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 that 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 a gaseous state, the calorific value is labelled "inferior". See The Draft International Standard ISO/DIS 14532 for additional details.
The unit for the heating value has the dimension of energy per volume, or energy per mass, or energy per mole, 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.