Application

This guide provides background and support for the use of sequential-sampling plans for the classification of air cleanliness in cleanrooms and clean zones. Such plans are generally most appropriate for environments where air cleanliness is expected to qualify as ISO Class 4 or cleaner.

The sequential-sampling plan described herein has been developed to match the operating characteristic (OC) curve of the traditional single-stagesampling plan. As a result, the probability of passing or failing a given classification is about the same for both plans.

When compared to sampling times required by traditional single-stage-sampling methods, however, sequential sampling is found to be capable of saving about 80 percent of the sampling time when the air sampled is very clean, 35 percent when the air just meets the class limit, and more than 80 percent when the concentration of particles in the air exceeds 2.5 times the class limit.

Sequential sampling is especially applicable as an alternative to a single-stage-sampling scheme in cases where the upper confidence limit (UCL) on the grand mean of the particle concentration is not required or when, with some further development, it can be used to provide an equivalent estimate of the UCL.

Objectives The objectives of this guide are:

a) To establish a sequential-sampling plan that has an OC curve with the same a and ß as the OC curve of the single-stage-sampling procedure of ISO 14644-1; and

b) To investigate the savings in both sampling time and associated costs that might result.