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aluminum cylinders are so much thicker than steel that it would take a very deep corrosion pit to compromise the cylinder wall strength.

I don't know your definition of "very deep"...

Aluminum has (roughly) 1/3rd the tensile strength of steel so the walls are of course proportially thicker. But that doesn't mean pits can penetrate the walls all that much further. The standards I was given by PSI (I don't recall the source) have an allowable isolated pit depth of 0.06" for aluminum cylinders and 0.04" for steel. Those can be hard to measure precisely on the insides so generally speaking an inside pit of any substance condemns the cylinder.

Its not that uncommon to find pitting underneath the bands of double AL80s. Galvanic action plus marginal rinsing can take their toll quite fast. Or sometimes on the interior of AL80 bottoms if water gets injected during a fill.
 
I have no idea what the dewpoint of scuba air might be. An interesting topic for research.

The current grade E standards for scuba breathing air dictate a dew point of -65f which is roughly 24ppm water vapor. This is really low (at least for the volumes used) and its not hard for filters to become saturated and for a compressor to fail to remove sufficent water vapor. Increased water vapor saturates charcoal making it less effective at removing organic contaminants, leads to rust in steel tanks, and can lead to regulator feeze-ups while ice diving. Aluminum, being a bit more corrosion resistant than steel tolerates "wet" gas from marginal/overwhelmed filters a bit more. Its one reason why AL80s surpassed steel 72s as "the" recreational tank a couple decades ago.
 
The current grade E standards for scuba breathing air dictate a dew point of -65f which is roughly 24ppm water vapor. This is really low (at least for the volumes used) and its not hard for filters to become saturated and for a compressor to fail to remove sufficent water vapor. Increased water vapor saturates charcoal making it less effective at removing organic contaminants, leads to rust in steel tanks, and can lead to regulator feeze-ups while ice diving. Aluminum, being a bit more corrosion resistant than steel tolerates "wet" gas from marginal/overwhelmed filters a bit more. Its one reason why AL80s surpassed steel 72s as "the" recreational tank a couple decades ago.
It is true that wet activated charcoal is bad news - but that is also why there are moisture separators between compressor and filter and why most filters are now packed 60% to 70% full of something like Vaporshell that acts as a very efficient dessicant as well as removing some contaminants.

As for pits, the acceptable depth depends on the material, location and the particular engineering standard - 3AA, 3AL or SP.

AL tanks did displace 3AA steel tanks in the 70's, but the current crop of SP steel tanks along with improvements in compressors and fill practices mean that the trend is moving solidly in the other direction. Aluminum is for beercans, not scuba tanks. :D
 
I'm just getting used to my LP72s, and I have to say, it's too bad these tanks are not still made. What's not to like? They're small, lightweight, and neutral when empty.

What I would like to know is, what is the structural difference between the LP72, the current LP85, and the current X7 3442 PSI tanks? IOW, if someone today made a tank the same size and wall thickness of the LP72, could it have a 3000plus PSI service rating?
 
I don't know your definition of "very deep"...

Aluminum has (roughly) 1/3rd the tensile strength of steel so the walls are of course proportially thicker. But that doesn't mean pits can penetrate the walls all that much further. The standards I was given by PSI (I don't recall the source) have an allowable isolated pit depth of 0.06" for aluminum cylinders and 0.04" for steel. Those can be hard to measure precisely on the insides so generally speaking an inside pit of any substance condemns the cylinder.

Its not that uncommon to find pitting underneath the bands of double AL80s. Galvanic action plus marginal rinsing can take their toll quite fast. Or sometimes on the interior of AL80 bottoms if water gets injected during a fill.

The studies that I referenced in my previous threads showed that aluminum cylinders all suffered less corrosion than steel cylinders under identical conditions. In the Battelle study, even severe corrosion did not significantly affect aluminum cylinder wall strength. However, in the University of Rhode Island study, corroded steel cylinders were routinely found to have severely degraded wall strength and were in danger of exploding, whereas some of the aluminum cylinders under identical conditions were sent for, and passed, hydro testing!

Actually, in my experience, a pit of 0.04 inches is so deep that it is easy to tell it apart from minor corrosion pits that just need a bit of a tumble.

You're right about galvanic corrosion, that seems to be one of the biggest problems with aluminum cylinders. It's not so much of a problem in fresh water.
 
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I would also like to know more about the tank: manufacturer, coating process, last hydro, last ViP, last fill, where filled, history of the fill station (compressor, filters), etc. Basically, I want to know enough to convince myself that it can't happen to my tanks.

It CAN happen to your tanks. But, in my personal opinion, you can prevent it from happening by keeping your hydro and annual inspections up to date. Also, if you get a free-flow and drain your tank to zero PSI underwater, have your tank cleaned and inspected right away. I think it's really that simple.

Sorry, I don't have the detailed information that you seek. I obtained many of the references through inter-library loan and so i do not have them anymore.

We don't know that all of the rust was created during that last fill.

That's true. However, if you look at the University of Rhode Island studies, severe corrosion in steel tanks occurred when there was a lot of water in the cylinder (500 ml to be exact). That water is necessary to fuel the corrosion process, and the corrosion process is what consumes the oxygen. Without water, the corrosion process stops. In the U of RI study, all 500 ml of water was consumed in at least one of the cylinders.

So... I would make an educated guess that in the case of the hypoxia fatality mentioned above, most of that rust was created since the last fill. The victim had used the cylinder 3 months prior to the accident without incident, so we make the assumption that the amount of oxygen in the cylinder was sufficient to support life. Then he put the cylinder aside for 3 months, with only 300 PSI remaining. There had to have been a lot of water in the cylinder at that time for corrosion to lower the oxygen content to only 2% to 3%. That's my thought, anyway.

It seems to me that if 3 months were the universal truth, we would have a bunch of people dying from hypoxia. Yet, in all of recorded history, we have just one documented case (that we know of) or, at least, there are no CPSA warning labels required on every steel tank. In fact, other than this thread and the previous link, I have never even heard of a problem with steel tanks as long as they are not abused.

Maybe we do have a lot of people dying from hypoxia. Annual inspections are much more universal now than they were in the 1970s and annual inspections catch a lot of small problems before they become BIG problems. In fact, both the Battelle and University of Rhode Islands studies recommended annual inspections to eliminate many of the problems that they were finding in their studies!

Furthermore, take a look at the link above: Compressed breathing air - the potential for evil from within. The authors sugggest that death from "bad air" is more common than we think. The cause of death is usually "drowning" but the true cause of death ("bad air") may be going unrecognized.

You would think that if rust was forming at such a rate, tanks would fail hydro in high percentage.

Steel cylinder do fail inspections at much higher rates than aluminum. Do a search on this board and you'll see all the problems with steel cylinders. But again, annual inspections catch problems early so the steel cylidners can be tumbled instead of condemned....

I can look into the PSI tank inspection program and begin monitoring my own.

I am a cylinder inspector, too, and that is one of the reasons I started looking back at all of the original data.

Good luck! Hopes this makes you think more about your cylinders!

Safe diving!
 
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I agree that if it is your practice to put 500ml of water into a steel tank, that storing the tank at low pressure and draining it every 3 months would be prudent.

I also agree that steel tanks have the potential to oxidize to a greater degree than aluminum as the process is iopen ended in steel, where any oxidation in aluminum tends to seel the surface below from further oxidation. So in effect with an AL tank, you have to have liquid water in the tank to get significant oxidation where high humidity would be enough to cause issues in a steel tank as rust is both porus and tends to attract moisture.

That said, if you use grade e or better air, inspect the tank annually and tumble the tank when needed (maybe every 5-10 years) to remove accumulations of flash rust, a steel tank will outlast an aluminum tank due to the other material properties fo steel versus aluminum. Aluminum tansk fail the hydro portion of the requalification at a much higher rate than steel. In fact it is all but unknow for a steel tank to ever fail a properly done hydro test (some steel tanks "fail" as the tester does not follow the proper round out procedure) and instead they fail the vip portion due to rust that is not removed and overtime results in pits and/or because water entered the tank and resulted in a much more rapid development of pits.

I also agree with the poster above who does not see why generic steel 72's are no longer made - what's not to like?

Faber made a 75.8 that was in essence a generic steel 72 rated for 2400 psi (2640 psi with the 10% overfill) compared to the normal steel 72 2250/2475psi pressure ratings. But at the time, steel 72's were commonly available on the used market for $100 or less, so it was hard to market a look a like for twice the money when AL tanks were all the rage.

Now however it would be to do even better and produce a steel 72 sized tank to a newer special permit standard with walls only slightly thicker and perhaps only 2 pounds heavier (the same as an AL 80) that would provide about 95 cu ft of capacity at 3442 psi.
 
Doc Harry,
DA Aquamaster

Thanks guys! I am really looking forward to the PSI course. You have brought up enough points to get me interested in inspecting my own tanks before I send them in for LDS inspection. I wasn't in a hurry to buy an oxygen analyzer as I test the tanks when I pick them up. Now I think I'll take a more proactive view of measuring the contents before I dive. Paranoia is a good thing, it can save your life!

I may be the proud owner of an LP72 (if it passes hydro and viz). I had never really thought I wanted one of these old dogs but it turns out that only the LP72 and Al 50 will fit the 6.9" bands used with vintage harnesses (AFAIK). I am definitely building a set of doubles with a pair of Al 50s but I think I'll use the LP 72 with the single harness. The LP 72 will be more suitable for the Mistral and the double 50's will be used with the RAM. I'll use the PRAM with the HP 100s.

Richard
 
I am currently painting a set of 72's (they are downstairs drying as we speak) to put with my navy harness and bands and 1959 new old stock USD manifold. I'll have to post a picture when I get them assembled.

If you pump a steel 72 up to 2700 psi (a 20% overfill) it holds 77 cu ft, just like an AL 80. Compared to the normal cave fill in low pressure 3AA steel tanks with 3600 psi in a 2400 psi rated tank - a 50% overfill, 20% is fairly conservative considering the tank is engineered to the same 3AA standard.
 

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