AS 3543:2014 Use of standard Ringelmann and Australian Standard miniature smoke charts
A Ringelmann (S) number can be considered as ‘standard sky’ or ‘standard for instrumental measurement’. The Ringelmann (S) number complies as far as possible with the visually observed Ringelmann number. With a typical sky brightness behind dark smoke that corresponds to good conditions for the visual observation of Ringelmann number, a visually observed Ringelmann number 2 corresponds to 64% obscuration (optical density 0.44). Taking this sky brightness as standard, Ringelmann (S) number 1 is 52% obscuration (optical density 0.32), Ringelmann (S) number 3 is 76% obscuration (optical density 0.62) and Ringelmann (S) number 4 is 88% obscuration (optical density 0.92).
Since the instrument measurement is not made at the top of the stack, it is necessary to allow for any differences between the stack exit and the measuring point. The major factor is the relationship between the stack exit diameter and the path length in the flue gas for the opacity monitor light beam. If these are equal and there is no temperature change between the measuring point and the stack exit, the instrument reading can be converted directly into Ringelmann (S) number utilizing the numbers provided in the paragraph above.
4.4 Correction for velocity
Where there is a change in velocity that is not caused by a temperature difference between the measuring point and the stack exit, no correction is required.
4.5 Multiple sources discharging through one stack
Where a single stack is used to exhaust emissions from a number of flue gas sources, with an instrument for each source, the procedure is unchanged. If the sources read widely different Ringelmann (S) numbers, the Ringelmann (S) number for the stack can only be assessed by using optical densities and weighting the average with respect to the flue gas rates at the measuring points. If the flue gas quantities and the measured Ringelmann numbers are approximately equal, an arithmetical average will suffice. Each reading should be applied directly to the control of its own source.