During the Coulter Principle measurement, as a particle passes through the sensing zone when the liquid is drawn from the container, a volume of the electrolyte equivalent to the immersed volume of the particle is displaced from the sensing zone. This causes a short-term change in the resistance across the aperture. This resistance change can be measured either as a voltage pulse or a current pulse. By measuring the number of pulses and their amplitudes, one can obtain information about the number of particles and the volume of each individual particle. The number of pulses detected during measurement is the number of particles measured, and the amplitude of the pulse is proportional to the volume of the particle. Because this is a single particle measurement process, it yields the highest resolution that any particle characterization technique can achieve. The particle diameter can be determined at the resolution of voltage or current measurement which can be very accurately using current electronics technology. The distribution amplitude can be determined to the accuracy of a single particle.

The advantages of such high resolution are multiple. The most obvious one is to display details of a particle size distribution. In a particle size distribution measurement, typically each distribution, whether is displayed cumulatively or differentially, is composed of a few hundred data points in a pre-set size range. Each data points is called a bin. Since every particle is measured, each bin is a collection of particles in the size range that bin represents. Depending on the distribution broadness, the total size range can be reset to a finer division, therefore showing the distribution details, i.e., each bin can be pre-set to cover a smaller size range. Other advantages include fine differential between two particles and more accurate statistic values calculated from the distribution. The following figures show a sample measured using Beckman Coulter’s MS 3 instrument and displayed in different size ranges. The pulse data was resorted into a finer set of bins in the right figure in which more detail of the distribution is displayed.