1. Methods of Particle Size Measurement
There are many technologies available to determine particle size distribution of
materials. Of all the technologies available, laser diffraction has become one of the most
widely used and preferred methods. This article will look at some of the more widely
accepted techniques used in different industries today.
What is Particle Size?
Particles are three-dimensional objects. In order to provide a complete
description of a particle, three parameters are required — length, breadth and height.
Thus, it is impossible to describe a particle using a single number that equates to
particle size. Therefore, most sizing techniques assume that the material being
measured is spherical because a sphere is the only shape that can be described by a
single number, its diameter, thus simplifying the way particle size distributions are
represented.
Widely Accepted Particle and Surface Measurement Techniques
Adopting different measurement techniques can produce different results
when measuring non-spherical particles. That said, any instrument or technique used
for particle size analysis needs to generate data in a form that is relevant to the process.
The method also needs to be reliable, simple to use and able to generate reproducible
data.
1. Laser Diffraction
Laser diffraction is the one of the most widely used particle sizing
techniques and has become the standard method in many industries for
characterization and control. This type of particle size analyzer relies on the fact
that particles passing through a laser beam will scatter light at an angle that is
directly related to their size. When particle size decreases, the observed scattering
angle increases logarithmically. Scattering intensity is also subject to particle size,
diminishing with particle volume. What this means is that large particles scatter
light at narrow angles with high intensity while small particles scatter at wider
angles with low intensity.

2. Laser diffraction has a wide dynamic range, from 0.2 to 2000 microns and is very
fast and reliable. It is also very flexible as it can be applied to dry powders,
aerosols and emulsions. In addition, laser diffraction does not require calibration
but can be easily verified.
2-Sedimentation
This is a traditional method widely used in the paint and ceramics
industries. Equipment as simple as the Andreason pipette or as complex as
centrifuges and X-rays can be used in this method. The main advantage of this
technique is that it determines particle size as a function of settling viscosity.
However, as the density of the material is needed, this method is no good for
emulsions where the material does not settle or for dense material that settles too
quickly. It is also based on spherical particles, so can give large errors for particles
large aspect ratio.
3-Sieve analysis
This continues to be used for many measurements because of its simplicity, cheapness,
and ease of interpretation. Methods may be simple shaking of the sample in sieves until
the amount retained becomes more or less constant. Alternatively, the sample may be
washed through with a non-reacting liquid (usually water) or blown through with an air
current. Advantages: this technique is well-adapted for bulk materials. A large amount of

3. materials can be readily loaded into 8-inch-diameter (200 mm) sieve trays. Two
common uses in the power industry are wet-sieving of milled limestone and dry-sieving
of milled coal. Disadvantages: many PSDs are concerned with particles too small for
separation by sieving to be practical. A very fine sieve, such as 37 μm sieve, is
exceedingly fragile, and it is very difficult to get material to pass through it. Another
disadvantage is that the amount of energy used to sieve the sample is arbitrarily
determined. Over-energetic sieving causes attrition of the particles and thus changes
the PSD, while insufficient energy fails to break down loose agglomerates. Although
manual sieving procedures can be ineffective, automated sieving technologies using
image fragmentation software are available. These technologies can sieve material by
capturing and analyzing a photo of material.
4-Electrical sensing zone method – Coulter Counter
Instrument measures particle volume which can be expressed as dv : the
diameter of a sphere that has the same volume as the particle. The number and size of
particles suspended in an electrolyte is determined by causing them to pass through an
orifice an either side of which is immersed an electrode. The changes in electric
impedance (resistance) as particles pass through the orifice generate voltage pulses
whose amplitude is proportional to the volumes of the particles.

4. 5-Microscopy
Optical microscopy (1-150µm), Electron microscopy (0.001µ-)
Being able to examine each particle individually has led to microscopy being considered
as an absolute measurement of particle size.
•
Can distinguish aggregates from single particles
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When coupled to image analysis computers each field can be examined, and a
distribution obtained.
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Number distribution
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Most severe limitation of optical microscopy is the depth of focus being about
10µm at x100 and only 0.5µm at x1000.
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With small particles, diffraction effects increase causing blurring at the edges determination of particles < 3µm is less and less certain.
Manual Optical Microscopy
Advantages
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Relatively inexpensive

5. •
Each particle individually examined - detect aggregates, 2D shape, colour,
melting point etc.
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Permanent record - photograph
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Small sample sizes required
Disadvantages
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Time consuming - high operator fatigue - few particles examined
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Very low throughput
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No information on 3D shape
•
Certain amount of subjectivity associated with sizing - operator bias.
Conclusion
Fine particles play essential roles in determining the characteristics of both
natural and manmade materials and have considerable influence on processes such as
dissolution, adsorption and reaction rate. In the majority of cases, these effects are a
function of the size, shape, surface area or porosity of the individual particles or of an
agglomeration of particles. These particle-related characteristics must be controlled in
order to optimize the desired effects, and efficient control requires measurement. These
same particle characteristics are either the causes of, the results of, or a determining
factor in natural phenomena. In this category, understanding or exploitation rather than
control is more likely the objective and, again, measurements provide fundamental
information used in achieving the objective. As this article has illustrated, there likely are
multiple techniques for determining the same particle dimension and each has its
advantages and disadvantages. Selecting a technique that is inappropriate for the
application can have a profound impact on the quality of the measurement you obtain.