Understanding ISO 21501-4 (EL)


ISO 21501 has the title Determination of particle size distribution—Single particle light interaction methods. Included with ISO 21501 are four parts, of which only Part 4 will be discussed in this paper:

  • Part 1: Light scattering aerosol spectrometer
  • Part 2: Light scattering liquid-borne particle counter
  • Part 3: Light extinction liquid-borne particle counter
  • Part 4: Light scattering airborne particle counter for clean spaces

The documentation and approval of ISO 21501-2, -3, and -4 subsequently replaces and cancels ISO 13323- 1:2000. The ISO 21501 standard widens the scope of analysis to include methodology for airborne particle counting and liquid particle counting (both light scattering and extinction methods). Specifically, ISO 21501-4 provides a calibration procedure and verification method for airborne particle counters to minimize inaccurate measurements and reduce variations between different instruments. These new guidelines require pulse height analysis (PHA) for particle counter calibrations, which reduces inconsistencies.

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