For the purpose of testing and grading the density of smoke, the "Ringelmann Smoke Chart" as published and used by the United States bureau of mines, is hereby adopted as a standard for such grading. Smoke is declared to be dense when it is of a degree of density of number two (2) on such chart, or greater, for more than four (4) minutes in any one hour whether such period of time is consecutive or not. (1975 Code § 9.08)