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Basics

Certain factors are routinely considered for any particle counter purchase, such as price, warranty, quality, and expected laser lifetime. When selecting a particle counter for cleanroom certification it is important to consider additional factors. Of these, sensitivity and flowrate are the most important parameters, both of which determine the sampling time required to complete certification at the desired level of cleanliness. Particle counter saturation also must be considered. The sensitivity of an airborne particle counter is the size of the smallest particle the unit can detect. Particle counters available today that are appropriate for cleanroom certification typically have sensitivities of 0.1, 0.3, or 0.5 µm. The flowrate of a particle counter is simply the rate at which its pump draws the sample air through the sample chamber. The flowrates of the particle counters commonly used in cleanroom certification traditionally have been 0.1 CFM and 1.0 CFM. However, the growing recognition in pharmaceuticals both of the critical importance of large particles and of how difficult large particles can be to sample has greatly reduced the utility of 0.1 CFM units for certification. These factors, combined with the need to minimize labor costs, are driving the increased acceptance and use of 3.53 CFM (100 LPM) counters.

Cleanliness Levels

ISO 14644-1 is the accepted worldwide standard for classifying the cleanliness of the air in cleanrooms and clean zones. Table 1 gives the maximum number of particles allowed (per cubic meter) if the zone is to meet a specified ISO class of cleanliness. From this table one can observe how the particle concentration allowed for one size of particle differs from that of another size. For 0.3 µm, for example, Class 6 allows 10 times as many particles as Class 7. In fact, note that the ISO classes have been constructed such that this 10:1 ratio is true for all vertically adjacent ISO classes, regardless of particle size.

Click here to download the full paper: Click here to signup and download files (How to Select a Portable Airborne-Particle Counter for ISO Certification of Cleanrooms)