本文件规定了临界流量喷嘴（CFN）的几何形状和使用方法（在系统中的安装和操作条件），这些喷嘴用于确定流经系统的气体的质量流量，基本上无需校准CFN。它还提供了计算流速及其相关不确定度所需的信息。 本文件适用于气流在最小流动截面处加速至临界速度的喷嘴，且仅适用于单相气体稳定流动的喷嘴。当喷嘴达到临界速度时，在现有入口条件下，流过喷嘴的气体质量流率是最大的，而CFN只能在规定的限制范围内使用，例如CFN喉道与入口直径比和雷诺数。本文件涉及超环面- 以及圆柱形喉道CFN，已对其进行了足够数量的直接校准实验，以使得出的系数能够在一定的可预测不确定性极限下使用。

This document specifies the geometry and method of use (installation in a system and operating conditions) of critical flow nozzles (CFNs) used to determine the mass flow rate of a gas flowing through a system basically without the need to calibrate the CFN. It also gives the information necessary for calculating the flow rate and its associated uncertainty. This document is applicable to nozzles in which the gas flow accelerates to the critical velocity at the minimum flowing section, and only where there is steady flow of single-phase gas. When the critical velocity is attained in the nozzle, the mass flow rate of the gas flowing through the nozzle is the maximum possible for the existing inlet condition, while the CFN can only be used within specified limits, e.g. the CFN throat to inlet diameter ratio and Reynolds number. This document deals with the toroidal- and cylindrical-throat CFNs for which direct calibration experiments have been made in sufficient number to enable the resulting coefficients to be used with certain predictable limits of uncertainty.