High-efficiency particulate air (HEPA) filters and ultra low penetration air (ULPA) filters such as those used in cleanrooms and clean benches are frequently scanned or probed for leaks. Historically, filters were scanned with a light-scattering photometer that measures the light scattered from many particles simultaneously. While particle counting methods have been introduced, the photometer methods are still used in many cases. There has not been a documented basis for the scanning speed when using photometers. The photometer response time and the time that the probe is over a leak affects the magnitude of the meter response. This paper sets forth a model for the photometer response characteristics. Photometer response to a leak while scanning is modeled as a simple exponential process. The model is verified with experimental data. Time constants for the experimental system were determined. The effect of the response time on the ability to detect leaks is discussed. Methods to compenstate for the loss of sensitivity during scanning are presented.

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