In Chemical Engineering the most common descriptions of a chemical process are the pressure, temperature and flow rate.  Often a mass or volumetric flow rate must be converted to a molar flow rate in order to have meaning in an expression for a chemical reactor.  A volumetric flow rate at a given temperature an pressure will be different when these parameters change.  For example, even though the mass of material in a combustion process may be small the volumetric flow rate can be quite large if the combustion results a gas phase product at a high temperature.  For construction of a venting system it is important to use a worst case scenario to deal with the volume of gas that must be removed.