NF EN ISO 9300

Measurement of gas flow by means of critical flow nozzles

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.

To comply with a standard, you need to quickly understand its issues in order to determine its impact on your activity.

The Requirements department helps you quickly locate within the normative text:
- mandatory clauses to satisfy,
- non-essential but useful clauses to know, such as permissions and recommendations.

The identification of these types of clauses is based on the document “ISO / IEC Directives, Part 2 - Principles and rules of structure and drafting of ISO documents ”as well as on a constantly enriched list of verbal forms.

With Requirements, quickly access the main part of the normative text!

The Redline + service - standards comparator allows you to easily and simply identify major changes between the current standard and its last canceled version.

At a glance, you will be able to identify the additions, deletions or modifications to a text, table, figure and formula.


The Redlines + service is offered to you on the collection of French standards in force, in French language and in HTML and PDF format.

For an overview of the service, click on View a standard in redline format

- The UPSELL service allows you to easily update one of your standards.
- With a single click, add a new language, the Requirements or Redline+ service and add one or more additional users.
- Whether you are in the process of acquiring a standard or it is already available in your personal space, the UPSELL service is available at every stage to help you understand it and implement it within your organization.

COBAZ is the simple and effective solution to meet the normative needs related to your activity, in France and abroad.

Available by subscription, CObaz is THE modular solution to compose according to your needs today and tomorrow. Quickly discover CObaz!

Request your free, no-obligation live demo

I discover COBAZ