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Recreational scuba tanks are filled with compressed, purified air. This air contains about 20.9% oxygen. In my opinion, the average open water certified diver using a standard aluminum 80-cubic-foot tank on a 40-foot dive will be capable to stay down for about 45 minutes before surfacing with a safe store of air. So check your tank before you go for scuba diving, if your tank is not full, make it right. 
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