Hello gentlemen! I'm rather new to this whole complex world we call compressed air. :) Put simply, I have spent the past 4 hours trying to find this answer anywhere and I am just not informed enough about all these numbers to get what I need to find out. I have an air cylinder that is compressed to 150 PSI with a volume of 7.3 cu in. I am trying to find out how long it will take for the 150PSI to vent down to atmospheric pressure through an opening that is 1.77mm in diameter. I realize I need to find CFM, Cubic Feet per Minute. But everywhere I look the calculator asks for velocity of the air. For some reason I just can't grasp why you would need the velocity of the air to figure out how much is moving through the opening. Nor do I have any idea how to figure the velocity since all the velocity equations I found are asking for the CFM!! Thanks guys!