[Phys-L] Re: Weighing air (Was: electricity)

[Bernard Cleyet <anngeorg@PACBELL.NET>]

I've always wondered why barrels are hooped *. Now I know -- it's even
called hoop stress.

http://em-ntserver.unl.edu/NEGAHBAN/Em325/18-Pressure-vessels/Pressure%20vessels.htm

Anyway getting back to the plastic water bottles, this explains why,
reportedly, they split lengthwise.


* additionally because the staves aren't glued.

My old plastics encyclopedia gives tensile strength (rupture) as  11 =>
15 k psi

plugging into the above (Sigma (t/r), the chug (w/ the distracter)
displays 15 psi (using the max. value)  What gives?

bc, differently confused

p.s. assumed polyethylene terephthalate;  measured:  1.4" radius, and 14
mill thickness.


John Denker wrote:

> Bernard Cleyet wrote:
>
>
> > A method:  nearly fill soda bottle w/ water; cover with towels, or place
> > in strong box and pump up till it explodes (use tire pump that includes
> > gauge -- e.g. those combo 12 V. batts. and pump)  then pump another
> > identical (not quite the correct word) to somewhat lower pressure for
> > "weighing".
>
> That's 99% of the right idea.
>
> Slight refinement:  Suppose bottle #1 explodes at Px. Use that
> to tell you what ballpark you're in, nothing more.
>
> Hydrostatically test bottle #2 to (Px - 5psi), and then use
> it at (Px - 10psi).
>
> The point is that you don't need to rely on any questionable
> assumptions about "identicality" or uniformity.  Test the
> bottle(s) you're going to use!
>
> Also:  I think industry standards call for Px > 80 psi,
> and I've never had a problem at 80 psi, but don't quote
> me on it ... this is just ballpark info to tell you what
> size pump and gauge to buy.
>
> Also, you can wrap the bottle with fiberglass reinforced
> strapping tape in a few places, to make sure that if it
> blows, there won't be any big pieces of shrapnel.  In my
> experience, soda bottles split open approximately lengthwise,
> like a banana, so shrapnel isn't a problem, but tape
> can't hurt and might help.
>
> The split is sudden enough and (in the air-filled as opposed
> to water-filled case) violent enough that you would be well
> advised to keep the bottles behind a 1/4 inch or so of plexy.
> You definitely don't want to let museum-visiting kids handle
> them.