Does gas go bad / expire / go stale in a tank?

Pierre Siquet · May 22, 2004

Hi all,

Got steel tanks filled now for over a year. Is there any health problem in breathing this air? I know it is recommended to change every 6 months, but why?

Thanks for your ideas.

Pierre

Drew Sailbum · May 22, 2004

In steel tanks, if there is even a small amount of water rust can form. As rust forms, the oxygen in the breathing gas is consumed. This can reduce the oxygen concentration below that necessary to sustain conciousness at low pressures.

While you can empty the cylinder and then refill it, you could just test the oxygen content with an analyzer used for nitrox. Use a freshly filled cylinder to calibrate the sensor. If the oxygen content is below atmospheric, you should have your cylinder inspected for rust damage.

mustfajohnson · May 22, 2004

An answer depends on the the condition of the interior of your cylinders. Certainly you need to have your cylinders visually inspected. I don't know where you are diving but in the USA your cylinder should be visually inspected if it has been sitting idle for so long. Depending on the amount of rust, I believe figure is you can lose up to 6% of the O2 content monthly. At 6% it doen't take long before the O2 level is at dangerously low levels. I would have to drag out my manual to be sure but you can probably check Bill High's website http://www.psicylinders.com and try to send him an email. Why risk your life for the price of a VIP and air fill?

Pierre Siquet · May 22, 2004

OK, I got the point, Thanks a lot. I did not know it would be O2 depletion, which actually is very logic. Actually, my tanks are almost brand new (just couple of dives), so i believe it is OK. If I do not post within 6 months, then there was not enough O2.

Thanks again

Pierre

Al Mialkovsky · May 22, 2004

So where does the O2 go??

Sideband · May 22, 2004

mustfajohnson:
Why risk your life for the price of a VIP and air fill?

It's worse than that. It's a risk for just the cost of an air fill. He will have to have the VIP anyway to get them refilled. (assuming US diving anyway)

Joe

Sideband · May 22, 2004

Al Mialkovsky:
So where does the O2 go??

It is 'lost' when it bonds to the iron in the steel to create iron oxide, rust.

KidK9 · May 22, 2004

Hey all, bought a new AL80 last year (Luxfer), and wondering how long my 32% Nitrox is good for. It was filled and checked last September - so the air is around 8 months old.

archman · May 22, 2004

Now that's a question I've never heard before, and a very good one at that. I too await the posts by the compressed air technophiles.

miketsp · May 22, 2004

KidK9:
Hey all, bought a new AL80 last year (Luxfer), and wondering how long my 32% Nitrox is good for. It was filled and checked last September - so the air is around 8 months old.

Air/Nitrox in a cylinder is valid indefinitely. If you think about it after each dive you only top up the cylinder so you are always breathing some old air.
In practice you may or not have to empty it at each annual inspection - varies from country to country. Most countries require internal visual each year so tap has to be removed.

But of course you will recheck the O2 PP prior to the dive won't you?

Does gas go bad / expire / go stale in a tank?

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