I need to measure the pressure drop across a core( lets say a restriction). Air is flowing ( around 15 g/sec) in a 4"ID tube which is very long( irrelevant). Now there is a core ( or lets call it some restriction) in the pipe which is 10" long and around 3.92" wide which fills in the 4" tube. Now where do I place the pressure taps? Do i place right near the core/restriction around 1" in front of it and 1" behind it or do i place quite some distance from it , like 10" in front of it and 10" behind it. Any logical reason behind selecting where to put the pressure taps. Appreciate your replies. Thanks.