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Technology for Continuous Emissions Monitoring - Light Scatter

Continuous Emissions Monitoring with Light Scatter Technology – the Pros and Cons

Continuous emissions monitoring systems employing light scatter technology use the rate and duration of light flashes scattered by particulates present in the gas stream to measure particulate densities. Their overwhelming advantage over other technologies is that:

  •  the error caused by humidity in the gas stream is reduced to an inconsequential level (provided the temperature is above the dew point), and
  • they are also unaffected by changes in the velocity of the gas stream

Unless particulate levels are very low, there is no humidity, and the ambient light is high, particulate monitoring systems using light scatter technology will give more accurate results than those using opacity technology. In addition, the inaccuracies introduced by optical fouling are significantly diminished as compared to opacity instruments. The most serious source of measurement error in continuous emissions monitoring systems using this technology is changes in particulate shape, a problem common to all technologies.

Light scatter instruments come in two forms — intrusive and non-intrusive. The former interfere with the gas flow, and are more affected by the temperatures of the gas stream and fouling. The latter therefore require less maintenance and are more reliable.

Scattered light can be measured using back scatter or forward scatter. The former is simpler to install, but the latter can give better results if particulate levels are very low.

Historically, light scatter instruments have suffered from a small sampling ratio, but the patented design technology incorporated in Etr-Unidata´s Zeta 5000 continuous emissions monitoring system resolves this issue.