What's a cave fill?

[Meng_Tze]

..., er ...
I keep on singing no-matter-what-fill-I-get.

.

Do you analyze the gas..............

I hear you sometimes when I am in stealth mode and watch divers go by.

 

[georgios77]

So I guess that filling my tanks with 30-32% nitrox at 300-310 bar 4350-4500 psi
would be a bit extreme for some people, huh...

 

[Lukas Ircha]

A translation:

American fill-> Rest of World

‘LP’ fill = (2400 psi/ 165 bar): an empty tank. Good reason to go abuse the tank monkey.
'LP'? fill = (3000 psi/206 bar): a weak fill - complain unless they are using a tiny compressor
‘HP’ fill = (3400 psi/ 234 bar): a normal tank fill.
‘Cave’ fill = (4000 psi/275 bar): a better than normal fill.

Hell knows what you guys would call our normal ‘high pressure’ fills (300 bar/4350 psi).

Still – when my 300 bar DIN valve pops out and takes my skull off you may all nod you head knowingly and say ‘I told him so’.

Cheers,
Rohan.

So a 2250 psi tank should be no problem to fill to about 2800psi according to your experience

 

[tomfcrist]

So a 2250 psi tank should be no problem to fill to about 2800psi according to your experience

Do you realize you are responding to a comment that is 10 years old?

 

[Magnus Lundstedt]

A translation:

American fill-> Rest of World

‘LP’ fill = (2400 psi/ 165 bar): an empty tank. Good reason to go abuse the tank monkey.
'LP'? fill = (3000 psi/206 bar): a weak fill - complain unless they are using a tiny compressor
‘HP’ fill = (3400 psi/ 234 bar): a normal tank fill.
‘Cave’ fill = (4000 psi/275 bar): a better than normal fill.

Hell knows what you guys would call our normal ‘high pressure’ fills (300 bar/4350 psi).

Still – when my 300 bar DIN valve pops out and takes my skull off you may all nod you head knowingly and say ‘I told him so’.

Cheers,
Rohan.

You forget that we in the civilized part of the world use 300 Bar tanks, so 275 is a reason to complain.

 

[DA Aquamaster]

So a 2250 psi tank should be no problem to fill to about 2800psi according to your experience

Do you realize you are responding to a comment that is 10 years old?

To be fair the dates of the pots are not all that prominent and new members are faced with a catch 22. If they use the search function they create zombie threads and get criticized. If they do not use the search function and ask a question that's been asked before, they get criticized for not using the search function.

So let's just cut Lukas some slack.

-----

Amazingly enough, despite the passage of 10 years the answer to his question hasn't changed.

Yes, the short story is that 2800 psi is a pretty reasonable fill number for a properly qualified and inspected 2250 psi service pressure 3 AA steel 72 in excellent condition with no rust, pits, etc. Condition is important and rust is the major killer of steel tanks.

The longer story is that a 2400 psi 3AA steel tank has a hydro test pressure of 4000 psi (5/3rds the service pressure). That leaves a 3600 psi cave fill at 90% of the test pressure (and in practice during the fill or in a hot car the tank pressure is right up there around 4000 psi).

A 3AA steel 72 is designed to the same 3AA standard and has the same 5/3rds test pressure - in this case 3,750 psi. 90% of that test pressure is 3,375 psi. In other words, and equivalent "cave fill" for a 2250 psi steel 72 would be 3,375 psi. In actual practice however I don't know anyone who fills them past 3,000 psi.

A steel 72 nominally contains 71.2 cu ft at 2,475 psi (it's 10 percent over fill plus rated pressure). That means you have .0288 cu ft per psi, and at 2,800 psi that results in 80.5 cu ft of gas - a bit more than the 77 cu ft you actually get in an AL 80 at 3000 psi, which is one of the reasons why 2800 psi is where most divers who want an over fill in one actually stop.

The disclaimer here is that all steel 72s are pretty old tanks, not that age itself means anything as steel tanks age differently than aluminum tanks, but condition matters.

Aluminum tanks have a fatigue life and some of that life is used up on every fill. The higher the pressure the greater the impact on the fatigue life. Luxfer tests their tanks with rapid pressure cycling to 5000 psi (the test pressure for a 3000 psi 3ALaluminum tank) to 10,000 cycles and the fatigue life at 3000 psi pressure cycles is somewhere well north of that, but no body with a lick of sense ever significantly over fills an aluminum tank (maybe 3300 psi max, during a hot fill). That's partly due to fatigue life concerns, but it's also due to the fact that aluminum tanks fragment when they explode and the resulting shrapnel is very lethal.

In comparison, (and in very simple terms) 3 AA steel tanks do not have a fatigue life - they will stretch within their elastic limits and return to their normal shape theoretically forever without failing. But...if the fatigue limit is exceeded, then the tank will become progressively more brittle and eventually fail as each one of excessive stress events eats away at the fatigue life of the tank until it will eventually fail. Given that cave fills have bene the norm on 2400 psi 3AA tanks for over a quarter century and that there have been only a handful that have failed the hydrostatic test portion of the requalification done every 5 years, the results in the field support the theory that 2400 psi 3AA tanks are sufficiently over engineered that the fatigue limit of the steel is not exceeded with 3600 psi cave fills. Now...if people keep pushing fill pressure up around 4000 psi, we may well start to see tanks failing hydro tests.

The important thing to note however is that there is not this same extensive experience in overfilling steel 72s to the equivalent 90% of test pressure, so it's a bit more of a gray area and limiting an over fill to 2800 psi is much more prudent than pushing it to 3000 psi or 3375 psi.

The other factor in favor of overfilling 3AA steel tanks, compared to 3AL aluminum tanks is that if they rupture, they tend to peel open without fragmenting. The lack of shrapnel greatly reduces the lethality of a catastrophic steel tank failure. I saw a video a couple weeks ago where a steel tank exploded about 2 ft behind the tank monkey. he was blown over, but got up and ran out of the shop. If it had been an aluminum tank, he'd have been lucky to survive, let alone ever stand on both legs again.

A final word here is that E (exempt) or SP (special permit) series steel tanks (normally with service pressures of 3442 psi) are not designed to the same very conservative engineering standard as 3AA steel tanks, and they are not tolerant of over fills.

----

And of course the bottom line is that unless you're willing to sit on the tank while it's being filled, don't ask a tank money to over fill your tank. Similarly, if the tank monkey says "no", his word is the final word.