ISO 21501-4: Light Scattering Airborne Particle Counter for Clean Spaces

ISO 21501-4 covers instrument calibration procedures to ensure air particle counter sampling measurement inaccuracies are reduced. A Counting Efficiency Test is also included as a final confirmation of air particle counting performance.

All GMP cleanroom operators/users must ensure the air particle counters used to classify and monitor their cleanrooms are compliant with ISO 21501-4 requirements.

ISO 21501-4 Parameters  Target Limit 
 Size calibration  ±5%
 Counting efficiency at first channel  50% ± 20%
 Counting efficiency at particle size 1.5-2.0 times first channel  100% ± 10%
 Instrument resolution (at manufacturer specified size)  ≤ 15%
 Zero count test   ≤ 1 count in 5 mins
 Max particle number concentration (specified by manufacturer)  ≤ 10%
 Sampling flow rate (volumetric)  ± 5%
 Sampling time  ± 1%
 Calibration interval  ≤ 1 year


DelsaMax light scatter instruments use light scattering detection to determine the size distribution of particles within a sample population. Light scattering is a sensitive technique biased by larger particles, making it very useful for detecting particle aggregation and sample contamination. It is important to note that light scattering, similar to laser diffraction, is an ‘ensemble’ technique which does not measure individual particles but determines size distributions within a given population.

DelsaMax light scatter analyzers offer incredible speed, reliability and precision for particle and nanoparticle characterization. Each instrument allows operators to gain valuable insight into samples through non-destructive testing of even the smallest quantities of materials.