Where does an air damper cause less resistance in a supply duct?

[Yuri B.]

Where in an air supply duct (after the fan) an air damper (or any other resistance to flow) will have less Δ P : closer to the fan, or closer to the outlet ?

 

[Q_Goest]

For any given flow rate and restriction in a flow stream, the irreversible drop in pressure across that restriction will be lowest where density is highest.

 

[Yuri B.]

Thank you very much !
(all is clear now).

 

[Yuri B.]

One little qwestion more please. For the known volume (for the known dP) to find the mass flow, what pressure (and RH) do we take : one of before the restriction, after, or a mean ?