Ok here is my problem. We have a static value that we use for determining the cost of air based on the pipe size and the CFM used. What I have is the size of the pipe openings and what my constant static pressure is. The air is being blown into a vacuum. System details: Lead pipes is 4 inches. It feeds two one inch pipes that for all intensive purposes is 100 % open. It discharges into a 12ftx12ftx30ft vacuum hopper that maintains a 16 inches of water vacuum in the chamber. My problem: I have figured my total discharge area out of the system to be 1.57 square inches of discharge. My system maintains 90 psi with significant fluctuation. The vacuum system will drop at most .5 inches of water pressure during a hit. The discharge time is 1.5 seconds. What I don't know how to do is how to figure out is the volume of air use I have.