A popoular size of scuba diving take is 71.2 ft3 because this is the volume that the compressed air inside the tank would occupy at a normal surface pressure of 1 atm. The air in the tank is at a pressure of about 2250 lb/in2, so that tank itself can have a volume much less than 71.2 ft3. How large does thee tank need to be to hold 71.2 ft3? Have to use Boyle's law: PV = K. 1 atm = 14.7 lb/in2