Superior calorific value: The energy released as heat energy by the complete combustion in air of a defined quantity of gas, in such a manner tha the pressure p_c at which the reaction takes place remains constant, and all the products of combustion are returned to the same specified temperature T_c as that of the reactants, all of these products being in the gaseous state except for water formed by combustion, which is condensed to the liquid state at T_c. If the water is also returned to the gaseous state, the calorific value is labelled Inferior. See draft International Standard ISO/DIS 14532 for additional details.
The unit for the heating value has the energy per volume or energy per mass or energy per mole dimension, i.e. it always refers to a specific quantity of gas, as described above. The base heating value is reported at standard conditions for the metering of the quantity of the gas and at standard conditions for combustion.