Ringelmann scale. A evaluate of smoke color. Invented by Professor Maximilian Ringelmann in Paris in 1888, it is basically a card with squares of black, white and shades of grey that's held up as well as comparative grayness from the smoke judged. Electrostatic precipitation. The smoke is handed by https://broadfar.com/oraco-global-trends-in-heat-not-burn-products/