ISO 9300:2005规定了临界流量文丘里管喷嘴（CFVN）的几何形状和使用方法（系统中的安装和操作条件），用于确定流经系统的气体的质量流量。它还提供了计算流量及其相关不确定性所需的信息。它适用于文丘里管喷嘴，在文丘里管喷嘴中，气流加速到喉部的临界速度（这等于局部声速），并且仅适用于单相气体稳定流动的情况。

ISO 9300:2005 specifies the geometry and method of use (installation in a system and operating conditions) of critical flow Venturi nozzles (CFVN) used to determine the mass flow-rate of a gas flowing through a system. It also gives the information necessary for calculating the flow-rate and its associated uncertainty. It is applicable to Venturi nozzles in which the gas flow accelerates to the critical velocity at the throat (this being equal to the local sonic velocity), and only where there is steady flow of single-phase gases.