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Four dust measurement principles explained


Animation series explains the four measuring methods transmission, scattered light (forward/backward) and extractive 

Hamburg, 24th of Januar, 2023 – Dust is one of the air pollutants for which measurement is mandatory in many regions of the world. Operators of industrial plants are therefore required by law to report the daily average value emitted to the authorities. The limit values vary depending on the type of fuel and plant and are expressed in milligrams per cubic meter (mg/m3). In our animation videos, we present four variants used by products of the DURAG GROUP.

Basically optical measuring devices (using the transmission and scattered light principle) are suitable for dry flue gases up to 95 percent relative humidity. The detection of concentrations at a relative humidity of more than 95 percent requires an extractive measurement.

Transmission principle

In some countries, measurement of opacity is required, for example in power plants and refineries. This cannot be determined by the scattered light principle, but only by the transmission principle. In the transmission principle, a light source emits white light into the flue gas duct, which is reflected back by a reflector on the opposite side. Thus, it encounters dust particles that scatter and partially absorb the light, reducing its intensity. This value is measured by the detector and provides information about the dust concentration. 

  • Our products: D-R 220 & D-R 290 
  • Application: Continuous, contactless dust and opacity measurement at medium to high concentrations in dry flue and process gases.

Scattered light principle: forward scattering

Particularly low dust concentrations in flue gas, such as that produced in waste incineration plants, can best be determined using the scattered light principle. Here In the scattered light principle, a laser beam is sent into the stack where it encounters dust particles that scatter the light in different directions. In forward scattering, the detector measures the light intensity that is scattered by the dust particles in the direction of the laser. 


  • Our product: D-R 808
  • Application: Continuous measurement of low to medium dust concentration in dry flue and process gases. Single-sided installation.

Scattered light principle: backward scattering

As with forward scattering, the laser beam encounters dust particles in the stack. In backward scattering, a detector is used to measure the intensity of the light that is scattered backwards by the dust particles.  

  • Our product: D-R 320
  • Application: For continuous, contactless measurement of low to medium dust concentrations in dry flue and process gases. Also suitable for corrosive gases and potentially explosive atmospheres. Usable for example in refineries.

Extractive measurement

Extractive measurement is the solution for process gases with a relative humidity content above 95 percent. It is also suitable for all flue gases with temperatures below the dew point of water and acids. Dust concentrations in flue gases containing liquid from cleaning by means of wet scrubbers must also be measured this way. For this purpose, a probe is used to continuously extract flue gas from the duct, diluted and heated. This produces a dry flue gas whose dust concentration is measured in the measuring device outside the duct using the scattered light principle. 

  • Our products: D-R 909
  • Application: Flue or process gases with a moisture content of more than 95 percent; for monitoring small to medium dust concentrations. Usable for example in petrochemical industry, power plant and steel industry as well as waste incineration.