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ČSN EN ISO 15970 This international Standard gives requirements and procedures for the measurement of the properties of natural gas that are used mainly for volume calculation and volume conversion: density at reference and at operating conditions, pressure, temperature and compression factor. Only those methods and instruments are considered that are suitable for field operation under the conditions of natural gas transmission and distribution, installed either in-line or on-line, and that do not involve the determination of the gas composition. This International Standard gives examples for currently used instruments that are available commercially and of interest to the natural gas industry. NOTE - Attention is drawn to requirements for approval of national authorization agencies and to national legal regulations for the use of these devices for commercial or official trade purposes. The density at reference conditions (sometimes referred to as normal, standard or even base density) is required for conversion of volume data and can be used for other physical properties. Density at operating conditions is measured for mass-flow measurement and volume conversion using the observed line density and can be used for other physical properties. This International Standard covers density transducers based on vibrating elements, normally suitable for measuring ranges of 5 kg/m3 to 250 kg/m3. Pressure measurement deals with differential, gauge and absolute pressure transmitters. It considers both analogue and smart transmitters (i.e. microprocessor based instruments) and, if not specified otherwise, the corresponding paragraphs refer to differential, absolute and gauge pressure transmitters without distinction. Temperature measurements in natural gas are performed within the range of conditions under which transmission and distribution are normally carried out (253 K < T < 338 K). In this field of application, resistance thermometer detectors (RTD) are generally used. The compression factor (also known as the compressibility factor or the real gas factor and given the symbol Z) appears, in particular, in equations governing volumetric metering. Moreover, the conversion of volume at metering conditions to volume at defined reference conditions can properly proceed with an accurate knowledge of Z at both relevant pressure and relevant temperature conditions.