SpudFiles

mopherman wrote:

sandman wrote:mythbusters exploded one at 150 so you are very safe at 35

mythbusters is a cool show, but dont use them as a source of info for spudguns. remember the giant ball valve cannon? inconsistant opening times and a horrible cb ratio made it a horrible cannon for testing.

back on topic: yeah its safe up to about 50 psi.

They have made decent cannons before.

I have used soda bottles at 125 and they held the pressure overnight and never blew, until I shot it.

jimmy101 wrote:I'm sure that the Coca Cola™ company tells their bottle manufacturer that the nominal pressure at normal temperature is such and such. But, the bottle must not fail at the pressure it'll have at 150 F, or something like that. The inside of a car will easily hit 150 F on a hot sunny day. I'm also sure there is a many page document that specifies in gory detail the failure statistics at various pressures and temperatures.

Naturally, bottlers are not interested in telling anybody what the actual testing data is or what the derived ratings are. Why should they? It would just be ammo for liability lawyers.

The pressures I quoted are significantly higher than the limit specified for room temperature (all tests are carried out at 20-25 degrees celcius) - it is common for bottles to be blown on-site, just before being filled (it's much easier to store and transport these than the full size bottles) so the bottler has some say in how well the bottle exceeds the specifications.

Preforms are sold by weight, it's possible to make the same sized bottle of out of different weights of preform which ultimately varies the wall thickness of the bottle, now manufacturers are aways interested in being close to the limit as lighter preforms are of course cheaper, easier to transport etc, so it depends how stingy who's making the bottle too.

One other thing, failure at 300 psi doesnt mean the bottle calmly sits there until it develops a small leak - au contraire, it starts to swell dramatically to the size of a balloon (indeed we used to enjoy marking smiley faces on the bottles that progressively got more worried as the bottle expanded ) until finally rupturing, and the blast is usually enough to tear the bottle in half. In the test chamber bottles are filled with water then pressure is applied using nitrogen.

More discussion of the mythbusters cannons... oh well.

My personal opinion of the mythbusters is that although they may make (very) good TV, a quite often their science and reasoning is clearly flawed (to someone with a reasonable science knowledge at least.)

If I had their budget (...contacts, legal lenicency, leverage, and huge warehouse), I reckon I could put together a show that could be at least on a par.

lol, i wonder if they will sell the chicken gun for my (small) wealth of knowledge with cannons, hehe

anyway if a show has good TV and some science it is deemed good by me, mythbusters is on that list

joannaardway wrote:My personal opinion of the mythbusters is that although they may make (very) good TV, a quite often their science and reasoning is clearly flawed (to someone with a reasonable science knowledge at least.)

At least they've been willing on several occasions to go back and revisit their experiments when mistakes have been pointed out.

jackssmirkingrevenge wrote:

jimmy101 wrote:I'm sure that the Coca Cola™ company tells their bottle manufacturer that the nominal pressure at normal temperature is such and such. But, the bottle must not fail at the pressure it'll have at 150 F, or something like that. The inside of a car will easily hit 150 F on a hot sunny day. I'm also sure there is a many page document that specifies in gory detail the failure statistics at various pressures and temperatures.

Naturally, bottlers are not interested in telling anybody what the actual testing data is or what the derived ratings are. Why should they? It would just be ammo for liability lawyers.

The pressures I quoted are significantly higher than the limit specified for room temperature (all tests are carried out at 20-25 degrees celcius) - it is common for bottles to be blown on-site, just before being filled (it's much easier to store and transport these than the full size bottles) so the bottler has some say in how well the bottle exceeds the specifications.

Preforms are sold by weight, it's possible to make the same sized bottle of out of different weights of preform which ultimately varies the wall thickness of the bottle, now manufacturers are aways interested in being close to the limit as lighter preforms are of course cheaper, easier to transport etc, so it depends how stingy who's making the bottle too.

One other thing, failure at 300 psi doesnt mean the bottle calmly sits there until it develops a small leak - au contraire, it starts to swell dramatically to the size of a balloon (indeed we used to enjoy marking smiley faces on the bottles that progressively got more worried as the bottle expanded ) until finally rupturing, and the blast is usually enough to tear the bottle in half. In the test chamber bottles are filled with water then pressure is applied using nitrogen.

Remember like PVC, these bottles aren't all made identical, there properties are very slightly different between each bottle produced, also like PVC, the bottles can fail, and even more like PVC when they do fail, boy do they fail, they shrapnel.

I remember a few years back a boy in NZ bought a Coke at a Pedda(petrol) station and after a hour or two in the car with it the bottle exploded, piercing his skin with hundreds of tiny plastic fragments.

NZ doesn't have particulary hot weather, 28*C on a good day in summer, so in a car you could get up to 30-45*C, which isn't that hot either.

So for some reason the bottle failed at a lower tempreature then it really should've, it may of been a mixture of a faulty bottle, pressure and heat that caused it to fail.
That's why I wouldn't trust one in a pneumatic.

I remember a few years back a boy in NZ bought a Coke at a Pedda(petrol) station and after a hour or two in the car with it the bottle exploded, piercing his skin with hundreds of tiny plastic fragments.

Do you have factual evidence of this happening? In my experience, the bottle would tend to tear rather than fragment, usually along the centre where the distribution of PET is at its thinnest, and many times the tear would be big enough for the top and bottom halves to separate. The caps on the other hand do fragment, check out this video of a PET bottle being hit by a marble from my 3/4" pneumatic:

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Note that the overpressure caused by the incompressible water being parted by the projectile's shockwave is enough to shatter the cap into many small fragments (it was screwed on tightly at the time of impact), but the bottle stays in one piece.

One for the mythbusters, perhaps?

jackssmirkingrevenge wrote:

I remember a few years back a boy in NZ bought a Coke at a Pedda(petrol) station and after a hour or two in the car with it the bottle exploded, piercing his skin with hundreds of tiny plastic fragments.

Do you have factual evidence of this happening? In my experience, the bottle would tend to tear rather than fragment, usually along the centre where the distribution of PET is at its thinnest, and many times the tear would be big enough for the top and bottom halves to separate. The caps on the other hand do fragment, check out this video of a PET bottle being hit by a marble from my 3/4" pneumatic:

Yes there is evidence, it was covered in a news story on two news channels, i'll see if I can dig it up, it should be in article form as well.

It sounds a little suspicious to me. Is it possible it was a glass coke bottle rather than a plastic one?

Nonetheless, TV programmes do sometimes run incorrect stories by mistake.

joannaardway wrote:It sounds a little suspicious to me. Is it possible it was a glass coke bottle rather than a plastic one?

Nonetheless, TV programmes do sometimes run incorrect stories by mistake.

NZ only has plastic bottles in the size of 600ml, We do have a few glass bottles but they're only about 330ml and the one in the story was a 600ml bottle.

well I still don't see how the bottle would rupture explosively, especially with water in it.

Well it was coke but practically the same thing pressure wise.
Yeah as i said it could've been a faulty bottle, all I know is that it happened, I looked for the article but couldn't find it.

MrCrowley wrote:NZ only has plastic bottles in the size of 600ml, We do have a few glass bottles but they're only about 330ml and the one in the story was a 600ml bottle.

Okay then. But having seen enough urban legends, this sounds as though there may be a bit of exaggeration involved. (Not on your part of course. From the original source most likely)

joannaardway wrote:

MrCrowley wrote:NZ only has plastic bottles in the size of 600ml, We do have a few glass bottles but they're only about 330ml and the one in the story was a 600ml bottle.

Okay then. But having seen enough urban legends, this sounds as though there may be a bit of exaggeration involved. (Not on your part of course. From the original source most likely)

Yes, I will agree with you 6 o'Clock News progams aren't 100% true but they did have picture etc.

MrCrowley wrote:...but they did have picture etc.

Ah, in that case... When there are photos, normally the story is true. Not invariably, but usually.

Having heard that, I'm instantly more concerned.