The DSL-340 Dust Monitor is an optical instrument designed to measure the concentration of dust or particulate matter in exhaust gas passing through a duct, stack or flue. The DSL-340 uses the innovative Dynamic Detection Principle (DDP) which measures fluctuations in the intensity of a light beam, using a folded beam Transceiver/Reflector arrangement. Increased dust or particulate density in the stack causes the amplitude of these fluctuations to increase.