Pressure tap location to measure pressure drop?

[Sycorax]

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 6" in front of it and 6" behind it.

Any logical reason behind choosing a particular location? appreciate your reply. thanks.

 

[BigInch]

The tap opening should avoid being placed where it is exposed to varying pressures accompanying eddy swirls. The tap should be placed a certain number of diameters away from the orifice. I don't remember how many, but that should be easy for you to research that yourself.

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