The Coulter Multisizer III employs changes in electrical impedance as particles are swept through a small cylindrical aperture (electrozone sensing) of known diameter and length to determine the particle size distribution. Two electrodes are submerged in a conductive solution and separated by a glass aperture tube. The cylindrical aperture is in a ruby window fused onto the aperture tube. The changes in impedance to current through the aperture as a particle enters the aperture are proportional to the particle's volume. Thus, the diameter of the particle can be calculated. The measurable particle size range is from about 0.4 µm to 1200 µm depending on the aperture tube selected. The particle size range for each aperture diameter is roughly from 2 to 60 percent of the aperture diameter. Apertures range from 15 µm to 2000 µm in diameter. The assumption of cylindrical particle geometry is used in developing the impedance to size relationship in the cylindrical zone of the aperture. The model cylindrical particle is then mathematically transformed to an equivalent spherical particle in interpreting the pulse heights due to the impedance changes in terms of particle diameter. This technique was originally developed to do blood cell counts.

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