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Alex: I haven't used one for years, so I don't know if they are still in
use. However, you might try the California Air Resources Board at
http://www.arb.ca.gov/homepage.htm. I notice that Ringelmann is still shown
in their glossary. At this site they have a (contact us).
Hope this helps.
Western Regional Pollution Prevention
P.O. Box 806
Sacramento, CA 95812-0806