Methods of Particle Size Measurement

The terminal velocity in the case of fine particles is approached so quickly that in practical engineering calculations the settling is taken as a constant velocity motion and the acceleration period is neglected. Equation 7 can also be appHed to nonspherical particles if the particle size x is the equivalent Stokes diameter as deterrnined by sedimentation or elutriation methods of particle-size measurement. 

Table 1.2. Some Methods of Particle Size Measurement...

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. 

British Standard 1377, The Hydrometer Method of Particle Size Measurement, 384... 

Stream scanning methods of particle size measurement... 

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. ... 

It is well known that particle shape affects many secondary properties relevant to powder handling such as the bulk density, failure properties or particle-gas interaction. For non-spherical particles, the results obtained with different methods of particle size measurement are, in general, not comparable. From the point of view of powder handling, flaky or stringy particles like wood shavings, mica or asbestos fibres are known to be difficult because they interlock and form obstructions to flow. 

Particle size is generally measured by sedigraph, which calculates a particle s size based on relative settling rate. Though this method assumes a spherical geometry (and very few papermaking minerals are spherical in nature), it is the most commonly used method of particle size measurement within the pigment industry. 

Table 2.10. Some methods of particle size measurement and their aproximate useful size ranges...

Sedimentation techniques are amongst the oldest methods of particle size measurement. Sizes are derived from the rate of settling of particles under the action of gravitational or centrifugal forces. One sedimentation technique is the disk centrifuge, which is used to measure particles in the range 0.1-50 pm. 

Different methods of particle size measurement determine different measures of size (see the list in Table 2.7) and great care must be taken when... 

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). 

The early efforts to achieve on-line measurement were largely devoted to trying to adapt laboratory methods of particle size measurement to an on-line use, usually confined to on-line monitoring. 

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