A Microorifice Uniform Deposit Impactor (MOUDI): Description, Calibration, and Use

An eight-stage cascade impactor, with cut-sizes ranging from 0.056 to 18 μm at a flow rate of 30 L/min, has been developed, calibrated, and tested in field programs. This impactor, called a microorifice uniform deposit impactor, has several features not normally found in cascade impactors. Ultrafine particles as small as 0.056 μm are collected while keeping the pressure drop moderate by utilizing multiple nozzles (up to 2000) as small as 52 μm in diameter. At each stage the collected particles are deposited uniformly over the entire impaction plate by rotating the impaction plate relative to the nozzles and by the correct radial placement of the nozzles. The particle cutoff characteristics (collection efficiency curves) of each stage and interstage losses were determined by means of monodisperse aerosols. The cutoff characteristics were found to be sharp and similar for all stages.