The volume of compressible fluid changes with change in pressure and temperature. The mass remains constant irrespective of changes in pressure and temperature of fluid. It is therefore convenient to measure mass flow rate.
Volume flow rate also can be measured provided the reference conditions of temperature and pressure are well defined. The volume flow rate is then computed for these conditions thus avoiding the errors due to changes in pressure and temperature of fluid.
Steam flow meters display the reading in terms of mass, i. e. Tons/hour or Kg/hour etc. while Air or Gas flows measure flow in terms of Nm³ or Sm³ where N and S stands for reference conditions.