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Electrical Zone Sensing (Coulter Principle)

The electrical zone sensing (EZS) technique is based on the Coulter principle. In this method, the powder is dispersed at a very low concentration in an electrolytic (i.e., conducting) solution, which is then drawn through a small orifice in an insulating wall on either side of which electrodes are placed. As each 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 this 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.

In order to relate the registered pulse amplitude to a specific particle size or volume, the instrument must be calibrated using particles having a narrow size distribution (i.e., monodisperse). Alternatively, the instrument can be calibrated using the test powder itself, if the entire size range of the powder is covered in the measurement. If particles are present that are too small to register a pulse, then the fraction of undetected particles can be determined by comparing calibration results from both methods.

Different size apertures can be used depending on the size range of interest; multiple orifices can be employed for powders having a very broad distribution of sizes, like cement. In the latter case, the orifice detection ranges should overlap. A range of orifice sizes are normally available, from 10 µm up to 2000 µm. The applicable range of particle sizes that can be measured using any given orifice is from about 2 % to 40 % of the orifice diameter [11]. Therefore, the maximum applicable size range is from 0.2 µm to 800 µm, although a lower limit of 0.6 µm is probably more realistic for normal operating conditions [10]. The lower detection limit arises due to electrical and thermal interference. For particles with a density similar to the liquid, there is no theoretical upper limit. For denser particles, the ability to keep the larger particles suspended is a limiting factor. The use of higher viscosity liquids can help in this case.

Coincident passage of two or more particles through the sensing zone is a potential source of error in EZS measurements. Coincidence can cause the instrument to count the combined pulse height of multiple small particles as a single large particle, thereby skewing the size distribution towards the high end. A pulse discrimination system can be adjusted to correct for this effect by rejecting distorted pulses. Another potential problem is particle asymmetry. Asymmetric (flaky) particles, commonly found in cement, rotate as they pass through the orifice. Since it is the volume swept out by the particle that is measured, this can also lead to oversizing. Porous particles (e.g., fly ash) are generally unsuitable for EZS measurements, because their effective densities are not known.

Next: Scanning Electron Microscopy Up: Description of Methods Previous: LASER Diffraction