On the Performance of the Berner Low Pressure Impactor

Particle collection characteristics of the compressible flow stages 1-6 of the Berner low-pressure impactor (BLPI, Hauke Aeras 25-4/0.015) have been studied. A new, real-time impactor stage efficiency measurement system has been developed. It is based on the use of two electrometers, one connected to the collection plate of the impactor, and the other to the backup filter. This method is used with singly charged, monodisperse dioctyl phthalate (DOP) particles to determine the shape of the collection efficiency curves of the BLPI. In addition, particle collection characteristics have been studied using fluorometric analysis. A new method for substrate film greasing is also introduced. Experimental cut diameters of stages 1-6, as determined with DOP particles using electrical measurement method, are 0.034, 0.066, 0.094, 0.17, 0.33, and 0.55 /un. They are clearly larger than corresponding values given by the manufacturer. The average of the square root of the Stokes number at 50% collection efficiency, based on experimental cut diameters and evaluated at stage conditions, is 0.465. For solid ammonium fluorescein particles the collection efficiency of high-velocity stages (1-6) was poor, although the substrates (Mylar film) were greased with Apiezon L vacuum grease. Using experimentally determined stage pressures and calculated cut diameters it was established that the compressible flow stages of different units of the same BLPI model may have significantly different cut diameters. The effect of temperature and pressure on cutoff diameters at atmospheric conditions near ground level is also discussed.