Coulter principle, particle sizing using

Various techniques and equipment are available for the measurement of particle size, shape, and volume. These include for microscopy, sieve analysis, sedimentation methods, photon correlation spectroscopy, and the Coulter counter or other electrical sensing devices. The specific surface area of original drug powders can also be assessed using gas adsorption or gas permeability techniques. It should be noted that most particle size measurements are not truly direct. Because the type of equipment used yields different equivalent spherical diameter, which are based on totally different principles, the particle size obtained from one method may or may not be compared with those obtained from other methods. 

The electrozone sensing technique, also called the Coulter principle, was originally developed for biomedical applications for counting blood cells. This method counts and sizes particle based on changes in the electrical resistance caused by nonconductive particles suspended in an electrolyte. It presently finds uses in a wide variety of industries, including the food, environmental, coatings, ceramics, and metals industries. 

By using orifices of various diameters, different particle size ranges may be examined and the resulting data may then be combined to provide size distributions extending over a large proportion of the sub-millimetre size range. The prior removal from the suspension of particles of sizes upwards of about 60 per cent of the orifice diameter helps to prevent problems associated with blocking of the orifice. The Coulter Counter and the Elzone Analyser work on this principle. 

This section contains a general description of the principles by which the Coulter Model N4 Sub-Micron Particle Analyzer, used in this study to characterize artificial gas-in-water emulsions (see Section 10.4), determines sample particle size. The measuring principles are based on the theory of Brownian motion and photon correlation spectroscopy (ref. 464,465 see also Sections 10.2 and 10.4). 

The most widely used stream scanning technique employs the Coulter principle (Figure 9.1a) where the interrogating field is electrical and particle size (volume) is proportional to the change in electrical impedance as the particles pass through the field. 

The Coulter principle is also standard for dry toners [8,9] and an accepted method for aluminum oxide powder [10], chromatography media [11], polymeric powders [12], plutonium [13], filter evaluation [14], catalytic material [15] and comparing particle size distribution using alternative types of particle counters [16]. In ASTM method C-21 it states that the experience of several laboratories indicates that the method is capable of a repeatability of 1% and a reproducibility of 3% at the 95% confidence level. Operating procedures for this technique are also covered in BS3405 [17]. The method is also the subject of an international standard [18]. 

One of the earliest electronic particle sizing devices available was developed in the late-1940 s and was originally designed to count blood cells (Coulter, 1956). The Coulter Counter rapidly gained immense popularity in many industry and research applications, including the Earth sciences, and is still a very commonly used instrument in sedimento-logical laboratories. The Coulter Principle is sufficiently well established to be included in many A.S.T.M. (American Society for Testing Materials) reference method standards. 

Coulter Counter A commonly used particle-sizing instrument employing the principle of electrical resistance. 

Other methods of particle size measurement are also widely used in the characterization of suspensions, e.g. particle counters or the Coulter principle in filter rating, microscopy for general particle investigations and screening for coarse solids (above 75 pm). 

Introduced in 1983, this Part gives recommendations for the electrical sensing zone method (the Coulter principle). It is recommended that the primary calibration technique is that of "mass integration", where a known volume of particles under test is used to calibrate the volumetric size response directly. This allows the method to be self-calibrating and to approach being absolute. 

The Coulter principle has been applied to floes but is severely constrained by the very severe shear stress that occurs within the vortex of fluid passing through a Coulter orifice. Indeed the Coulter Counter has been used as an on-line method for studying the disruption of floes. If a floe were entirely disrupted just before or in the orifice it is reasonable to assume that the additive volume of the fragments would be detected as one coincident group of particles. This would however assume that the fragments were of the same particle volume concentration as the parent, an unsafe assumption for floes. However breakup in an orifice is not normally complete and the observed size may also depend on the amount of distortion the floe experiences in the orifice. From the behaviour of other types of porous particle in die Coulter Counter it is often assumed that the instrument measures the external envelope volume if the floes are not disrupted. Attempts have been made to measure individual well characterised floes under realistic orifice conditions but the experiments are very difficult to do and the results remain ambiguous. ... 

Measured size distributions depend not only on the physical dimensions of the particles but also on the method of size analysis used. Size distributions by the Coulter Principle will only agree with sedimentation data if the particles are spherical. Indeed the difference is a measure of particle shape. Since classifiers separate particles on the basis of their Stokes sizes a sedimentation method of size analysis should be used to determine their grade efficiency. Sedimentation analyses are also applicable to many other industrial situations. 

Xu, R., and O. A. Di Guida (2002). Particle Size and Shape Analysis Using Light Scattering, Coulter Principle, and Image Analysis . In World Congress on Powder Technology. Sydney. 

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