Measurement unit for the degree of blackening as defined by DIN 51402 Part 1.
The soot content of flue gas is determined by means of an individual measurement, namely quantitative retention on a filter of silica fiber material followed by measurement of the reflectance. The smoke spot number is then obtained by conversion according to VDI 2066, Sheet 8.
The German Clean Air Law (TA Luft) prescribes that combustion systems in the 5 MW to 25 MW range fired by light fuel oil must be equipped with a continuous measurement system capable of maintaining smoke spot number 1 with sufficient reliability. This is usually done with scattered light measurement either directly in the stack or using extractive sampling. The latter method protects the photometer against heat exposure and corrosion by the flue gas.