The sensitivity, resolution, and sample rate of optical particle monitors, counters and spectrometers are described. Important differences in conceptual design of each instrument class are detailed. Optical particle counters and spectrometers require uniform sample volume illumination and have been used in monitoring aerosol microcontamination for many years. Spectrometers have the highest resolution and the greatest number of size channels. Counters may have high intrinsic resolution but it is lost because of the fewer number of size channels provided.

Monitors are a relatively new class of instrument which do not provide uniform sample volume illumination. They are becoming widely used in liquid monitoring. Monitors are simpler, less expensive devices and are characterized as having poor resolution but providing the highest sample flow rates and delivering the largest database. When used on fluids with normal populations having an exponential size distribution, monitors show little size distribution distortion. When modal populations or deviations from exponential size distributions are encountered, counters or spectrometers are required. Filter penetration tests generally demand the highest resolution offered only by spectrometers.

Data are presented from field visits to 17 semiconductor plants having deionized (D.I.) water processing facilities where monitors and spectrometers were used simultaneously to characterize watcr quality. The data set provides an interesting comparison within the industry as well as an opportunity to compare the low-resolution monitors with high-resolution spectrometers under field conditions.

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