ABSTRACT
The relation expressed by Eq. (11.49) is known as the Hagen-Poiseuille law for laminar flow, i.e., for Reynolds numbers smaller than 2100. This relation is used for viscosity measurements from the measured volumetric flow rate q versus the pressure drop dpjdx. It is important that in these tests the laminar flow must be in the steady state (i.e., constant q or constant dpjdx) and smoothly developed (i.e., the pressure drop measurements must be made at distances at which the disturbances at the tube inlet are minimized).
