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Smoke and smoke opacity meters are instruments measuring the optical properties of diesel exhaust. These instruments have been designed to quantify the visible black smoke emission utilizing such physical phenomena as the extinction of a light beam by scattering and absorption. In general, smoke and opacity meters are much simpler (some of them very simple) and less costly in comparison to most of the instruments described in the preceding sections. They are used to evaluate smoke emissions in locations outside the laboratory, such as in maintenance shops or in the field. In fact, the smoke opacity measurement is the only relatively low-cost and widely available method to measure a PM-related emission parameter in the field. For this reason, opacity limits are used in most inspection and maintenance programs for diesel engines. Smoke opacity limits may be also included as auxiliary limits in new engine emission standards.
In view of the demands of advanced, low emission diesel engines, the following areas of concern can be identified in the smoke opacity meter technology:
There have been various attempts to improve the sensitivity of diesel opacity meters, for instance by using multiple light path systems with mirrors . The NO2 cross-sensitivity could be eliminated by switching to a different light wavelength, e.g., to red light. Red, however, is less sensitive to small particles than green. A switch from green to ultraviolet, in turn, could improve the “visibility” of small particles . In view of these conflicting solutions, a simple switch to a different wavelength would not be sufficient. Rather, future opacitymeters will likely be multi wavelength devices, utilizing a range of sensors to separate the various effects.
Smoke opacity readings generally do not correlate well with other PM measurement parameters. Numerous correlations between opacity or smoke readings and PM mass that have been developed can provide only approximate results. Since opacity readings may be affected by sulfates, HCs, water vapor, as well as by PM composition (SOF) or physical conditions (e.g., coagulation), no accurate correlation is possible.