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The Bay Area Air Pollution Control District adopted a regulation limiting plume visibility in terms of the Ringelmann Chart and also in terms of obscuration of the observer's view. Since both limitations were adopted in terms requiring the judgment of an observer, preparation for enforcement required development and calibration of a training unit, so that inspectors could be qualified to testify as expert witnesses under this regulation. This paper summarizes the design, calibration and use of the training unit developed for this purpose.

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