In contrast to liquids, gases can be compressed. The gas density changes depending on the pressure and temperature. Pursuant to the ideal status equation for gases
the volume in the example changes from 1 m3 upstream of the compressor to 0.172 m3 downstream of the compressor. Since there is a flow in the example, the volume is specified dependent on the time (volume flow). The figure shows the air mass flow. This remains constant at 1.205 kg/h over the entire distance. The density changes from 1.205 kg/m3 upstream of the compressor to 7,001 kg/m3 downstream of the compressor. In general the mass flow is specified as a standard volume flow, in other words, in the form of a volume flow as defined in DIN 1343.
Diagram of volume flow change, depending on the measurement position