The purpose of the recovery test is to determine the amount of time that is necessary for nonunidirectional cleanrooms or clean spaces and their systems to reach a specified steady state cleanliness level after a brief particle generation event within the clean space. It is not recommended for unidirectional airflow clean spaces nor in ISO Class 8 areas.
Note: There is potential for residue from the aerosol to be deposited on the surfaces within the cleanroom.

  • An aerosol generator or airflow visualization generator.
  • A discrete particle counter. An aerosol photometer may be used as an option.


  • The number of points and the location of the points is to be determined.
  • Ensure that the cleanroom HVAC systems are in operation.
  • Set up the particle counter or photometer in accordance with the manufactures instructions.

Note: An aerosol dilutor is recommended when using a particle counter to avoid coincidence loss and contamination of the optics.

  • The sample probe should not be setup directly under the HEPA filter or room air supply.
  • Identify poor airflow areas, turbulent zones, etc. within the test area. The sample probe(s) should be slightly above work surface height or the height of potential product exposure.
  • Up to four sample locations can be measured using one remote instrument. Sample tubing up to 8 m (26 feet) in length can run out of the cleanroom to the particle counter / photometer. Samples will be taken at each location every 15 seconds by sequentially sampling from each tube.
  • Generate a particulate challenge in the cleanroom or clean space to be tested while the air handling units are in operation. Raise the initial particle concentration 1,000 to 10,000 times the target cleanliness level.
  • Stop the aerosol challenge, record the start time, and begin aerosol concentration measurements. Record the aerosol concentration for a 6 second to 12 second sample period for each minute until the particle count is returned to ten times the target cleanliness level.
  • Record and plot the particle concentration versus time for each sample location.
  • The recovery time is the time, in minutes, for the particle concentration to decrease two orders of magnitude, i.e., the time for each location to recover from 1000 times the target concentration to ten times the target. The slowest recovery location in the cleanroom defines the room recovery rate.

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