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I recently watched a Mythbusters episode where they tested decompression in a tethered diving suit (which I believe is a Standard Diving Dress). When they removed the air pressure, the suit crumpled up and forced the body into the helmet. They had the suit 300ft underwater, and said that was something like 135psi.

My question is, why wasn't the body crushed by the air pressure keeping the suit inflated? If the air needs to exert 135psi to keep the suit inflated against the water, why does that not crush the person in the same way the water does?

I know that in this case they made a body which isn't "airtight" in quite the same way that a human is, but live people use these suits as well.

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Real bodies aren't airtight. In fact, one of the most important principles guiding the evolution of multicellular organisms is that materials (such as oxygen) need to be exchanged throughout the organism, and so many interfaces are actually conducive to equilibrating pressure.

When the ambient air pressure increases, the air pressure in your lungs increases just as much, since there is no barrier separating these things. This is a good first step to making sure your body isn't crushed (and conversely is why, should you ever find yourself suddenly in a vacuum, you should exhale rather than try to maintain pressure in your lungs). The same holds for the air pockets in your middle ear, which equilibrate with ambient pressure via the Eustachian tubes.

But it doesn't stop there. Your body is mostly liquid (as opposed to being a rigid solid) and so pressure differences will equilibrate, as per Pascal's law. And water is rather incompressible - large changes in pressure result in only small changes to volume. So the pressure everywhere in your body increases without much else changing.

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  • $\begingroup$ In addition to this, it is worth noting that if you were to increase the pressure in the suit, it would have more of an effect on your body. Eventually, that air pressure would crush you. $\endgroup$ – jkeuhlen Jul 6 '14 at 2:35
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What tends to do damage to soft flexible objects is pressure pushing outwards.

The human body is pretty complex, so lets start with something simpler, an inflated and tied off balloon.

Lets say we put our balloon in a pressure chamber. As we crank up the pressure the balloon will deflate such that the pressure inside the balloon remains slightly above the pressure outside it. The balloon will not be damaged.

Now lets say we put our balloon in a vacuum chamber. This time the balloon will expand to maintain the pressure inside slightly above the pressure outside. Eventually it will expand to the point of bursting.

Now lets say we fill the balloon with water instead of air. In this case it will survive in both the pressure chamber and the vaccum chamber, chamber

Now lets say instead of one balloon we have two, one inside a pressure chamber one outside it and the balloon are connected by a pipe running through the chamber wall. As we increase the pressure in the chamber the balloon inside the chamber will shrink and the out outside will expand and explode. This will apply regardless of whether the balloon are full of air, water or some mixture of both.

We can think of our body as being like a series of balloons, most of them are full of water but some of them (the lungs) are full of air and can be opened to the air.

IF someone dives without breathing apparatus then the lungs will compress but this doesn't damage them. If someone dives with breathing apparatus then the breathing apparatus will keep the pressure in their lungs at roughly the same pressure as the water pressure while allowing the lungs to retain their normal size. If someone descends using breathing apparatus and tries to hold their breath on the ascent then damage is likely.

In the mythbusters experiment the fluids in the body are pushed from the body of the suit to the helmet, bursting anything that tries to stand in their way.

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The diving scenario is made worse because there's a little hole in the top of the helmet where air escapes until your head seals it, and then only fluid going up the line can fix things.

This is much worse than the scenario where air (or water) flows all over your body and presses everywhere equally. You have to have one spot where the pressure is NOT equalized to get spectacular body-squeezed out like toothpaste from a tube. The hole in the helmet is where the "toothpaste" comes out. It's not the pressure of water vs. air, so much as having the toothpaste tube with hole rigged up at low pressure, and otherwise, you're the toothpaste at high pressure.

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