## sucks to be a crab

Crab vs. Pipe

"This is a video taken in 6000 feet of water. An undersea robot is sawing a 3mm wide slit (1/10th of an inch... remember that width) in a pipeline. The pressure inside the pipeline is 0 psig, while the pressure outside is 2700 psi, or 1.3 tons per square inch. Then a crab comes along...."

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### 24 Responses:

1. ralesk says:

Hmmm, what is psi?  For pressure, I only know Pa (pascal, N/m2)

• klarfax says:

pounds per square inch

• ralesk says:

Interesting abbreviation :D Thanks!

• ciphergoth says:

Pounds per square inch.

1 psi ~=~ 6894.757 Pa

2700 psi ~=~ 18.5 MPa

atmospheric pressure is around 0.1 MPa.

• ralesk says:

Thank you as well ^^

2. baconmonkey says:

I've seen that movie before, but for some rason I think Sigourney Weaver was there, and it wasn't really a crab.

• feren says:

Is it just me, or was that scene the most horrifying symbolic abortion ever?

• baconmonkey says:

I dunno, I thought it was the most horrifying violation of the laws of physics.
14 psi to 0 psi is far different than 2700 psi to 0 psi.

3. chaobell says:

Holy shit!

I just watched it three times. The third time, I made the appropriate sound effects as co-workers looked on.

4. insomnia says:

That is almost exactly what I thought would happen, actually... though I'm still wondering what exactly the point is of having a leaky pipeline full of cracked crab.

• cetan says:

Feeding tube for Brando?

5. psymbiotic says:

Great! Now they got crab bits in the pipeline to clean out.

Egan >:>

• jwz says:

Well.... the opening was 1/10th inch, so I don't think it's so much crab bits as a crab mist...

• Sort of like the end of Alien: Resurrection.

• jwz says:

Which was total crap, BTW. The pressure difference between inside the ship and outside was (kinda by definition) one atmosphere. Human skin regularly resists five or ten atmospheres every time someone goes scuba diving (pressure doubles every 10M under water.) This crab met a pressure differential of somewhere around 180 atmospheres.

• swoobysnacks says:

Almost... pressure increases by 1ATM each 10M, not doubles. A 66 foot deep dive is at 3 ATM, and a 100 foot dive is at about 4.

• jwz says:

Huh, I thought I read that it doubled. Well, either way -- people don't just implode!

• swoobysnacks says:

So you get 1 ATM for each 33 feet of freshwater, and 1 ATM for each 32 feet of saltwater (denser stuff).

My old dive instructor used to dive to about 350 feet in freshwater, which would over 10 ATM more than at the surface.

However, skin doesn't really "feel" 11 ATM of pressure, because we are essetially big incompressible bags of water. The only parts of our bodies that get squished are airspaces, specifically lungs and everything they're connected to. The scuba regulator's job is to deliver air into those spaces at ambient pressure, so your body is equalized from the inside out.

Now, there is one other interesting case: free divers. There are some nutjobs who take a really deep breath and dive down as far as they can without any breathing equipment (other than a snorkel). They don't get the benefit of scuba equalization, and they have gone down to over 300 feet just holding their breath. 10 ATM, easy-like. :)

• ivorjawa says:

Freedivers don't breathe. They do have to equalized their ears all the way down and up, though.

JWZ -- diver's don't feel a pressure differential. The regulator provides air at the pressure you're diving, which is why you can burn through a tank in five minutes at (some depth, I don't have my dive tables handy), while you can breathe on one for 60 minutes near the surface.

What killed the crab (and the alien in Alien 4) was a pressure differential. I once had a chemistry teacher do a demonstration where the atomosphere was sucked out of a small gas can. 1ATM of differntial pressure was more than sufficent to completely crush the can.

Likewise, a human exposed to vacuum completely will be basically freeze-dried, not explode, because the gas will be sucked out of his lungs and there won't be any pressure differential. But a small hole in a space suit will be both messy and painful.

• ciphergoth says:

185 atmospheres - did you independently work that out, or work from the sums I posted here?

And pressure *definitely* increases linearly.

1) Imagine a weighing scales with a weightless tube on top in a vacuum. As you pour water into the tube, the height of the column, the weight of the column, the reading on the scales and the pressure on the scales all increase linearly.

2) If it were exponential and there were no atmosphere, the pressure on the surface would be zero, so the pressure at all depths would be zero.

3) If it were exponential, the pressure at the bottom of the Marianas Trench would be obviously ludicrous - easily enough to ignite a star down there I would guess.

• psymbiotic says:

I showed this to a co-worker of mine; his response was:

"Pipe Wins! Fatality! Flawless Victory!" (he's a bit of a Mortal Kombat fanatic).

Egan >:>

6. retrodiva1 says:

OMG that's hilarious I posted the same thing in my journal this morning without having read yours. I even thought of you when I posted it, figured it was something you would get a kick out of. :o) Demented minds think a like...

7. rasp_utin says:

I simply can't fathom what the sensation would be like, were I in the crab's place... During that brief moment of getting stuck, it must have been the epitome of excruciating pain.