Why use volumetric units for mass flow rates?
Following the logic above, a mass flow rate should be expressed in units of mass such as g/h, mg/s, etc. Most users, however, think and work in units of volume. That’s fine, and to use density in converting mass flow to volume flow, we must pick a set of specific pressure and temperature conditions at which we use the density value for the gas. Worldwide, there are quite a lot of these standard reference conditions for conversion.
The following reference conditions are used by Bronkhorst:
BLOG: Reference conditions
- When the mass flow rate is expressed with subscript n as in mln/min or m3n/h, this means that a fluid density at a temperature of 0 °C and a pressure of 1 atm (1.013 bar) are selected for conversion from mass flow rate to volumetric flow rate. The subscript n represents normal reference conditions in European style.
- This corresponds to the prefix "s" in sccm (standard cubic centimetres per minute) or slm (standard litre per minute), which refers to American standard conditions at a temperature of 0°C (32°F) and an absolute pressure of 1 atm (1.013 bar, 14.69 psia).
- As an alternative, a temperature of 20°C and a pressure of 1 atm (1.013 bar) are used to refer to European standard reference conditions, indicated by the subscript s in the volumetric units (mls/min, m3s/h). These values resemble average temperature and pressure conditions at sea level.