Note:
In most cases the capillaries are operated laminar due to their small diameters. In this case the volume flow correlates very linear with the differential pressure between inlet and outlet and the mass flow increases approximately in proportion to the applied inlet density of the medium.
In rare cases of turbulent operation (Re > 2000) the very linear laminar flow is superimposed by the mass flow of an orifice. Thus a flow portion is added, which is in proportion to the square root of the product of differential pressure between inlet and outlet and the inlet density of the medium.