Abstract
An aerodynamic particle sizer (APS) can be used to make real-time measurements of the aerodynamic particle size distribution over the range of 0.5 to 32 µm. This instrument is very useful in conducting health-related aerosol measurements involving aerosol generation, respirator efficiency, and particulate sampling efficiency. One of the two signal processors within the APS can create spurious or phantom particle counts that can significantly affect relative measurements and calculated mass distributions. In the APS, particle size measurement is based upon a particle's transit time between two laser beams that are perpendicular to an accelerating airflow. The signal processors measure each particle's transit from the time between the two pulses of scattered light that are generated as the particle passes through the two laser beams. When only a single pulse from a particle is detected, another pulse can cause the recording of a randomly sized phantom particle. The small particle processor(SPP), which measures particle transit from the times in digital increments of 4 nanoseconds, can create phantom particles; the large particle processor (LPP), which measures particle transit times in digital increments of 66.67 nanoseconds, is designed to prevent the creation of phantom particles. These two processors overlap in the range of 5.2 to 15.4 µm. The difference in particle counts in this overlap region can be used to estimate an upper limit to the number of phantom particles in each channel of data from the SPP. The user needs to exercise some caution in using this correction because the LPP of the APS responds to coincidence by underestimating the particulate concentration. Furthermore, this correction does not cleanse the data of the statistical noise caused by phantom particle creation.