (water) pressure is 5 bar, just as the air inside the suit is 5 bar and your body fluids are also equalized to 5 bar, all in equilibrium (equalized). It’s definitely not an unbearable pressure!
————————————————
But still dry suit squeeze happens, and it’s a real thing, why?
I think the best explanation by far has to do with the material of the suit, rakkis and ClayJar commented that in the OG thread. I’ll try to make a summary here:
When you go down, pieces of the fabric touches your skin creating air pockets, it’s these air pockets that have their volume reduced UNTIL the fabric becomes so packed that it becomes rigid, then the only way of reducing the volume is pulling your skin into the air pockets, hence the bruises. This effect of the fabric becoming rigid is what makes you unable to move your arms and legs and also your belly/chest muscles to breath. Just as infieldg exampled here on top!
See how it’s not an “extreme pressure” exerted by the suit? The pressure is still 5 bar for everything in here.
TLDR:
I - suit touches your skin leaving air pockets
II - under pressure, air pocket reduces in volume until the fabric becomes rigid (due to compression)
III - since suit can’t reduce in volume anymore it pulls up your skin into the air pockets, leaving bruises.
I just wanted to compile all the correct answers and debunk the physically wrong explanations here. Hope this helps!
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Well, when I was too busy having fun in a neoprene suit to push the button during a negative entry
I began to become vacuum packed, with most all the air whooshing past the neck seal, and then as
the pressure increased far too much like being in a baloon without air about to be crushed to death
I inflated my suit, so any attempt at technification of the physics and biology is complete poppycock
I began to become vacuum packed, with most all the air whooshing past the neck seal, and then as
the pressure increased far too much like being in a baloon without air about to be crushed to death
I inflated my suit, so any attempt at technification of the physics and biology is complete poppycock
began to experience drysuit squeeze
which does not happen during descent
- unless, of course, you were carrying exorbitant amounts of weight, too much for the drysuits physical volume and the BCD to compensate for (this must have been an user error, or else you would be dead)
This is actually interesting! How much weight did you carry? Did you inflate the BCD at all?
which is impossible here
"The open-sea diving depth record was achieved in 1988 by a team of COMEX and French Navy divers who performed pipeline connection exercises at a depth of 534 metres (1,750 ft) in the Mediterranean Sea as part of the "Hydra 8" programme" - Wikipedia
That is about 53 atm overpressure.
Not sure about bar vs atm vs psi as the number is large enough to make rounding errors relevant.
How deep would you need to go to be actually crushed to death? Breathing gas related issues such as HPNS and possibly some pressure inflicted joint problems would become a problem much before.
Physics and personal experience are measured on different scales.
Physics is a hard fact.
Experience is a feeling.
Both are important but neither cancels the other.
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