ISO 20765的这一部分规定了天然气、含有合成混合物的天然气和类似混合物的体积和热量特性的计算方法，该混合物只能作为气体存在。 该方法适用于正常进行输配操作的压力和温度范围内的管道质量气体。对于体积特性（压缩系数和密度），计算不确定度约为±0.1%（95%置信区间）。对于热量特性（例如焓、热容、焦耳-汤姆逊系数、声速），计算的不确定性通常更大。

This part of ISO 20765 specifies a method of calculation for the volumetric and caloric properties of natural gases, natural gases containing synthetic admixture and similar mixtures, at conditions where the mixture can exist only as a gas. The method is applicable to pipeline-quality gases within the ranges of pressure and temperature at which transmission and distribution operations normally take place. For volumetric properties (compression factor and density), the uncertainty of calculation is about ± 0,1 % (95 % confidence interval). For caloric properties (for example enthalpy, heat capacity, Joule-Thomson coefficient, speed of sound), the uncertainty of calculation is usually greater.