Detection of the scattered light intensity permits the measurement of low dust concentrations in air. Its practical uses include flue gas measurement, determination of visibility in road tunnels, and the monitoring of filter installations for pollutants (insecticides, dyes, chemicals) coming from manufacturing processes. Aerosols of liquids such as sulfuric acid mist in exhaust gases and oil mist in ship engine rooms are also measured this way.
The Sigrist dust instrument, which measures at a scatter angle of 15°, covers a nominal range from 0..0.1 mg/m3 to 0..1000 mg/m3 PLA. PLA stands for polystyrene-latex aerosol, which is used for the factory calibration. It consists of spheroidal particles with a diameter of 1µm.
The instruments are calibrated at site with the actual exhaust air. In this connection, it is worth noting that black particles also scatter light. Elementary carbon in its various forms (such as carbon black, graphite, or activated charcoal) produce scattered light intensities of the same order of magnitude as white diatomaceous earth (particle color).
Sampling is done extractively, which means the measurement takes place outside of the process and thus protected from heat, corrosion, and contamination. The flow cell is equipped with a purge air system that prevents dust from building up on the optical parts - as the above Figure illustrates. Together with the sample, the blower draws in ambient air that, after being filtered and heated, surrounds the sample stream with a protective shroud. It is also introduced at points S2 to promote flow towards the flow cell outlet.