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. Increasingly, particle detectors that count individual particles are used to scan filters. The equations that govern the scanning of high-efficiency filters with particle counters have been derived. The assumptions that bound the applicability of the equations are stated. The relationships provide the means to determine the appropriate linear scan rate and to understand the effect of the test parameters on the duration and accuracy of the test. The linear scan rate is derived in terms of a leak flow rate but a method is presented to relate the leak flow to the traditional leak penetration. Further, a method is described to standardize the definition of leak penetration to eliminate the dependence of measured leak penctration on the instrument used to measure it. The linear scan rate is shown to depend on the number of particle counts used to characterize the threshold leak. The statistical impact of the count used to calculate the linear scan rate is described.

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