Old SCUBA Cylinder

[vern]

Can I make anything out of an old aluminum SCUBA cylinder that will go boom.  I hate to throw this thing out but its days as a SCUBA cylinder are over.

Vern

[GGaskill]

What's wrong with it as a SCUBA cylinder?  The things that make it useful as an air tank are the same that make it useful as a cannon barrel.

I don't like aluminum for gun barrels but you might be able to make a steel chamber plug and use the aluminum as the thinwall portion of a Coehorn design.  What are its dimensions?

[vern]

It's a standard 80 cubic foot tank.  I retired it due to the type of alloy used back in the 1980's.  Apparently, it is prone to stress cracks arond the threaded portion where the tank valve gets inserted.  Probably another good reason not to use it for a cannon barrel, right?

[GGaskill]

Any idea what the wall thickness is?  What's the pressure rating?

[vern]

wall thickness approx 0.5", pressure rating 3000 p.s.i., length 26", diameter 7.25"

from the net: About six million aluminum cylinders have been made from the 6351 alloy, and to date, 19 tanks have exploded. Of those, 12 were scuba cylinders. Since June of 1988, all U.S.-manufactured aluminum cylinders have been made from 6061 aluminum alloy, which does not appear to be susceptible to sustained load cracking.

This cylinder was tested last year and found to be free of any cracks, I just feel nervous using it even though it tests ok.

[Rickk]

from what the guy at the welding shop told me when he failed an O2 tank of mine, most steel tanks that get rejected fail around the threads as well. Considering what a tank costs, I would question how many mortars are made from good cylinders.

Scuba tanks run pretty high... over 2000 PSI... CO2 is only about 500 PSI on a really hot day. Oxygen is up there at about 3000 or so PSI.

Tanks are pressure tested at twice the working pressure for the intended use.

However, check this link out and decide for yourself :

http://www.napsd.com/cscuba.htm

Rick

[GGaskill]

My BB mortar is made from a extra high pressure gas cylinder that was condemned by visual inspection.  It is steel, with about 3/4" wall thickness.  The ID is about 8.5" but it is not perfectly round.

Your aluminum one is substantially smaller than bowling balls, so you would need to find something to use as shot that is a good fit before going farther anyway.

[Double D]

Just keep in mind that you are going to have to build a steel powder chamber.

[Artilleryman]

I see various pressure ratings posted here for these tanks.  The question that is unanswered is what is the pressure that is generated in a mortar or cannon?  This obviously is dictated by the kind and amount of powder and type and weight of projectile.  So if you know the rating for the tank you only know half the information needed. 

[CU_Cannon]

I’m not sure if I would use it.  The tank sounds like it would work but I’m not comfortable with the aluminum. 

I don’t mean to sound rude but, why would you use a tank that you don’t feel is safe for its original purpose for something it was never designed for?

[BloomGrad]

I would not have any problems using that cylinder for diving.  NASA tested the life of aluminum tanks back in the early 1980's and came up with a figure around 100 to 110 years as long as pumped to proper pressure, visually inspected once a year (this should also inspection of threads at a good SCUBA dive shop) and not repainted or subject to heat.  Failure rate of 19 from 6 million says it all.

I have been diving since Dec 1983.  I stopped logging every single dive when I hit 400+ hrs bottom time with 1000+ dives.  Now I just log vacation trip dives.  Instruction dives with students or quick fun dives with friends now number so many the brain would explode.

If you want cash for that tank just go down to your dive shop during a dive gear swap meet and I can tell you I will sell.  Somebody always wants or needs an extra tank for a spare stage or decompression bottle.  Backyard tinkers can convert it to a pressure bottle for sprayers or air tools REAL easy.

As Ebay says there is money in your stuff.

[BloomGrad]

Oh yeah I forgot about my cannon thoughts.

100% faliure on the first and only time you set it off.  Spiitsville.  Aluminum is no good for a barrel.  Old scuba tank or new.  The stength of the tank comes from the base, top and sides for the aluminum stucture as a whole unit. 

If I was with you setting that aluminum thing off I would be so far behind you (with the first aid kit) and behind a big tree that a walkie-talkie is the only way we could talk.

Go with something, anything, of good old thick steel.

[vern]

You're probably right, but when the seed is planted in your head that the alloy is no longer used and could fail, it gives you a weird feeling when 3000 p.s.i. is next to your spine.  I think I may keep it for filling my car tires at reduced pressure of course.