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This document provides the means for energy determination of natural gas by measurement or by calculation, and describes the related techniques and measures that are necessary to take. The calculation of thermal energy is based on the separate measurement of the quantity, either by mass or by volume, of gas transferred and its measured or calculated calorific value. The general means of calculating uncertainties are also given. Only systems currently in use are described. NOTE Use of such systems in commercial or official trade can require the approval of national authorization agencies, and compliance with legal regulations is required. This document applies to any gas-measuring station from domestic to very large high-pressure transmission. New techniques are not excluded, provided their proven performance is equivalent to, or better than, that of those techniques referred to in this document. Gas-measuring systems are not the subject of this document.