A worksheet has been constructed within the Microsoft Excel software system for converting raw particle-fallout data into percentages of obscuration; that is, percentages of areas obscured by particles on surfaces. These obscuration levels are needed for assessing contamination levels in nominally clean manufacturing and assembly facilities. The raw data are obtained by an automated instrument that scans witness plates, determines the sizes of particles that have fallen onto the plates by analyzing their light-scattering characteristics, and counts the number of particles in each of six size ranges. For a given witness plate, the user enters the number of particles counted in each size range into a designated space on the worksheet. Within each size range, the unknown particle-size distribution is assumed to be characterized by a log-normal slope represented by a line that connects the cumulative counts at the limits of the size ranges. The worksheet software uses the counts and this assumed distribution to compute an overall particle-size distribution, which is then used to compute the percent obscuration.

This work was done by Christian J. Schwindt, formerly of I-Net, and Eugene N. Borson of Swales & Associates, Inc., forKennedy Space Center. KSC-11956


NASA Tech Briefs Magazine

This article first appeared in the September, 1998 issue of NASA Tech Briefs Magazine.

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