Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 126

Transmission of light through a flue gas that contains particulate matter will be reduced through a combination of scattering and absorption processes. Scattering and absorption of light by a stack plume gives rise to its opacity, its opaqueness to light transmission.

If light is not able to penetrate through a plume, the plume is said to be opaque – the opacity of the plume is 100%. Transmittance and opacity can be related as follows:

(4‐7)

Therefore, if a plume or object is 100% opaque, the light transmittance through it is zero. If it is not opaque, but has zero percent opacity, the light transmittance would correspondingly be 100%. A stack gas or plume will rarely have either zero or 100% opacity, but will have some intermediate value. Opacity standards are typically set at 20%, but can range from 0 to 40% opacity, depending upon the policies of the regulatory agency.