Filter testing can be performed for the finest particle diameters (0.01 to 0.5 micrometers) using a series of monodisperse aerosols generated by an atomizer and an electrostatic classifier (EC). The polydisperse aerosol generated by the atomizer passes into the EC; only those of a very narrow diameter range exit through a silt at the bottom of the EC. The size of the particles exiting can be changed by changing the voltage applied to the EC. Particles are then detected upstream and downstream of the filter with a condensation nucleus counter (CNC). A CNC first grows submicrometer aerosols to supermicrometer alcohol droplets and then measures the droplet concentration. The CNC is insensitive to particle size, shape, and refractive index, so virtually any challenge aerosol can be used. It operates over a concentration range of 0.01 to 107 particles/cm³, making it capable of measuring a wide range of filter efficiencies.

Units: SI