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Characterization of particle sizeand charge distribution has many applications in the fields of powder coating, electrophotography, electrostatic separation of minerals, electrostdic spray processes, and pharmaceutical powder dispersion and delivery. Monitoring of the electrostatic charge and size distribution on a single particle basis can provide insight of the electrodynamics of the process, and can provide empirical tools for optimizing the process. We present here development and performance of newly-improved Electronic Single Particle Aerodynamic Relaxation Time (E-SPART) analyzer, that provides particle size and electrostatic charge distributions in real time and on a singleparticle basis with precision and accuracy. The instrument uses an AC electric drive to oscillate the particles and to measure the particle size and distribution, as opposed to using an acoustic field superimposed on a DC electric field. Application of an acoustic field in the earlier E-SPART version produced acoustically generated d r flow fields that interfered with the measurement of electrostatic charge distribution of particles. In addition to AC drive, the new instrument uses a digital signal processor to derive the phase-lag of the individual particle motion with respect to the AC drive at a frequency of 1 W , and to measure the amplitude ratio of the oscillation of the particle with respect to the amplitude of the AC drive. A C program runs on the DSP chip to detect the arrival of particles, sample the data, and store the sample data in DSP board memory; it also signals the PC to begin processing the data. Another C program runs on the PC, and reads the data, de-modulates the signal, and calculates the particle's aerodynamic diameter and electrostatic charge 0-7803-4943-1/98/$10.00 0 1998 IEEE (both magnitude and polarity) from the measured phase-lag and amplitude ratio of the particles. Several applications, including powder coating, electrophotography, coal beneficiation, pharmaceutical powders, are given as examples demonstrate the utility of real-time analysis of size and charge distribution of airborne particles while keeping them in a suspended state.

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