Why not overfill Aluminum cylinders?

[Basking Ridge Diver]

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

I over fill both - aluminum by 10 to 15% - steel by 20 or 25%. I am not worried about the next owner - I am going to get my monies worth out of my tanks while I still can - once I am gone or done diving I could care less about the tanks. I am not in the sport to resell or make money on my equipment.

 

[Doppler]

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

Hi Charles... for what it's worth, when there is a need for a greater volume of gas than my primary cylinders can supply within the pressure suggested by the manufacturer, I carry a/several stage bottle(s). I am no longer as comfortable as I was a couple of decades ago, when I started to cave dive, with the concept of overfilling anything: steel, aluminum, cardboard, or wood.

A couple of posts here and some responses to my blog post on FB prompt me to make clear to everyone reading this that:

• I am not suggesting anyone follows the guidelines I follow. I don't make the rules and certainly have no desire to police them, understanding fully that my methods will seem odd to some;
  
• I do not profess to be an expert at anything;
• I am not trying to sell anything;
• I AM interested in what others think, but have little interest in argumentum ad hominem;
• Think that LowVis posted an interesting and valid topic well worth "talking" about

 

[scubadada]

So I posted earlier that I commonly get AL cylinders filled 10-20% over working pressure. Are there any implications here or is this below worrisome pressures? These are cool pressures, not immediately after filling.

 

[lowviz]

Charles2,
There is a change of gears, here.

I originally attempted to answer your OP in a tone and level of detail meant to match your question. As we all learn/remember, this discussion gets ever more detailed. However, this is a worthwhile concern and I'll give it my best shot here in this post. I've gone back to my precious few material science courses and rediscovered the basis for my concerns with aluminum.

First and foremost it isn't all about aluminum the element. It is about aluminum the alloy. Aluminum forms the weirdest conglomeration of alloys of any metal that I know about. Frozen rivets

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. ...

Nope, not backing off.
Elastic limit ...Look in there for 'aluminium does not exhibit a well defined "knee" on its S-n graph'

...//... 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? ...

There is another problem to consider,
Sustained%20Load%20Cracking]Sustained Load Cracking
I spoke loosely in my initial answer, no longer

...//... 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. ...

Not just with cyclic loading.

...//... 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. ...

"adequate life" is now replacing elastic limit?

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

Yeah, if you say "similar" then I will agree.

...//... 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' ...

I asked that question, no answer yet.

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

Overfilling with respect to service pressure or with respect to elastic limit?

The following links were all chosen to make a progression of points. I'll not bore the casual reader with the details:
6061

6061 T6

6061-T6

Fracture

Changes over time

Failure

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

Let's be clear on this one. You are asking ME why I don't overfill aluminum cylinders.

Simple, I don't trust aluminum past its recommended maximum specs.

 

[kelemvor]

Hi Charles... for what it's worth, when there is a need for a greater volume of gas than my primary cylinders can supply within the pressure suggested by the manufacturer, I carry a/several stage bottle(s). I am no longer as comfortable as I was a couple of decades ago, when I started to cave dive, with the concept of overfilling anything: steel, aluminum, cardboard, or wood.

A couple of posts here and some responses to my blog post on FB prompt me to make clear to everyone reading this that:

• I am not suggesting anyone follows the guidelines I follow. I don't make the rules and certainly have no desire to police them, understanding fully that my methods will seem odd to some;
  
• I do not profess to be an expert at anything;
• I am not trying to sell anything;
• I AM interested in what others think, but have little interest in argumentum ad hominem;
• Think that LowVis posted an interesting and valid topic well worth "talking" about

I think that jablonski DIR dude has some wood or cardboard tanks. I'm pretty sure I saw a promo video with him flipping the tanks over his head. I'm not sure what their service pressures are

 

[Nemrod]

The reason we cave dived with overfilled, sometimes very overfilled, steel 72s is that is what we had, that is all there really was and we had bunches of them and we had no money to buy anything else even if there had been.

There is a thread, ongoing about coke bottling a tank, do not know about that, but I have seen steel 72s filled until they looked like a pregnant watermelon. Not much exaggeration.

I have no fear of aluminum tanks, I do not overfill them, I intend to use them and if worried about them convert them to scrap metal. They are so cheap I do not care about resale value.

N

 

[Charles2]

Is the elastic limit of steel tanks defined? What is the limit for the LP Worthington tanks for example?

The 'elastic limit' of any material is very difficult to determine and so most folks use the term 'yield strength'. Yield Strength is usually defined as the stress at which the material exhibits a permanent deformation of 0.2 percent. 4130-N alloy steel normally has a Yield Strength of 63,100 psi and an Ultimate Strength of 97,200 psi. I suspect that if Worthington uses 4130-N, it will have strength properties very close to those numbers.

 

[Compressor]

The 'elastic limit' of any material is very difficult to determine and so most folks use the term 'yield strength'. Yield Strength is usually defined as the stress at which the material exhibits a permanent deformation of 0.2 percent. 4130-N alloy steel normally has a Yield Strength of 63,100 psi and an Ultimate Strength of 97,200 psi. I suspect that if Worthington uses 4130-N, it will have strength properties very close to those numbers.

My goodness

What is the same numbers for AL please?
Thanks

 

[lowviz]

The 'elastic limit' of any material is very difficult to determine and so most folks use the term 'yield strength'. Yield Strength is usually defined as the stress at which the material exhibits a permanent deformation of 0.2 percent. ...//...

I don't care what you do to your aluminum cylinders. It is a personal safety/extended-dive/economic trade-off for all of us. I'm no better or worse, just what works for me.

But let's be as accurate as possible from here on, you played that card:

Steel:
e·las·tic lim·it
nounPHYSICS
the maximum extent to which a solid may be stretched without permanent alteration of size or shape.


Aluminum alloys:
yield strength
nounPHYSICS
(in materials that do not exhibit a well-defined yield point) the stress at which a specific amount of plastic deformation is produced, usually taken as 0.2 percent of the unstressed length.

Elastic vs Plastic

Edit:
Apologies, in a rush when I posted and neglected to include something important. It isn't just the absence of a well-defined "knee" but this: Section 18: Nonlinear Materials | Nastran In-CAD | Autodesk Knowledge Network

 

[scubafanatic]

The reason we cave dived with overfilled, sometimes very overfilled, steel 72s is that is what we had, that is all there really was and we had bunches of them and we had no money to buy anything else even if there had been.

There is a thread, ongoing about coke bottling a tank, do not know about that, but I have seen steel 72s filled until they looked like a pregnant watermelon. Not much exaggeration.

I have no fear of aluminum tanks, I do not overfill them, I intend to use them and if worried about them convert them to scrap metal. They are so cheap I do not care about resale value.

N

I make sure to never buy used steels from anyone anywhere near 'cave-country' ( ie., Florida, for example) as the odds are high those tanks are being sold because the owners have seriously over stressed those tanks and they're trying to dump them on unsuspecting buyers after MANY sessions of being 'rode hard and put away wet'!