After a big mistake in this PF, I try to post my question here, hoping it is ok. I'm looking for a more "simple" relation than Darcy's law. I try to determinate what is the relation between air speed and differential pressure in a tube where there is a restrictor. A kind of relation like V=kP, where V in m/s and P in Pa or mBar and k a constant. I do not want to try experimentation to determine this relation, I'm looking for theoretical relation to calculate air speed with only a differential manometer with the + connected before the restrictor and the - after it. In the Darcy's law, there is too much parameters I can't know. Thank you in advance for your help, and rather impatient to read you.