The product of cross-sectional pipe area and average fluid velocity is the volumetric flow rate of
the fluid through the pipe (Q = Av). This tells us that fluid velocity will be directly proportional
to volumetric flow rate given a known cross-sectional area and a constant density for the fluid
flowstream. Any device able to directly measure fluid velocity is therefore capable of inferring
volumetric flow rate of fluid in a pipe. This is the basis for velocity-based flowmeter designs