Optical Dust Measurement

Optical dust measurement is often made with an in-situ dust meter to be mounted directly into the flue gas duct. The dust concentration in the process is either measured by transmission (converted to opacity), absorption or reflection. The transmission, opacity and absorption meters are all cross stack meters, which are to be installed with a sender and receiver unit opposite each other on the stack. Light is sent out from the sender unit, and the transmission or absorption is measured to determine the dust concentration in the process.

The scattered light meters are delivered with the sender and receiver units gathered in one unit. Light is sent into the process by a well-defined angle in relation to the flow direction. The sender/receiver unit is also installed in a well-defined angle in relation to the flow direction. The light that the dust particles in the process reflect back onto the receiver unit is an expression of the dust concentration in the process.

Optical in-situ dust meters are not able to operate in processes where the flue gas temperature is very close to or below dew point. This is due to the fact that water drops/aerosols will reflect the light or influence the transmission/absorption, and this might cause erroneous measurements. A common feature for all types of optical dust meters is that they must be calibrated by an accredited test laboratory in order to relate the relative size of the reflection, transmission, absorption or opacity into mg/m³.

Optical dust measurement is a relative and not an absolute measurement because the dust particles do not have a well-defined density and because the reflecting/absorbing qualities will vary depending on the application.

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