TargetTalk

How long can the gas in a scuba cylinder be stored ? Please provide your valuable opinions.

Well, I'd have no qualms using 20 year old air, as long as it's at 200 bar...

I assume you mean for the purpose of using it in air guns and if that is correct the answer is as long as the pressure is good you are good to go. For Scuba use not so. I used to SCUBA dive and long ago i heard of a case where someone used a tank of air for breathing and suffocated. I think some bacteria or some type of little critters in the cylinder used up all the oxygen.

Ah, ok, I agree - for the purpose of diving (sorry, didn't think that far, thought this was about shooting here) I'd only use fresh stuff, too :-D

There is a lot of gas in a cylinder at 200 bar, at say 20% oxygen this means 40 times the cylinder volume is pure oxygen! It would take a significant volume of bacteria to chew this up, and in the process of converting to CO2 would require a source of carbon (nutrients). Also the dry environment would inhibit bacterial growth.
Could this be another "urban myth"?

I'd be more likely to suspect rust as the culprit. It is iron doing a slow burn, and the reason the dive shops do a visual inspection.

I'm sure that I would not sweat using it in an AP.

I would think that if the air is dry and has no impurities (as it should be), there shouldn't be a problem.

BUT... for the few dollars it takes to refill the cylinder, do it each year (on your birthday) then you know it's OK.