Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2024-02-09.
Two optical techniques measure particulate concentration: attenuation and scattering. Transmission instruments place a light source on one side of the duct and a detector on the other; the more dust in the flue gas, the more light is absorbed or scattered on the way, and the less reaches the detector, whose signal is converted and calibrated into a dust concentration. A variant puts a reflector opposite the source and returns the light along the same path, so that detector and source can share one housing, keeping the sensitive optics and electronics together; the arrangement also doubles the optical path, which improves accuracy at low dust concentrations. The same transmission principle is used in some ambient applications, mainly for visibility, for example at airports. Scattering instruments look at the light scattered sideways: a higher detector signal means more dust, and in dense industrial ducts white light and the glow reflected from the particle cloud may be enough. Particle counters go further, drawing a controlled flow through a laser focus and detecting the flash of Mie scattering as each particle passes, with brightness indicating size, so that a size distribution and then a concentration can be derived.
Chinese particulate CEMS commonly use scattering or transmission (opacity) principles, and the same reference framework applies to both: GB/T 16157 and HJ 836 for manual gravimetry, HJ 75 and HJ 76 for the continuous system, HJ 212 for data upload. Under ultra-low emission limits the difficulty shifts from measuring high values to staying stable at low ones, because fouling on a window or mirror changes the zero and the span response directly; purge air quality, window cleaning and periodic calibration and comparison therefore become the maintenance core. High moisture close to saturation adds a positive bias from droplets and condensation — extractive systems need heat tracing, in-situ systems need attention to condensation on the optical path. On the ambient side the reference method is filter gravimetry, with beta attenuation and TEOM as continuous analysers and light-scattering sensors used mainly for dense networks and screening, which is a different duty from an industrial duct in range, calibration and interference control. In every case particle size and density are not constants: an optical instrument reports an equivalent concentration tied to its calibration conditions, so recalibration after a process change matters more than a marginal datasheet advantage. Two practical details are easily missed: match the optical path to the range, since a doubled path improves low-range resolution but demands alignment and clean mirrors on both sides, and judge data validity together with purge pressure, window transmission and calibration deviation rather than from the concentration curve alone.
Inside the Particulate Monitor: Optical Methods — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2024-02-09. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
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