Part 4: Test methods for particle content

ISO 8573-4 introduces several test methods to determine the content of particles in compressed air, describing the typical range of applications and limitations of each method. The result is a concentration by number.

Applying one of the test methods of ISO 8573-4 is mandatory for the allocation of a purity class according to ISO 8573-1.


Most common is the partial flow method using an optical particle sizing and counting instrument, i.e.

  • for a defined time period a sample flow is taken from the turbulent compressed air main flow applying isokinetic sampling and is guided to another downstream sampling line or directly into a diffusor
  • selection of a suitable measurement device (optical aerosol spectrometer (OAS) according to ISO 21501-1 respectively optical particle counter (OPC) according to ISO 21501-4)
  • ensuring a valid calibration (using certified and NIST traceable polystyrene latex microspheres (PSL) or requested by ISO 21501-1 or ISO 21501-4, respectively)
  • ensuring the avoidance of coincidence errors (possibly further dilution required offered by an additional sampling line with isokinetic sampling)
  • using a suitable diffusor in combination with typically non-pressure resistant particle sizing and counting instruments

ISO 8573-4 test methods cannot differentiate between solid and liquid particles and thus measure both – solid and liquid particles.

ISO 8573-4 test methods require specialized technical competences and professional measurement devices and thus should be carried out by according specialized institutions. There are countless sources of error to be avoided and several device-specific features to be considered – especially when a class 1-3 for particles according to ISO 8573-1 shall be measured.


Any kind of particle measurement shall just be carried out when no more liquid particles or condensable vapors are present in the compressed air. Both contaminants in the compressed air may result in a substantial measurement error.


Pressure regulators SHALL NOT BE USED for pressure expansion.


Within ambient air there are much higher particle concentrations than those that can be measured in the compressed air. Thus, any kind of contamination from the ambient air results in a substantial measurement error.