Overview
ISO 8573-9:2004 - “Compressed air - Part 9: Test methods for liquid water content” defines standardized test methods to determine the liquid water mass concentration in compressed air. It is one part of the ISO 8573 series that harmonizes measurement of air contaminants and supports assigning air purity classes in accordance with ISO 8573-1. The standard specifies sampling techniques, measurement procedures, evaluation, uncertainty considerations and reporting requirements for the air purity parameter liquid water (distinct from water vapour, which is covered in ISO 8573-3).
Key topics and technical requirements
- Scope and purpose: Measure liquid water (aerosol + wall flow) as mass concentration in compressed air; specify method limitations and reporting.
- Selection of method: Method selection depends on the expected liquid water concentration; the standard lists appropriate methods for different concentration ranges.
- Sampling: Samples shall be taken at or near actual system pressure and at a constant flow rate. Sample parameters (pressure, temperature, velocity) must remain within instrument limits.
- Two principal measurement methods:
- Gravimetric method: Collect condensate, separate water from oil if present, and measure volume or mass. Test equipment includes separators, high‑efficiency coalescing filters, collecting vessels and measuring columns. Recommendations include collecting at least 100 ml total condensate for good uncertainty. Formulas for converting collected volume/mass and sampled air volume to g/m³ are provided.
- Vapourization method: Measure water vapour content before and after transforming liquid water to vapour (via heating/pressure change); the difference yields liquid water content. Requires accurate pressure, temperature and humidity sensors and a heater capable of full vaporization.
- Measurement accuracy: The standard states instrument accuracy expectations (e.g., pressure and differential pressure ≈ ±2%, temperature ≈ ±1 °C, flow ≈ ±5%, mass/volume measurement ≈ ±2%).
- Evaluation & reporting: Procedures for result evaluation, uncertainty analysis and standardized test reporting are defined (including a sample test report in an annex).
Applications and users
ISO 8573-9 is used by:
- Quality/control laboratories testing compressed air purity
- Compressor and filtration manufacturers for performance verification
- Facility engineers and maintenance teams in industries sensitive to water contamination (pharmaceutical, food & beverage, electronics, petrochemical, painting/coating)
- Consultants and auditors specifying air purity for processes and for compliance with ISO 8573-1 purity classes
Practical benefits include consistent measurement, comparable results across labs, and clear basis for selecting dryers, separators and filters.
Related standards
- ISO 8573-1 - Contaminants and purity classes (reference for purity class assignment)
- ISO 8573-2 - Test methods for aerosol oil content (sampling references)
- ISO 8573-3 - Measurement of humidity (water vapour)
- ISO 1219-1, ISO 3857-1, ISO 5598 - Symbols and vocabulary used in compressed air testing
Keywords: ISO 8573-9:2004, compressed air, liquid water content, test methods, gravimetric method, vapourization method, air purity, sampling techniques, ISO 8573 series.
