When the flow medium flows through the measured tube, the pressure will force on the target because of the flow kinetic energy. It is known that the pressure value forcing on the target can be expressed as：
F = cd ρ v2 At / 2
where
F = force on the target (N)
cd = overall drag coefficient obtained from empirical data
ρ = density of fluid (kg/m3)
v = fluid velocity (m/s)
At = target area (m2)