The Cleanroom Recovery Test
The purpose of the test is to establish how long it takes for the cleanroom to recover from a challenge concentration to the Target Cleanliness Level. The ISO 14644-3 document recommends that the recovery test is only performed on non-unidirectional airflow systems and should be carried out upon an installation in the as-built or at-rest state. It is not recommended to conduct this test “in production” (for obvious reasons) and further it is not recommended that the test be applied to ISO Class 8 or ISO Class 9 environments due to the excessive challenge concentrations that would be required. The standard also highlights that where an artificial aerosol is used to challenge the cleanroom that residue contamination of the installation should be avoided. This is a critical point, particularly when testing the recovery performance of ISO Class 7 cleanrooms or cleanrooms with a high background particle count or baseline, as the recovery test requires that an aerosol challenge be established in at 100 x the “Target Cleanliness Level”. The data presented in the graph shown in Fig. 1 depicts the Recovery Test being conducted in a ISO Class 7 Cleanroom. As described in the standard, the particle measurements were made inside the time range where the decay of particle concentration is described by an exponential decay, indicated by a straight line on a semi-log chart. Note that the particle concentrations are plotted on the ordinate by a logarithmic scale and the time values on the abscissa by a linear scale.
Figure 1: Recovery test conducted in a ISO 7 class cleanroom
After the particle counting equipment is set up and running in the cleanroom, the room’s background or “baseline” should be established. In the example above, this ISO Class 7 Cleanroom has a calculated average baseline of 65,000 particles per cubic meter of equal to or greater than 0.5um.