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Selecting an Airborne Particle Counter
Understanding which features you need in a particle counter is a difficult task. This article will identify the various features available and help you choose which are relevant to your application. The right particle counter will depend on the monitoring environment, communications, monitoring purposes, desired flow rate, and particle size you choose to monitor. Pharmaceutical manufacturing has special monitoring requirements that are not completely addressed here.
Selecting an airborne-particle counter requires some basic understanding of the terminology and procedures. All airborne particle counters sample air at specified volumetric flow rates, which are measured in cubic-feet-per-minute (CFM) or liters-per-minute (LPM). Since particle counters are calibrated at a specific flow rate, the sizing accuracy is dependent upon that flow rate.
To meet most contamination specifications, particle counters must sample defined volumes of air; this provides confidence in the particle data and is often stated as statistical significance. The ISO standard (ISO 14644) refers to cubic meters; to meet this standard, a 1 CFM particle counter must sample for 35 minutes (1 cubic foot multiplied by 35 equals 1 cubic meter). Particle counters with faster flow rates can meet the ISO specifications in less time. A particle counter with a flow rate of 50 LPM can sample one cubic meter in only 20 minutes.
The ISO standards prescribe limits for contamination and the limits can be converted to the obsolete Federal Standard 209E (FS-209E). In most cases the ISO limits are available for common particle sizes such as 0.1, 0.3, and 0.5 µm.
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