Let me start by stating its been years sense I've dealt with fluid mechanics. I'm currently working on a project at work that involves leak testing a pressure vessel. The area in which the test will be done does not have access to shop air. Therefore it will be required to use a portable air tank to pressurize the pressure vessel. I am trying to determine how long it will take before a 200 gallon tank @ 120 PSI (This is what we have available to tow around) becomes insufficient to supply the pressure vessel through roughly 13 feet of 4 inch ductwork with 35psi of air regulated by means of an in line pressure regulator, knowing that the vessel has a leak of 4.0 GPM. (This is a rough estimate of what is to be our max allowable leakage) Any input is appreciated.