Question regarding tank fills

[keyshunter]

A local dive shop (now gone) which I frequented filled thousands of AL 80's a year. The fill pressure gauge had a black mark at 3400. All the tank monkeys had to do was fill to the line and tanks would cool to about 3000--no thinking involved.

 

[Divin'Papaw]

Any reason why you bleed the Al tanks down to 3100? Is there anything wrong with adding a little extra to an Al tank? I pump mine to 3300 and it cools a little, so 3100 is usually around where it falls. I'm just curious your take?

Thanks!

As long as they cool to 3100 or so I leave them. It’s only if cool they are much more than that do I bleed some off. AL aren’t as tolerant of overfilling as Steel.

 

[Charles2]

AL aren’t as tolerant of overfilling as Steel.

I am not in favor of significantly overfilling any tank; but, why aren't 3AL tanks as tolerant of overfilling as 3AA tanks?

 

[BRT]

I am not in favor of significantly overfilling any tank; but, why aren't 3AL tanks as tolerant of overfilling as 3AA tanks?

Steel can stretch and return to shape much better than aluminum.

 

[CT-Rich]

I am not in favor of significantly overfilling any tank; but, why aren't 3AL tanks as tolerant of overfilling as 3AA tanks?

Not a tank expert, but steel is much more elastic than Aluminum. That is why steel is used in making springs. The metal in in steel tanks will not fatigue like aluminum. Early steel tanks actually could get “+” rating which meant they could be over filled by ten percent. Add to that the unfortunate AL tanks that were made from an inferior alloy back in the 1970s and 80s. Some exploded and tanks with those alloys are no longer considered safe. But because of the general properties of Aluminum, overfilling them is not considered good form and may shorten the service life of the tank.

 

[Charles2]

Steel can stretch and return to shape much better than aluminum.

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.

 

[Charles2]

Add to that the unfortunate AL tanks that were made from an inferior alloy back in the 1970s and 80s. Some exploded and tanks with those alloys are no longer considered safe. But because of the general properties of Aluminum, overfilling them is not considered good form and may shorten the service life of the tank.

Agreed - - The use of Aluminum alloy 6351 certainly has given the aluminum tank (3AL) a bad rap.

 

[CaptainPanda]

Holy cow, did this thread blow up.

I went back to the LDS and mentioned that I appeared to have gotten a low fill, and asked if they would top it off for me. They grabbed a pressure checker and confirmed, yep, totally a low fill in this tank, we'll fix it for you right away. Super gracious and apologetic, topped off quickly, and they even offered a free fill in the future for having to bring it back in. Their best guess is that some of the new people working at the center had only filled LP steel tanks before, and hadn't realized that mine was the high pressure 3442 instead of the low pressure 2400. If that is the case, hopefully they'll make up a sign or remind people to check the pressure stamp before filling.

All in all, resolved to my satisfaction, and I'll continue to patronize the shop in the future. I'll just remember to bring my SPG with me next time and check before leaving. Thanks everybody!

 

[Divin'Papaw]

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.

You would be wrong. They don’t. There is a myriad of data to this effect. Search & read both on ScubaBoard & the internet. The two metals behave very differently to overfilling.

If you aren’t a fan of overfilling, than don’t have your tanks overfilled.

 

[tridacna]

You would be wrong. They don’t. There is a myriad of data to this effect. Search & read both on ScubaBoard & the internet. The two metals behave very differently to overfilling.

If you aren’t a fan of overfilling, than don’t have your tanks overfilled.

Do you have a link to a good reference? I’m genuinely interested.