ISO 6145-6:2017

Overview

ISO 6145-6:2017 - "Gas analysis - Preparation of calibration gas mixtures using dynamic methods - Part 6: Critical flow orifices" defines a dynamic method for preparing calibration gas mixtures from pure gases or pre‑mixtures using critical flow orifices. Published in 2017 (third edition) as part of the ISO 6145 series, the standard is intended mainly for non‑reactive gases and covers multi‑component and binary mixtures with flow rates from 1 ml/min to 10 l/min.

Key Topics

• Principle of critical flow orifices: flow becomes independent of downstream pressure when the critical pressure ratio is reached - enabling stable, reproducible dilution.
• Design and operation: use of multiple critical flow orifices to create multi‑component mixtures and achieve large dilution ratios (up to 1 × 10^4).
• Pressure requirements: mainly for atmospheric pressure mixtures; preparation at pressures above atmospheric is possible provided upstream pressure is at least 2× downstream pressure.
• Flow calculations: mass flow rate, molar and volume flow rate calculations under ideal and real conditions, including use of stagnation pressure/temperature.
• Key parameters: discharge coefficient, critical flow function/coefficient, isentropic coefficient (γ), compressibility factor (Z), throat Reynolds number (Re) and dynamic viscosity.
• Uncertainty analysis: identification and estimation of uncertainty sources for flow and composition; procedures for uncertainty propagation.
• Calibration & verification: methods to calibrate the mixing system (flow and concentration), and verification procedures. Informative annexes provide worked examples (e.g., toroidal orifices, nitrogen calibration).

Applications

ISO 6145-6:2017 is used by:

• Calibration laboratories and national metrology institutes preparing primary and secondary calibration gas mixtures.
• Manufacturers of gas analyzers and flow control equipment designing critical flow orifice systems.
• Environmental and emissions monitoring labs that require accurate, traceable standards for air pollutant and greenhouse gas measurements.
• Industrial process control and quality assurance teams needing on‑site dynamic generation of multi‑component gas standards.

Practical benefits include repeatable, scalable dilution, the ability to prepare multi‑component mixtures with relatively simple hardware, and reliable traceability when combined with proper calibration and uncertainty evaluation.

Related Standards

• ISO 6143 - Comparison methods for checking composition of calibration gas mixtures
• ISO 9300 - Measurement of gas flow by critical flow Venturi nozzles
• ISO 12963 - Comparison methods for gas mixture composition based on calibration
• ISO 16664 - Handling of calibration gases and gas mixtures - Guidelines
• ISO 7504 - Gas analysis - Vocabulary

Keywords: ISO 6145-6:2017, critical flow orifices, calibration gas mixtures, dynamic preparation, gas analysis, mass flow rate, discharge coefficient, uncertainty evaluation.