No person shall cause, suffer, or allow to be emitted into the open
air from any fuel burning equipment during normal operation, smoke the shade
or density of No. 2 of the Ringelmann Chart, except for three minutes in any
fifteen-minute period which the smoke may be equal to No. 2 Ringelmann Chart,
and except for one minute in any thirty-minute period, during which the smoke
may be greater than No. 2 Ringelmann Chart, and except as hereinafter provided.