Stale Air and Proper Tank Storage?

[tufb98]

I was reading through some posts yesterday and I heard the term stale air.

Does stale air occur when sitting in a tank for too long? How long does this process take and what happens to the air mix while sitting in the tank?

Also, I heard about people storing their tanks at ~500 psi. I used to dive most weekends and I would keep tanks full so that I could grab and go. However, I have recently been sidelined with surgery for a few months. Should I be storing the tanks at that lower level?

Thanks for the help.

 

[GrumpyOldGuy]

I would not change a thing for a few months hiatus. In colder climates, many divers have off seasons and routinely have tanks of air 6-9 months old. If you know and trust the source of the air to be clean and dry when filled then there is not much to go bad.

 

[Eff]

I have heard a few times of air being stored in steel tanks for several years, in which the tanks also had some moisture in them. Over the time period, apparently the rusting action inside the tank consumed most of the oxygen in the fill!

Then again, this was over a long timeframe, and it was a story from a friend of a friend each time...



As for storing at lower pressure, if I'm not diving for a few weeks I try to keep it at 1000 or less. Dunno if it actually makes a difference in valve/tank life but it sure makes me feel better.

 

[GrumpyOldGuy]

I have heard a few times of air being stored in steel tanks for several years, in which the tanks also had some moisture in them. Over the time period, apparently the rusting action inside the tank consumed most of the oxygen in the fill!

If you do a search, someone with better chemistry than myself did the math and posted it last year. The bottom line was the amount of oxygen used up was minimal for typical rust / oxidation.

As for storing at lower pressure, if I'm not diving for a few weeks I try to keep it at 1000 or less. Dunno if it actually makes a difference in valve/tank life but it sure makes me feel better.

This gets interesting.

If the tank is full and involved in a fire, the safety disk hopefully will rupture and safely release the air before the metal gets too hot where it becomes weak.

If the tank is near empty (few 100PSI) even if the tank ruptures, it should not be catastrophic.

If the tank is at a mid-pressure, the heat may increase the pressure while weakening the metal and allow the tank to burst with catastrophic consequences before the burst disk rupture. I really don't know the answer and not claiming it will happen, but it is interesting to ponder.

 

[Eff]

Thanks for the correction on the rusting process, all I had heard before were rumors.

Good point on the fire thing! Never thought about it like that before.

 

[bleeb]

I have heard a few times of air being stored in steel tanks for several years, in which the tanks also had some moisture in them. Over the time period, apparently the rusting action inside the tank consumed most of the oxygen in the fill!

Then again, this was over a long timeframe, and it was a story from a friend of a friend each time...

People might want to have a look at this thread, which includes some extensive write-ups about the topic: Scuba Cylinder Long-Term Storage: Fact and Fiction.

As others have said, it partly depends on how much you trust all the facilities which have filled your tank since it was last opened up, and how much water any of them might have gotten in (or all of them combined). IIRC in the above-mentioned thread, steel is more of a problem than aluminum, orientation can matter, there's been at least one fatality because of oxygen depletion resulting from rusting, and in some USN-funded experiments things managed to reach dangerous levels within a few months under somewhat extreme but not completely inconceivable conditions.

 

[tufb98]

I appreciate all the feedback. This turned into a more thought provoking conversations than I anticipated.

Thanks.