ISO 12213规定了在混合物只能作为气体存在的条件下计算天然气、含有合成混合物的天然气和类似混合物的压缩因子的方法。 它分为三个部分:这一部分ISO 12213-3:20 06规定了在已知优越热值、相对密度和二氧化碳含量以及相关压力和温度的情况下计算压缩系数的方法。如果存在氢气（通常是具有合成混合物的气体的情况），也需要知道氢气含量。 该方法主要适用于通常进行输配操作的压力p和温度T范围内的管道质量气体，不确定性约为+/-0.1%。对于更广泛的应用，结果的不确定性增加。

ISO 12213 specifies methods for the calculation of compression factors of natural gases, natural gases containing a synthetic admixture and similar mixtures at conditions under which the mixture can exist only as a gas. It is divided into three parts: this part, ISO 12213-3:2006, specifies a method for the calculation of compression factors when the superior calorific value, relative density and carbon dioxide content are known, together with the relevant pressures and temperatures. If hydrogen is present, as is often the case for gases with a synthetic admixture, the hydrogen content also needs to be known. The method is primarily applicable to pipeline quality gases within the ranges of pressure p and temperature T at which transmission and distribution operations normally take place, with an uncertainty of about +/-0,1 %. For wider-ranging applications the uncertainty of the results increases.