§ 93.21 DENSE SMOKE.
   (A)   It shall be unlawful to cause or permit the emission of dense smoke from any fire, chimney, engine, oil burner, or any other agency in the city so as to cause annoyance or discomfort to the residents thereof.
   (B)   (1)   For the purpose of testing and grading the density of smoke, the Ringelmann Smoke Chart, as published and used by the United States Geological Survey, shall be and hereby is adopted as the standard for such grading
      (2)   Smoke shall be, and is hereby defined as and declared to be DENSE when it is of a degree of number 3 of the chart, or greater, for more than 6 minutes in any 1 hour, whether the period of time is consecutive or not.
Penalty, see § 93.99