# ISO 6976:2016 pdf free

**ISO 6976**:2016 pdf free.Natural gas – Calculation of calorific values, density, relative density and Wobbe indices from composition

Likewise, for density and relative density, the methods use formulae in which, for all individual molecular species of the gas mixture, the tabulated value of molar mass is weighted in accordance with its mole fraction, all terms then being added together to obtain the mole fraction average of this quantity. Formulae are given that convert this mole fraction average molar mass into the ideal-gas density or relative density.

Values of the density and relative density for the real gas are then obtained by the application of a volumetric correction factor (compression factor), a prescription for the calculation of which is given.

For calorific values, conversion from the ideal-gas state to the real-gas state is in principle slightly less simple. The application should first be made of a small enthalpic correction (residual enthalpy) to the calorific value (gross or net) of the ideal gas on the molar basis, so as to obtain the calorific value of the real gas on the molar basis. For the purposes of this document, however, this enthalpic correction has been estimated as so small as to be justifiably negligible (see ISO/TR 29922).

In consequence of neglecting the enthalpic correction, the real-gas calorific values on the molar and mass bases are in effect set as equal to the corresponding ideal-gas values. To obtain the values of the real-gas calorific values (gross or net) on the volume basis from the corresponding ideal-gas values, however, the volumetric correction factor (compression factor) mentioned above is applied.

Finally, formulae are given for the calculation of Wobbe indices, for either the ideal gas or real gas, from the other properties considered herein.

For each of the natural gas properties for which formulae are provided as described above, the methods prescribed in GUM[5] have been applied so as to provide further formulae that enable an estimate of associated uncertainty.

The derivation of each such uncertainty formula is presented fully in ISO/TR 29922. In essence, each results from the analytical derivation of sensitivity coefficients, by means of partial differentiation of the relevant mixture-property formula with respect to each of the input quantities (namely the pure-component physical properties and component mole fractions) with which an uncertainty may be associated. The derivations also take into account the unavoidable correlations between the component mole fractions and the likewise unavoidable, but less obvious, correlations between the component molar masses (see also Clause 11).

For each property, the total variance (squared uncertainty) is obtained by addition of the independent contributions to the variance from each source of uncertainty and the overall uncertainty result is taken as the square root of this quantity. The formulae to be used are given in Annex B.

In Clause 12, tabulated values are given for the relevant physical properties and their associated uncertainties of the pure components of natural gas for each of the commonly used reference conditions.

Auxiliary data, including uncertainties, are given in Annex A. Example calculations are given in Annex D.ISO 6976 pdf download.