1.1 This International Standard specifies a method of determining whether an analytical system for natural gas analysis is fit for purpose. It can be used either to determine a range of gas compositions to which the method can be applied, using a specified calibration gas, while satisfying previously defined criteria for the maximum errors and uncertainties on the composition or property or both, or to evaluate the range of errors and uncertainties on the composition or property (calculable from composition) or both when analysing gases within a defined range of composition, using a specified calibration gas. 1.2 It is assumed that for evaluations of the first type above, the analytical requirement has been clearly and unambiguously defined, in terms of the range of acceptable uncertainty on the composition, and, where appropriate, the uncertainty in physical properties calculated from these measurements, for applications of the second type above, the analytical requirement has been clearly and unambiguously defined, in terms of the range of composition to be measured and, where appropriate, the range of properties which may be calculated from these measurements, the analytical and calibration procedures have been fully described, and the analytical system is intended to be applied to gases having compositions which vary over ranges normally found in gas transmission and distribution systems. 1.3 If the performance evaluation shows the system to be unsatisfactory in terms of the uncertainty on the component amount fraction or property, or shows limitations in the ranges of composition or property values measurable within the required uncertainty, then it is intended that the operating parameters, including the analytical requirement, the analytical procedure, the choice of equipment, the choice of calibration gas mixture, and the calculation procedure, be reviewed to assess where improvements can be obtained. Of these parameters, the choice of the calibration gas composition is likely to have the most significant influence. 1.4 This International Standard is applicable to analytical systems which measure individual component amount fractions. For an application such as calorific value determination, the method will be typically gas chromatography, set up, as a minimum, for the measurement of nitrogen, carbon dioxide, individual hydrocarbons from C1 to C5 and a composite measurement representing all higher hydrocarbons of carbon number 6 and above. This allows for the calculation of calorific value and similar properties with acceptable accuracy. In addition, components such as H2S can be measured individually by specific measurement methods to which this evaluation approach can also be applied. 1.5 Performance evaluation of an analytical system is intended to be performed following initial installation to ensure that errors associated with assumed response functions are fit for purpose. Thereafter, periodic performance evaluation is recommended, or whenever any critical component of the analytical system is adjusted or replaced. The appropriate interval between periodic performance evaluations will depend upon both how instrument responses vary with time and also how large an error may be tolerated. This first consideration is dependent upon instrument/operation; the second is dependent on the application. It is not appropriate, therefore, for this International Standard to offer specific recommendations on intervals between performance evaluations.