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This document gives the basic data needed, recommended procedures and illustrative calculations for computing heating values of natural gas mixtures from the composition of the mixtures and the properties of the components at commonly used reference conditions for gas measurement within the range 273.15 K to 298.15 K (0 deg C to 25 deg C). Symbols and terms used are defined, units of measurement are defined and conversion factors and physical constants are given. Selected values for standard enthalpies of combustion and heat capacities of the pure hydrocarbon gases C1 to C6 gases are tabulated at the reference temperatures 273.15 K (O deg C), 288.15 K (15 deg C), 288.71 K (60 deg F), anf 298.15 K (25 deg C) on a molar basis, and a volumetric basis. Both the dry gases and the ideal water-saturated gas are treated. The calculation of enthalpies of combustion of gas mixtures on a molar, mass, or volumetric basis is described. Second viral coefficients are presented as functions of temperature for the pure substances and for binary interactions with methane. Tables are given for molar volumes, enthalpic effects (H-H deg), and the values of the dry real-gas hydrocarbons on a molar mass volumetric bases at two reference conditions, 288.15 K (15 deg C), 101 325 Pa; and 288.71 (60 deg C), 101 560 Pa (14.73 psia). An analysis is presented of the effects of errors in the data on calculated heating values. (Author)