Federal Standard 209D (FED-STD-209D) for certifying the cleanliness of air in cleanrooms requires sampling at a minimum number of locations as determined by the Class Limit and the area of the flow. Gas sampling particle counters are used to sample only part of the area through which gas is flowing. When used in this way, they are carrying out a partial inspection sample, rather than a complete inspection. It is difficult to determine the area of flow to be associated with this point sample. Given the partial, rather than complete nature of the sample, the statistics of partial inspection sampling should be used. This approach can reduce the number of samples needed to ensure adequate gas cleanliness, especially when used to supplement more complete inspection.

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