Effects of instrument flow rate, sensitivity, and background noise on cleanroom particle count statistics

Detailed particle microcontamination measurements were made in Class 100 and Class IO [unidirectional flow, or vertical laminar flow (VLF)] environments used as final assembly and electro-optical assembly areas, respectively, at Particle Measuring Systems, Inc. (PMS), Two groups of measuring instruments monitoring the two areas included four laser particle counters (LPCs) and three condensation nucleus counters (CNCs). The utility of both types of measurements in these environments is discussed relative to sensitivity, flow rate, and background noise. These data indicate that the statistical advantage of CNCs over 1 cu ft/min LPCs can only be achieved at flow rates > 0.50 cu ft/min. The most prevalent range of particle sizes is limited to those near the most penetrating particle size, The higher count rare of lower flow rate CNC and LPC instruments was also found to be a factor in quantifying episodic events and transient populations. Outside ambient air was shown to be the major contributor to in-room particle counts under the conditions studied here.