Why not overfill Aluminum cylinders?

[rjack321]

I am also aware that my practice of selling cylinders after about six or seven years is conservative... I do so out of concern for the buyer... and because I can afford to be cautious.

When I teach a VIP course, I tell participants that in the final analysis, if they would not be comfortable sitting on a tank as it is being filled, they should not pass it. Ridiculously conservative? Possibly. But I would like to see all cylinders filled in containment stations. Odd perhaps, but I'm okay with that label.

You sell off (not recycle) 6 or 7 year old cylinders? Selling something with 3 or 4 years left before the 2nd hydro is required. I assume you are doing that to maximize the used value? What exactly is your concern for the buyer here? They aren't good enough for you to use anymore but they are good enough for someone else who's less well off financially - but can still afford to pay a decent price?

Best practice for filling cylinders is of course containment. But even without containment you don't go maximizing your risk just because you "feel like" its safe.

The contradictions in your personal practices and risk aversion are definitely strange to me.

(for the record I have a number of 3AL cylinders on their 3rd or 4th hydro. All are 6016 aluminum. I VIP and fill them myself, but I don't sit on them)

 

[lowviz]

...//... Again, if you are willing to pump a steel tank rated at 2250 psi to 3000 psi, why are you unwilling to pump an aluminum tank rated at 3000 psi to 4000 psi? After all, it's the same percentage of overfilling.

I had to think about this for a while. I will attempt a second answer.

Continuous overfills in a metal cylinder that are all still below its elastic limit (at overfill) is very different from the same percentage overfills in a metal cylinder that has no elastic limit. I believe that this is the root of the difference between the lifetimes of overfilled 3AA and 3A cylinders.

If so, a new question arises. Is the service rating only determined by the manufactured tank’s ultimate yield strength? Or does the elastic limit of a tank's material also have something to do with the tank's service rating?

 

[drrich2]

Doppler:

About how many times in 6-7 years do you fill one of those aluminum cylinders? Is that a big factor in your time to sell? Does that tank size affect your choice of when to sell?

 

[Doppler]

I assume you are doing that to maximize the used value? What exactly is your concern for the buyer here? They aren't good enough for you to use anymore but they are good enough for someone else who's less well off financially - but can still afford to pay a decent price?

Has nothing to do with money or getting the best price. The concern for the buyer, which is often NOT an individual, is that they are getting something that's not shagged out, and that shows minimal wear and tear. And it has NOTHING at all to do with them not being good enough for me to use anymore... embarrassing to admit, but it's more about cosmetics.

Doppler:

About how many times in 6-7 years do you fill one of those aluminum cylinders? Is that a big factor in your time to sell? Does that tank size affect your choice of when to sell?

Bailout cylinders may only be filled a couple of times a year! Stage bottles perhaps 20 - 30 times.

Probably aluminum 40s turn over faster than 80s, but I am not certain. I did just scrap two of them that were hydroed (is that a valid verb?) in 2013.

 

[Compressor]

Relevant to this discussion is the longevity factors as well. I have steel tanks that are over 30 years old. My oldest Al is 29. My profile is not overfilling the tank; as such my tanks have never failed hydro. Doesn't mean they won't. I think this info is important for the beginner diver who might be reading this thread.

 

[vam]

My LDS routinely fills my AL80 to 3300 so that when it cools it is closer to 3000. If it was particularly low at the start this works out to about 2900, and if it was extra full I might have almost 3100 at the start of a dive. But it is usually pretty close. I'm still a fairly new diver and gas is the limiting factor to my bottom time, so I want all the air I can safely put in there.

Is this a common practice? Another local shop doesn't do this and I do miss the extra air.

I've been thinking about moving to an HP100 for my next tank, or just making the leap to BP/W with doubles as a better solution to the air hog problem.

 

[Compressor]

Hi Vam: How many dives have you made. As you dive more, you will tend to be more relaxed thus reducing your air consumption rate.
Do you know how much your surface air consumption is? Don't change too many equipment just as et until you get more experience.

 

[vam]

Thanks @compressor - I'm looking forward to #14 this evening. I haven't worked out my SAC. I'm 6'2" and about 230lbs (little of that is muscle) so I don't imagine it will be good. Scuba has motivated me to try to improve my fitness.

 

[Compressor]

Welcome Vam;
No issue not knowing your scr.
I suggest you dive a lot; as you gain more experience and expertise, you can then adapt your equipment to your needs.
Happy Diving

 

[Charles2]

Continuous overfills in a metal cylinder that are all still below its elastic limit (at overfill) is very different from the same percentage overfills in a metal cylinder that has no elastic limit. I believe that this is the root of the difference between the lifetimes of overfilled 3AA and 3A cylinders.

If you are implying that aluminum does not have an elastic limit, I think you are wrong. I seem to be able to find an elastic limit value in just about any aluminum alloy that I look up. I asked in an earlier post if perhaps you meant the 'endurance limit'; however, I did not see an answer from you. Could you help me here?

I think that you may be confusing the concepts of 'elastic deformation' and 'fatigue failure'. Fatigue failure occurs with cyclic loading while the material in question is usually within its elastic limits. The Wikipedia article that you referenced illustrates that.

It is true that when steel has been loaded below its endurance limit it is relatively immune to cyclic loading. Aluminum does not share that property and the designer always has to be concerned with cyclic loading when using aluminum and its alloys and must select an allowable stress that gives adequate life on parts that are cyclically loaded.

Steel, when loaded above its endurance limit, exhibits the same fatigue failure modes as aluminum. Your Wikipedia link also illustrates that.

I suspect that when a steel cylinder is filled to its 'service pressure', it is very close to or perhaps less than its 'endurance limit' and can therefore withstand an almost unlimited number of refill cycles. I also suspect that overfilling by any significant amount causes the tank to exceed its endurance limit and therefore enter the region where fatigue failure is eventually possible - just like aluminum.

In addition, I also think that overfilling whether steel or aluminum limits the overall life expectancy of the tank.

Now, back to the original question - If you are willing to overfill steel cylinders, why not overfill aluminum.