Particle size, measurement sieving

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. 

Various grades of sorbitol are available that differ in their pcuticle size distribution. Most of the particle size measurements reported for sorbitol are by sieve methods [1,7,26,27], and permit direct comparison with the vendor s specifications. For the samples used in this study, the vendor specified sieve limits for the various grades are listed in Table 1. 

Schweyer (1942) compared various methods of particle-size measurement (except centrifuging). He found excellent agreement between pipette and hydrometer methods. He considers the former the best method for determining the particle-m/.c distribution of sub-sieve material by sedimentation, and prefers the hydrometer as a rapid control procedure. 

Various techniques are utilised to measure particle size range. Sieving is the most common technique, particularly for suspension resins, and can be used to measure particles above 30 microns. For particle sizes below 100 microns, techniques such as sedimentation, optical and electrical sensing can be used. 

In many industries, particle size measurements have been carried out historically by sieve analysis and light scattering instruments are increasingly replacing this. In order to correlate with historic data banks some manufacturers have software to manipulate the data so as to present the size distribution in terms of sieve diameter. 

Stork Screens (1982), Neth. Appl. NL 82 04 381, 218 Anon., (1991), Particle Post, November, publ. ATM Test Sieves Inc., 218 Whitby, K.T. (1958), Symp. Particle Size Measurement, ASTM Sp. Publ. No 234, 3-25, 218... 

The measurement of particle size is a key issue in the formulation of many pharmaceutical products. Particle size distribution is known to directly influence physical properties of powders, such as dissolution rate, powder flow, bulk density, and compressibility. Conventional methods of particle size measurement include sieve analysis and laser diffractometry. ... 

The most difficult part of particle size estimation is related to the determination methods themselves. Particle size determination is complicated by size distribution, the presence of particle associations, and the shape of particles. If particles are not spherical, more than one parameter is needed to describe them and if the shape of the particle is irregular, numerous parameters are needed to express their dimensions. The method used for particle size determination (sieving, light scattering, microscopy, etc.) determines what dimensional aspects are measured. In addi-... 

Physical characterization of TBO includes particle size and size distribution measurement by laser diflraction (macroporosity) as well as specific surface area measurement (microporosity). Particle size measurement by FSSS (Fisher Sub-Sieve Sizer), as also sometimes used, is misleading, because of the porosity of the TBO particles. An empirical relationship between FSSS and particle size measured by laser scattering can, however, be detected if the microporosity of the samples is uniform (constant blueing conditions). 

Washington (1992) has discussed the concepts and techniques of particle size analysis and its role in pharmaceutical sciences and other industries. There are many different methods available for particle size analysis. The techniques most readily available include sieving, optical microscopy in conjunction with image analysis, electron microscopy, the Coulter Counter and laser diffractometers. Size characterization is simple for spherical particles, but not for irregular particles where the assigned size will depend on the method of characterization used. Table 6.2 lists particle size measurement methods commonly used and the corresponding approximate useful size range (Mullin 1993). 

Spectroscopy provides the laboratory analyst, and the production engineer, the ability to perform particle size measurement over a very broad range of sizes in a timely manner. Spectroscopy in the form of diffraction and dynamic light-scattering technology have been demonstrated to provide information that can be applied to research and process studies as well as to the routine measurement demands of the plant. Examples of particle size measurement capability for the plastics industry from 0.003 to 3000 pm were presented, as well as a short discussion of means to convert the data from the newer technology to the classical sieve measurements. A final comment on the advancing science of spectroscopic particle size measurements was also presented. 

There are several particle size measurement techniques in use, such as optical imaging, electron imaging, optical diffraction and scattering, electrical resistance changes, sieving, sedimentation, and ultrasonic attenuation [4j. 

Nathier-Dufor et al. Comparison of Sieving and Laser Diffraction for the Particle Size Measurements of Raw Materials used in Food Stuffs, Powder Technology, 76 (1993) 191-200. 

Probably the simplest technique for particle size measurement is sieving. Conventional woven-wire screens can be used to analyze powders, most of whose particles fall into the range of 50 jum to 1 cm. The range can be extended downward to 10 fim by using electroformed screens, which are more accurate than woven wire screens up to about 200 m. It is advisable to... 

This last point is important in all particle size measuring techniques and it may be necessary to use a liquid sieving technique, together with dispersing agents, in difficult cases. 

The mass transport influence is easy to diagnose experimentally. One measures the rate at various values of the Thiele modulus the modulus is easily changed by variation of R, the particle size. Cmshing and sieving the particles provide catalyst samples for the experiments. If the rate is independent of the particle size, the effectiveness factor is unity for all of them. If the rate is inversely proportional to particle size, the effectiveness factor is less than unity and

experimental points allow triangulation on the curve of Figure 10 and estimation of Tj and ( ). It is also possible to estimate the effective diffusion coefficient and thereby to estimate Tj and ( ) from a single measurement of the rate (48). 

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