Importance

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Abstract

Smoke density measuring apparatus comprising a duct (1) through which the smoke passes, a light source (4) and a photodetector (6) located on opposite sides of the duct adjacent openings (2,3) in the side walls of the duct so that the source (4) directs a beam of light across the duct through the openings to the photodetector (6), a gas supply (10) associated with the openings (2,3) in the side walls of the duct so that gas flows through each opening into the duct, gas flow control valve (13) for varying the rate of flow of gas through said openings, sensor (17) for monitoring the rate of flow of smoke through the duct, and a control circuit (FIG. 4) for controlling the gas flow control valve (13) in a manner dependent on operation of the sensor (17) so that the rate of flow of gas through the openings is reduced when the rate of flow of smoke falls below a predetermined value. The control circuit (FIG. 4) operates to reduce the flow of gas from a first to a second predetermined flow rate when the rate of flow of smoke falls below the predetermined value. The smoke flow rate sensor (17) comprises a smoke temperature sensor. Smoke density assessing circuitry has inputs from the photodetector (6) and temperature sensor (17) and includes correction device (20) that operates to multiply the smoke density measurement by a correction factor that increases with smoke temperature. The correction factor takes account of changes of gas density with temperature, the occurrence of smoke particle agglomeration with decreasing exhaust gas temperature, and changes of effective optical path length.

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First Claim