Flow expresses the quantity of fluid (liquid, gas or steam) that passes through a section in a unit of time. It can be expressed in volumetric (m3/h, l/min, l/h, etc.) or mass (kg/s, Kg/h, etc.) flow units. The main factor for flow measurement is the mean flow velocity that is used to calculate the Reynolds Number (Re), which subdivides the flow of the fluid into two categories: Laminar (typically with Re < 2000) and Turbulent (typically with Re > 10000).