Ringelmann scale

[1][2] It was developed by a French professor of agricultural engineering Maximilien Ringelmann of La Station d'Essais de Machines in Paris, who first specified the scale in 1888.

[3][4] The scale has 5 levels of density inferred from a grid of black lines on a white surface which, if viewed from a distance, merge into known shades of grey.

They were introduced to the United States in an article published in the Engineering News of November 11, 1897, with a comment that the author had learned of the Ringelmann scale in a private communication from a Bryan Donkin of London.

It was subsequently adopted by the Technologic Branch of the U.S. Geological Survey (which later became the U.S. Bureau of Mines) and used to study smokeless combustion in St. Louis in 1904.

The apparent darkness of a smoke depends upon the concentration of the particulate matter in the effluent, the size of the particulate, the depth of the smoke column being viewed, and natural lighting conditions such as the direction of the sun relative to the observer while the accuracy of the chart itself depends on the whiteness of the paper and blackness of the ink used.