Maximizing Usable Air from Decompressing a Scuba Tank

[qwerty1785]

Homework Statement

I need to find the amount of usable air from decompressing a scuba tank from 3000psi to 100 psi. The tank is a standart 80cu ft scuba tank and the air is being drawn from the tank at a rate of 1.5 cubic feet per minute and 100 psi at 20 celsius.I know that air is not an ideal gas but am not sure if I can assume it will behave similar here so ideal gas law might not work. would van der waals equation be what I need? I know the air decompressing that much will get very cold and not be as much volume as ideal, but how much is it exactly?

Homework Equations

The Attempt at a Solution

 

[DaveC426913]

You're going to end up with 80 cubic feet of air when it reaches room temp.

 

[denverdoc]

The actual volume of the tank is is .39 cu ft.

If the total volume of room air is 80cu ft, one can figure out the volume of air remaining in the tank at 100 psi. Hint: use Boyles law and the fact one atmosphere=14.7psi

Then subtract this from the entire capacity of 80.