Visible emissions history
Before EPA Method 9, inspectors judged smoke against printed grids of black lines. The Ringelmann chart is still referenced in some rules, so it helps to know what a Ringelmann number means.
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The Ringelmann chart is where visible emissions measurement started. Knowing how it works helps when an older rule still quotes a Ringelmann number.
French engineering professor Maximilien Ringelmann developed the chart in the 1880s to grade smoke from boilers. It was in use in the United States by 1897, and the U.S. Bureau of Mines later published the standard printed version.
The chart has grids of black lines on white. Ringelmann No. 0 is all white, No. 1 is 20% black, No. 2 is 40%, No. 3 is 60%, No. 4 is 80% and No. 5 is solid black. Seen from a distance, each grid blends into a shade of gray.
The observer held the chart at arm’s length or placed it at a distance and compared the darkness of the smoke with each grid. The closest match became the Ringelmann number.
The chart only grades dark smoke and depends on the printed card. EPA Method 9 replaced it with trained, certified observers who read both black and white plumes as percent opacity. Ringelmann No. 1 is often treated as 20% opacity.
If a permit or local rule still uses Ringelmann numbers, check with your agency how readings should be made and recorded today. Our resources page has the blank VEO form and other observer handouts.
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Ringelmann No. 1 is a grid that is 20% black. Smoke as dark as that grid is often treated as equal to 20% opacity.
Some older state and local rules still express limits as Ringelmann numbers. Most modern opacity limits use EPA Method 9, which requires a certified observer.
No. Where a permit or regulation requires Method 9, the reading must be made by an observer who holds a current Method 9 certification.
No. It only grades dark smoke. Method 9 covers both black and white plumes.
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