(A) No light industrial use may emit more than ten smoke units per hour, per stack or smoke in excess of Ringelmann No. 2. However, once during any 24-hour period, for soot blowing, process purging and fire cleaning, each stack may emit an additional ten smoke units, and, during that time, it may emit smoke up and including Ringelmann No. 3.
(B) No heavy industrial use may emit more than 30 smoke units per hour, per stack or smoke excess of Ringelmann No. 2, however, once during any six-hour period, for soot blowing, process purging and fire cleaning, each stack may be permitted an additional ten smoke units, and, during that time, it may emit smoke, up to and including Ringelmann No. 3.
(C) For the purpose of this section, the term RINGELMANN NUMBERS means the number of the area on the Ringelmann Chart that most nearly matches the light-observing capacity of smoke. The Ringelmann Chart is described in the U.S. Bureau of Mines Information Circular 6888 or amendments, on which are illustrated graduated shades of gray for use in estimating smoke density. Smoke below the density of Ringelmann No. 0 and Ringelmann No. 1 shall be considered as no smoke.
(D) For the purpose of this section, SMOKE UNIT means the number obtained when the smoke density in Ringelmann number is multiplied by the time of emission in minutes. For the purpose of the calculation, a Ringelmann density reading shall be made at least once a minute during which it is observed. The products so computed shall than be added to give the total number of smoke units observed during the entire observation period.