Question regarding tank fills

[Gone for diving]

Like I said if my math is right...
Need to use few more place values....
5000 psi on an aluminum tank seems awful high. A couple hundred psi over 3000 psi doesn't seem unreasonable...
But you probably don't want to be 200-300 over and in the hot sun...

 

[tridacna]

Like I said if my math is right...
Need to use few more place values....
5000 psi on an aluminum tank seems awful high. A couple hundred psi over 3000 psi doesn't seem unreasonable...
But you probably don't want to be 200-300 over and in the hot sun...

I doubt that a 3AL will burst in the hot sun when overpressured by a few hundred psi. I say this because it gets done every day by hundreds of shops all over the world. The AL is a reliable workhorse that may have its life shortened by those practices but will not turn into a fireworks display.

 

[CT-Rich]

News to me. Please cite a source.

Re-reading the source, they talked about DOT, then transitioned to some shops having a 20 year policy. I misread a poorly written section of the article.

 

[tridacna]

Re-reading the source, they talked about DOT, then transitioned to some shops having a 20 year policy. I misread a poorly written section of the article.

Not my store. We have a vast selection of 3ALs going back at least 30 years. Still working fine.

 

[boulderjohn]

Re-reading the source, they talked about DOT, then transitioned to some shops having a 20 year policy. I misread a poorly written section of the article.

I have made many an error of this kind in my time. It is great that you come right out and say it.

IMO, the reason that some shops have for that rule is a classic example of a common problem in human thinking--strictly adhering to a rule without understanding why it was created or if the reasons for it still apply. Since 1985, concerns have been raised about the potential of sustained load cracking in aluminum tanks created by a variety of manufacturers using the 6351 alloy. Dive shops needed to evaluate those tanks carefully, and in the mid 2000s, some shops decided they would eliminate the problem by refusing to fill tanks over 20 years old. By doing that, they assured themselves that none of the tanks they filled would have the 6351 alloy.

It has now been 35 years since any tanks have been made with that alloy, but some of those shops still keep that policy. BTW, it is still safe to fill those tanks if they have been properly evaluated.

PSI-PCI - A short Review of 6351 Alloy Aluminum Cylinders

 

[tridacna]

BTW, it is still safe to fill those tanks if they have been properly evaluated

This is true but I still get a nervous feeling when filling them even though we've done an Eddy Current test and recent hydro. And I assure you, those never get an overfill! Thankfully the numbers of those tanks are declining slowly - a few fail Eddy tests every year.

 

[CT-Rich]

I have made many an error of this kind in my time. It is great that you come right out and say it.

IMO, the reason that some shops have for that rule is a classic example of a common problem in human thinking--strictly adhering to a rule without understanding why it was created or if the reasons for it still apply. Since 1985, concerns have been raised about the potential of sustained load cracking in aluminum tanks created by a variety of manufacturers using the 6351 alloy. Dive shops needed to evaluate those tanks carefully, and in the mid 2000s, some shops decided they would eliminate the problem by refusing to fill tanks over 20 years old. By doing that, they assured themselves that none of the tanks they filled would have the 6351 alloy.

It has now been 35 years since any tanks have been made with that alloy, but some of those shops still keep that policy. BTW, it is still safe to fill those tanks if they have been properly evaluated.

PSI-PCI - A short Review of 6351 Alloy Aluminum Cylinders

The shop owner has liability for what their employees do. Easy to understand and clear criteria take the guess work out of the decision tree. All businesses make policies like this. Whether to service old regulators where the service packages are no longer available or to accepting personal checks. The kid at the counter shouldn’t be deciding if a customer can purchase two thousand dollars worth of gear with a personal unless he has been given a clear policy. It may be an expensive mistake or piss a customer off. The fallout should go to the owner, not the $10/hr clerk.

 

[boulderjohn]

not the $10/hr clerk.

That in itself is part of the problem.

When you go into a scuba shop and look for equipment advice, it should not come from someone making $10 an hour. The kind of competent, professional help you need will not work at that wage. I know people who started working at shops because of their love of scuba and quit because of the miserly pay.

 

[Divrtim]

There is a lot to be said for a Faber steel LP 95. Low fill pressure ( 2400 + 10%). At 2640 psi the tank holds a little over 100cf. I like the negative weight too. The down side is that the tank is a little wider and increases drag slightly.
The other cats meow is a set of old Scubapro steel 72's doubled. Again, these low pressure tanks can be pumped up 10% and provide a little over 150 cubic feet. I like them doubled because they are only 7 1/2 inches in circumference each and have a very low drag profile.
Don't skip the deco stops...........

 

[BRT]

I am not quite sure this statement is true. Keep in mind that the question was "why aren't 3AL tanks as tolerant of overfilling as 3AA tanks?" You're generalized response does not address those two specifications. Steel and aluminum are different materials with different properties. It is my contention that those differences in properties were taken into consideration before the final design of 3AA and 3AL cylinders and that result is that both cylinders will respond similarly to overfilling.

It may be your contention, I invite you to show me pictures of a steel tank that has blown up while within hydro. Now look for the same in aluminum. Steel and aluminum are different materials with different properties. One of the differences is steels ability to stretch and then return to shape.