Overview
ISO 9300:2022 - Measurement of gas flow by means of critical flow nozzles (CFNs) - is the international standard that specifies the geometry, installation, operating conditions and calculation methods for using critical flow nozzles to determine the mass flow rate of single‑phase gases, generally without the need for individual nozzle calibration. The third edition updates discharge‑coefficient models, adds new nozzle/configuration options, and provides calculation and uncertainty guidance (including use of REFPROP for gas properties).
Key technical topics and requirements
- CFN types covered: toroidal‑throat and cylindrical‑throat CFNs (directly calibrated data support their use). The edition also introduces quadrant CFNs and detachable diffusers.
- Geometry & materials: standardized nozzle geometry, throat and contraction/diffuser requirements to ensure predictable performance.
- Operating limits: applicability only when flow reaches critical (choked) velocity at the throat and for steady single‑phase gas flow; use is constrained by throat-to-inlet diameter ratio, Reynolds‑number ranges and downstream/back‑pressure conditions.
- Measurement inputs: upstream/downstream pressure tappings, temperature and density measurement, and recommended installation configurations for pipes and chambers.
- Calculations: theoretical and practical formulas for mass flow, discharge coefficient (Cd), critical flow function (C*), conversions to stagnation pressure/temperature, and viscosity determination.
- Uncertainty analysis: methods to estimate measurement uncertainty, including correlated uncertainty components and practical computation approaches.
- Informative annexes: empirical discharge‑coefficient curves, critical flow function values for pure gases and air, diameter correction methods, and data‑fitting procedures.
Practical applications and users
ISO 9300:2022 is intended for organizations and professionals who require high‑accuracy gas mass flow measurement where CFNs are suitable:
- Flow metrology and calibration laboratories
- Gas custody transfer and billing systems where mass flow must be determined reliably
- Process and utility plants (natural gas, industrial gases) needing traceable flow measurement
- CFN manufacturers and test facilities designing or qualifying nozzles and diffusers
- Research institutions performing experimental gas flow testing
Benefits include a standardized, calibration‑minimized approach to mass‑flow determination, transparent uncertainty budgeting, and reproducible installation and calculation practices.
Related standards and context
ISO 9300:2022 was prepared by ISO/TC 30 (Measurement of fluid flow in closed conduits) and references general ISO/IEC editorial and conformity procedures. For complete implementation, consult companion ISO flow‑measurement standards and national metrology guidance.
Keywords: ISO 9300:2022, critical flow nozzle, CFN, gas flow measurement, mass flow rate, discharge coefficient, critical flow function, choked flow, flow metrology, REFPROP, uncertainty.