Stale air?

[scubadobadoo]

Personally I wouldn't dive my steel (or Al) tanks after three years of just sitting around. Will the air still be good? Probably, no reason to think otherwise. Will the tank still be good? That is the problem, and you won't be able to know without a vis unless the air tastes bad. I wouldn't want to suck rust into my regulator or my lungs. You probably wouldn't get hurt but who wants to be the test boy?:shakehead

 

[ClayJar]

Are you a rocket (or scuba) scientist? That's some deep stuff!

Kudos to you for knowing it! I'm in awe at that answer!

Chemical engineering degree, but that was actually just freshman chem. Hehe, you know, the class that nobody but chemists and chem.e's pays much attention to...

 

[ScubaBabe22]

Chemical engineering degree, but that was actually just freshman chem. Hehe, you know, the class that nobody but chemists and chem.e's pays much attention to...

you are one of the people that made me hate chem class, aren't you?

 

[lamont]

Chemical engineering degree, but that was actually just freshman chem. Hehe, you know, the class that nobody but chemists and chem.e's pays much attention to...

sophmore chem class in high school for me...

 

[ClayJar]

you are one of the people that made me hate chem class, aren't you?

Hehe, nah. If you hated chem, you weren't on my side of the segregated class. (In honors chem, the professor tells you all sorts of interesting stories, not a single one of which I could possibly repeat out loud... except maybe the one about the chemical in a fungus that grows on old peanuts and... um... okay, not that one either.)

Anyway, so I'm not completely off topic, I certainly agree that the state of the cylinder is almost certainly going to be the important factor when it comes to whether you should use vintage fills. That being the case, since the aluminum alloys corrode differently than steels, if I had to dive out of Poseidon, and I had my choice between a vintage 1990 steel fill or an equally old aluminum fill (both with the same usable volume, of course -- nice of them to be simple like that), I'd take the aluminum bottle.

(Sadly, the captain would probably keep me from diving, since I don't have a wreck de-penetration C-card.)

 

[gangrel441]

of course if it's sitting more than a year, it's due for a viz by then...

Yep. And to do a vis, they gotta' get the air out of the tank, so better go breathe it down.

 

[skinnydogdives]

hi all
i have been told by the chap that maintains our compressor that if the tank was filled with filtered air as all should be the air will last for at least 17 years he showed me results from test that had been done and this was the longest that they had kept a tank for . they have no data on anything longer than that. but he rekond that the air should stay good indefinitly. as a tank must get a vis every 2 and hydro at least every 4 years its kinda irrelevant
with regard to agvantages of AL over FE i would opt for steel every time. i know they rust but as an ex toolmaker who has worked with raw AL every time you get a cut it gets infected and can take weeks to heal .i wont even use AL frying pans or saucepans for cooking that stuff is not good for you

 

[ClayJar]

Assuming it's filled with nice, dry air, I would imagine the neck O-ring would go before enough of the oxygen is consumed by cool, dry air oxidation of the metal. Of course, if it hasn't gone yet, you may as well breathe it down.

If you were concerned about it, and you have an analyzer handy for checking your nitrox mixes, you could always check the oxygen fraction. If it's still 20.9% (again, assuming it was air) and it doesn't smell or taste, it's fine. Air in a sealed clean metal container won't go bad. Oxygen could be consumed by oxidation (corroding the metal, for example), and the seals (neck O-ring, for example) will break down and eventually fail (but you'd see that on your SPG), but other than those, you can consider the air basically "inert" for the purposes of storage.

Then again, if it had been *years* since you'd been diving (a shame, that would be), you should probably have your regulators checked and serviced, your tank(s) vis'd, etc... In that case, $5 or so of air is almost rounding error.

 

[usmc4x4]

The only information I've ever read on "stale air" was a passing mention noting that corrosion (which consumes oxygen) could decrease the oxygen fraction in the air. Obviously, I just had to try some numbers out...

Let's start with a fictional "Iron 80". Now, since we know that 1 mole of gas occupies 22.4 liters of volume at STP, and using the conversion 28.3 liters per cubic foot, we find out our ~200 bar I-80 is a conveniently-sized 100 mole tank. Since it's been there a while, we'll assume it was filled with standard air (since we would've certainly used a nitrox fill by now), and we'll simplify things by rounding to 21% O2 / 79% N2.

In NAUI's Nitrox Diver course materials, they state that the EAN dive tables can be used with gas mixtures within 1% of the stated fraction of O2, so we'll assume our old air will be good if it still has 20% oxygen by the time we use it. Doing the numbers, that means we can "use up" 1.25 moles of O2 to form 0.83 moles of rust (Fe2O3, the red, flaky rust), which is 133 grams, 0.29 pounds, or about 4 and 3/4 ounces of rust. (Incidentally, the I-80 would've lost only about 40psi from the oxygen used up to make the rust, assuming the rust was the same volume as the iron from which it came.)

So, basically, if you shake your tank and hear a big pile of rust rattling around inside it, don't use it, but stale air should likely be the least of your worries. I'd be more concerned about getting crud into my regulators.

Your wicked smart yo!!! I don't believe I have seen anything as clear or well versed on any board. Thanks.

 

[bugsmashers]

I just bought 4 steel 72's that have been stored with 500PSI since 1981. I was curious so I took a swim in the pool and couldn't tell any difference. The bottles were not rusted inside at all.