Volume flow Vs. Mass flow

The key difference in mass and volume flow comes to the dependence on the pressure and temperature conditions. While volume flow rate may alter with the varying pressure and temperature, mass flow rate remains constant regardless of pressure and temperature conditions. Hence for practical, routine applications we rely on the volumetric flow readings as they correspond to realistic conditions. Mass flow on the other hand remains constant despite effects of pressure and temperature. In various industrial settings where flow measurement with high degree of accuracy is required, mass flow rate readings are considered. Volumetric flow devices are generally employed at places where we deal with specific volume of fluid with nominal accuracy levels. This makes mass flow measurements and hence mass flow meters expensive compared to volume flow devices. Owing to stable nature of mass flow, their measured readings show negligible to zero variations with temperature and pressure. While calculating volume flow readings, a reference temperature and pressure value is to be fixed. The reason for this is the gas equation showing pressure, temperature and volume interrelation.

The question is, which one of these to rely on? It entirely depends on your application and purpose of flow measurement. Both mass and volume flow devices are paramount in industrial settings, their usage lies on desired accuracy levels.