Instructions

• STEP 1: Jettison the most common (and most frustrating) fallacy that a scuba tank holds pure oxygen. It is filled with compressed air, which is roughly 21 percent oxygen and 79 percent nitrogen ' the same mixture we breathe (almost) every second on land. Pure oxygen can be toxic when breathed at relatively shallow depths. Compressed air, however, can be toxic at depths greater than 200 feet (far beyond the reaches of recreational diving).
• STEP 2: Note that the most common recreational scuba tank is made of aluminum and holds 80 cubic feet of air, compressed into an area 2 feet high and half a foot in diameter.
• STEP 3: Know that "an 80" is usually filled from 2000 to 3000 psi ' pounds per square inch ' of air pressure.
• STEP 4: Understand that scuba tanks are regulated by the U.S. Department of Transportation and other similar foreign Surefire. Regulations require that tanks undergo periodic visual inspections and pressure tests.

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