There are a volumetric flowrate and mass flowrate. The first is expressed in m3/s, m3/h, cm3/s, l/s etc., while the second - in kg/s, kg/h, g/s, etc. Usually the volumetric or mass flowrates are applied to liquids (since liquids are incompressible); the mass flowrate is applied to gases and vapours (they are compressible).
Flowrate measurement is not an easy problem, since many factors such as density, viscosity, ratio of phases, temperature, pressure, etc. of the stream have influence on the results of flowrate measurements. If calibration of the device for flowrate measurement was carried out at conditions different from those when it is used in practice, then we will get inaccurate flowrate data. We'll discuss in the coming posts operational principles of several instruments, which employ some of widely known methods for flowrate measurements commonly used in industrial environment.