Blocking the intake of an air flow station I built results in the FPM shooting up along with the vacuum. ie: wide open both ends 4" pipe with inline fan reads . 5" wc / -190 pa / 3700 fpm on instruments monitoring intake pipe. Blocking the intake increases both the vacuum to -470 pa and increases the velocity to 5400 FPM. MY QUESTION: Is their a conversion factor to plug in at the beginning of a CFM equation to compensate for vacuum on the intake side. ie: .0873(4"pipe) x 3700 fpm = 323 cfm This answer seems like it should be smaller using the same equation with blocked intake but it actually increases unless some allowance for the blocked intake is factored. ??????