In systems with vacuum pumps, blowers, compressors and heat exchangers, air pressure and temperature are constantly changing - so does the volume flow. This makes the specification of volume flow particularly prone to error and misinterpretation. But unless there is a leak, the mass / standard volume flow stays the same. It is more concise to discuss these systems in terms of mass or standard volume flow. Specify mass or standard volume ...be understood!
*These standard conditions are one choice among many standards. For more information see Wikipedia's page on standard conditions for temperature and pressure.
For more information on the relationship of temperature, pressure, volume,
mass and molar weight, see Wikipedia's page on the
ideal gas law.