It depends! 5 minutes to over an hour. The things that affect how long a tank lasts include:
-- size of the tank ... 80 Cu ft for a full size aluminum versus only 72 cu ft for the older steel tanks.
-- depth of the dive. As you go deeper, you use the air up in proportion to the pressure. 33 ft down in ocean water, your rate of air consumption doubles (2 atmospheres pressure), at 66 feet it triples, and so on.
-- how fast you breathe! Anxiety, cold, exertion, all increase your rate of air consumption.
cold, anxious, at 100 ft, you may find 10 minutes of air in the tank. At 30 feet down, relaxed, and not struggling, breathing calmly, your tank lasts longer than you want to (or should!) stay down!
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