Life of aluminum -- age or # of cycles based?

[rjack321]

Right -- in this particular instance I was planning on using it for a stage bottle and Catalina 80s aren't known to 'ride' very well, hence why I passed on the deal.

In salt water I can barely tell the difference between a Luxfer and a Catalina stage. And in fresh its not that big a deal either. Full with nitrox both hang like anchors. Empty both rear ends ride up. Everything in between depends on water type and mix.

Although it has a clearcoat finish which will get nasty fast. And it needs a proper DIN valve. Since its out of hydro its not really that great a deal at $60. $50 is more like it.

 

[halocline]

I'm not sure why nitrox would affect the buoyancy; is O2 that much heavier than air? Helium mixes I could see being quite a bit lighter.

I didn't notice the clearcoat, but looking at the photo again I think I see what you mean. I bought a used catalina that had clear lacquer on it, all banged up, and it was a real pain to get it off.

 

[rjack321]

I'm not sure why nitrox would affect the buoyancy; is O2 that much heavier than air? Helium mixes I could see being quite a bit lighter.

I didn't notice the clearcoat, but looking at the photo again I think I see what you mean. I bought a used catalina that had clear lacquer on it, all banged up, and it was a real pain to get it off.

For all practical purposes nitrox has the same density as air.

Sparticle said he wanted to use this as a stage. Most people put nitrox in bottom or deco stages (fairly common would be 36%, 50%, or 80% depending on your sensibilities). Sometimes a trimix backgas (15/55 being fairly common in this application - which would make it a bottom stage) and lastly a trimix deco gas might be used (like 21/35 or 35/25).

Probably the least likely gas anyone is going to put into either a bottom stage or a deco stage is good old air.

To me, the differences between catalina and luxfer 80s are way over-hyped. Certainly the mix inside is vastly more relevant to how they ride. If the price was right I would buy whatever is available.

 

[Bretagus]

You mentioned the tank has been sitting empty for a number of years? That would concern me, though if it was in his garage and the valve was closed... perhaps its alright?

 

[SparticleBrane]

You mentioned the tank has been sitting empty for a number of years? That would concern me, though if it was in his garage and the valve was closed... perhaps its alright?

Why would it concern you? Aluminum doesn't exactly rust... It was going to be O2 cleaned anyway.

rjack -- I found a place hear me that does hydros for $9.50, or at least that's what they charged me last year.

 

[muddiver]

Right -- I'm familiar with all that. I suppose my question came about from wondering if just sitting around empty, the "life" of aluminum cylinders was being 'used up' just due to some sort of natural aging process in the metal, or if their aging is caused by usage.

eg -- if I put a brand new Al80 empty in a closet for 50 years, and then brought it out to use, would it have the same service life, all things being equal, as an Al80 that just came off the assembly line?


Sometimes the rejection of a tank for fills at a shop, as wrong as it is, is based off of age and perceived danger rather than knowledge.

Back to your original question and peramiters: Cylinder stored empty (no internal presure) - it would be no different than tossing a lead weight into the closet, forgeting about it for 50 years and then picking it up and putting it on a belt for a dive. Neither aluminum or lead, under mild ambient conditions, rust. So it should be just fine. Also assuming that there was no water inside the cylinder.

 

[rjack321]

rjack -- I found a place hear me that does hydros for $9.50, or at least that's what they charged me last year.

Nice, that's half my cheapest local price.

New you might still be able to find an already clean luxfer 80 new and in hydro for $120 still. Considering all the hassles a clear coated, out of hydro, unclean tank poses, $60 more is actually a better deal.

 

[WD8CDH]

Tanks don't die, they are MURDERED.

 

[Doc Harry]

Wow, someone butchered my post so I re-did it. Mods ????

Metal bends and deforms.

However, it is my understanding that scuba cylinders are designed to be completely elastic at service pressures. In other words, repeatedly filling a scuba cylinder to service pressures does not weaken the metal. Luxfer advertises that it pressure-cycles its cylinders 100,000 times. These things are over-engineered for safety.

It isn't until you get into hydro service pressure that the metal deforms permanently (whether or not a cylinder passes hydro depends on the ratio of permanent expansion to total expansion). Nothing made by man is perfect, so despite the built-in safety margin, hydro tests whether or not a cylinder still expands elastically as it is supposed to.

So the life of a cylinder does not depend on pressure cycles, as long as it is not over-filled. And I am not aware of any lifetime limit based on time either.

 

[Scared Silly]

Just for clarification - while we say steel rusts and aluminum oxides. In reality both materials oxidize. Rust is a specific form of oxidation or the common name of iron oxide. Further, when steel oxides the oxidation process continues past the surface layer. However, when aluminum oxides (forming aluminum oxide) it forms a protective layer (passivation) that stops the oxidation process.