Crush Depth for an Empty AL80?

[Brandon]

No, meters

From what I was reading, crush/yielding depth is a function of wall thickness/outer diameter and material strength. The higher the ratio, the deeper you can go.

Compare possible ratios of a 7.25" OD scuba cylinder with a .5" wall thickness to that of a 30' OD modern submarine pressure hull made out of a couple inches of layered steel. Then consider that a submarine's pressure hull has numerous holes and access points cut into it, reducing its overall strength (though that is mitigated somewhat by internal bracing) and creating additional failure points that affect the overall depth rating. That said, steel is also much stronger than aluminum.

We're unlikely to get specific enough numbers for our hypothetical submarine to do the math accurately, though the back of the napkin math appears to check out. The submarine may be 50 times larger in diameter than an AL 80... But the hull is only 4 times thicker. Not inconceivable to imagine an AL80 surviving depths 5x greater than a modern submarine. Our AL80, after all, has a much smaller surface area.

AL80s are amazingly strong, all things considered.

 

[Superlyte27]

holy cow! don't you mean feet?

WWII subs had crush depths of around 300 meters, and modern subs something like 750 meters

Don't believe everything you read (or hear). They'll go all the way to the bottom.

 

[kelemvor]

Call up Jamie and Adam, and let's start a new thread... "I'd like to see a scuba tank explode... UNDERWATER." =)

But first, let's put it in the gas bucket and shoot fireballs at it. - Joe Dirte`

Or just shoot it with a regular ol gun.

 

[H2Andy]

so what would the crush depth be for a solid steel soccer ball?

 

[Brandon]

Spheres are stronger than cylinders... I really doubt you could crush a 8.5" steel sphere with pressures found in the ocean.

 

[g2]

I'd expect the proportions of a scuba cylinder to be much stronger than a submarine.

Yes, yes! This was exactly my hunch.

Using the WHOI/MIT formulas on the web site provided by Brandon (thanks!), it looks like the thickness/outside-diameter ratio is proportional to the cube root of the pressure. OMG, MATH!!

Or to put it another way: (t/OD)^3 = pressure.

This means that as your outside diameter gets bigger (think: submarine), your cylinder wall has to get a LOT thicker -- or stronger metal -- to withstand the same amount of pressure. This is why a small-diameter AL80 could withstand oodles more pressure than a submarine: it's structurally more robust.

The related Underwater Experiment Canister Project is almost exactly what I'm trying to do: find a cheap way to enclose sensors for very deep oceanographic use. I'm thinking I might be able to cut an AL30 in half, mill an end-cap (from another AL30?) and add O-rings or epoxy to make an economical enclosure.

 

[H2Andy]

last question:

so the smaller the object (advantage to the sphere) and the thicker its "wall" the more pressure it takes to crush it, right?

so a solid steel tennis ball would be far more resistant than a solid steel soccer ball?

because the less surface area, the less pressure that can be brought to bear, but also the less total mass as resistance ... so the decrease in total mass must be less than the decrease in pressure per surface area ...

 

[uncfnp]

You guys really have too much time on your hands.

 

[tursiops]

When I worked at Woods Hole Oceanographic Institution, we routinely used hollow glass spheres both for deep-sea flotation devices and to hold deep-sea instruments. No problem going to 6000m.
https://teledynebenthos.com/product/flotation_instrument_housings/flotation-glass-spheres

Stainless cylinders were often used to 12000m.

I'm not the least bit surprised by the 2000m calculation made earlier for a scuba cylinder.

 

[H2Andy]

I'm sure this is basic physics, but I'm an English grad