Electrical Zone Sensing is based on the Coulter principle. The powder is dispersed at a highly dilute concentration in a conducting liquid, which is then drawn through a small orifice in an insulating wall on either side of which are placed electrodes. As a particle enters the orifice, or sensing zone, the volume of solution displaced by the particle causes a transient change in the measured electrical impedance across the opening. The amplitude of the impedance pulse is proportional to the particle's volume. By accumulating pulses over time, a PSD is constructed. EZS is a particle counting method capable of producing a number-weighted or mass-weighted distribution of particle sizes and requires calibration. Different size orifices are used to capture broad PSDs. The applicable particle size range is from about 0.2−800 µm, although a lower limit of 0.6 µm is probably more realistic for normal operating conditions (Allen, 1990; ISO 13319:2000(E)). Errors can arise from coincident passage of multiple particles through the orifice and from high asperity particles, both of which skew the PSD towards larger sizes. A pulse discrimination system can be used to correct for the coincidence effect by rejecting distorted pulses. Porous particles (e.g., fly ash) are generally unsuitable for EZS measurements because their effective densities are not known. The dispersion procedure and solids concentration are critical parameters for accuracy in EZS measurements.