For many people, the thought of measuring and sizing particles (that are often too small to be visible) can seem completely baffling. Just the thought of how this could possibly be done can seem daunting.

These days there are several types of particle size analyser instruments available that operate using different measurement techniques. laser diffraction is one such technique that can give us great insight into the properties of different materials.

Of course, there is no single measurement technique that can be used to measure all materials and all particle sizes. As such, some industries use a range of different techniques to measure different materials.  The problem with this is that different techniques measure different dimensions of particles, so when they often yield different results.

In order to understand more about the different results that are obtained through particle measurement and analysis, it is useful to be aware of some commonly used methods. These include:

Sieves:

This is one of the oldest and most traditional of all the methods of measuring particle size and is so often used because it is cheap and reasonably effective when measuring larger particles. The measurement of large particles occurs in a number of industries, including the mining industry.

However, the use of a sieve makes it impossible to measure particles within sprays or emulsions.  Even dry powders become very difficult to measure when particle are very fine. A further disadvantage is that the ability to reproduce results is difficult, particularly when the technique of wet sieving is applied.

In order for particle size results from sieves to be useful and informative, the operating methods and measurement times need to be standardised.

Sedimentation:

This is another technique which has been around for a long time and has historically been used in certain industries.  Soil scientist have long used sedimentation to measure course grained soils (ie.sand) and it has also been widely used in the ceramics industry. While some industries have a long history with sedimentation, it has also provided these industries with some problems.

For a start, the calculation of size from sedimentation rate is only valid for spherical particles.  If the particles are not spherical then the results reported can be very different from reality.  It is also necessary to know the density of a material.  If density is not known, or if a sample contains a mixture of materials, it is difficult to obtain accurate results. It is impossible to determine the size of emulsions particles, where the material does not settle. Similarly, it is difficult to get accuratete measure of large and very dense materials where the material quickly settles.

Electrozone Sensing:

This technique is particularly useful for measuring the size of blood cells but it poses some problems as a technique for measuring industrial materials. Samples must be suspended in a salt solution so it is not possible to measure emulsions and many dry powders.  Measuring the particle size within sprays simply cannot be done.

Laser Diffraction:

A popular and preferred measurement technique, laser diffraction also has the advantage of being one of the most accurate ways of measuring particle size. Other advantages of laser diffraction include:

• It provides a great degree of flexibility for different materials
• Results and answers can be provided quickly (in less than one minute)
• It is an absolute method of particle analysis
• There is no need to calibrate instruments against a standard
• It is possible to measure dry powders, suspensions, emulsions, sprays and many other materials
• The technique provides a wide and dynamic range of measurement
• The measurement of an entire sample is possible
• The technique can be repeated easily.

While there are a range of ways to measure and analyse particle size, the most appropriate way of measuring any given material will and should depend on the type of material that is being measured.

As a technique of particle size analysis, laser diffraction is widely used and has many advantages. It offers a high level of precision, a fast speed of response, high potential for the repetition of results and a wide measurable particle diameter range. Particle size analysers offer a sophisticated way of measuring particle size and enable incredible insight and understanding of the particles that make up materials used in a range of industries.

Laser diffraction is an important technique for measuring particle size. Here we offer a range of facts about this technique:

• It is also known as Low Angle Laser Light Scattering (LALLS).
• Considered a standard method in many industries because of its ability to characterise particles and for reasons of quality control.
• Huge advances have been made in instrumentation over at least the last two decades.
• The method is founded on the fact that the angle of diffraction is inversely proportional to particle size.
• The instrument consists of a laser (as a source of coherent intense light that has a fixed wavelength), a detector (typically an array of photo sensitive silicon diodes) and a means of passing the sample through a laser beam.

Laser diffraction is so popularly and extensively used because it offers a number of advantages. These advantages include:

An absolute method grounded in fundamental scientific principles – In this method it is not necessary for an instrument to be calibrated against a standard. However, validation of equipment is possible to prove that it is performing to a standard that can be traced.

Wide and dynamic range – In measuring particle size, good equipment will allow the user to measure particles sized between approximately 0.1 and 2000 microns.

Flexibility – Laser diffraction offers new possibilities for measuring materials. It is even possible to measure the paint that is sprayed from a nozzle in a paint booth. The pharmaceutical and agricultural industries are two of the many industries that have benefited greatly from such advances.

Dry powders – Even dry powders can be measured through the technique of laser diffraction. Although this may result in a poorer level of dispersion than if a liquid dispersing medium was used, it is an advance that dry powders can be directly measured. In combination with a suspension analysis, it can support the assessment of the amount of agglomerated material in a dry state.

Liquid suspensions and emulsions – It is possible to use a recirculating cell to measure liquid suspensions and emulsions. This technique promotes a high level of reproducibility and facilitates the use of dispersing agents and surfactants to determine the primary particle size. If it is possible to do so, it is preferable to take measurements in a liquid suspension.

Sample measured – This technique allows for the whole of the sample to be measured. As the sample passes through the laser beam, diffraction is measured for all particles.

Rapid – This technique is so rapid that results can be derived in one minute or less. Feedback can therefore quickly be provided to plants and repeat analyses can also be made quickly.

Repeatable – This technique is highly repeatable and knowing that the results can be relied upon provides peace of mind.

A sophisticated method for determining particle size, laser diffraction is widely used and offers distinct advantages that are beneficial for many industries.

In many industries, the ability to determine the size of particles is not only useful but can be very important. A Laser diffraction particle size analyser is commonly used to provide this sort of information but the processes used to obtain particle size can be quite complex. Sampling, dispersion processes and the shape of materials all contribute to the complexity of particle size analysis.

Here we provide key information for understanding particle size analysis.

What does it mean to describe a particle?

When dealing with a three dimensional shape, one unique number cannot accurately be given to describe its size. While this remains true in so many situations and circumstances, it is particularly relevant when seeking to describe complex shapes such as a grain of sand or even a particle within a can of paint.

While many of us may question why this information is so important, it is imperative and influential for people such as Quality Assurance Managers within particular industries and organisations. For example, a Quality Assurance Manager may need to know whether the average size of particles has increased or decreased since the last production run.

The equivalent sphere

So often we seek to describe a shape by only one number. As has been mentioned, this is problematic as there is only one shape – a sphere – that can be accurately described by one number.

In order to arrive at a particular number to explain the size of a shape, equivalent sphere theory is frequently used. Using the equivalent sphere theory, some property of the particle is measured and it is then assumed that this refers to the diameter of a sphere to describe the particle. Essentially, this means that three or more numbers do not have to be used to describe a three dimensional particle. Although it is more accurate to describe three dimensional particles with three or more numbers, it is also inconvenient and can become unmanageable.

Using different techniques

When a particle is examined under a microscope, a two dimensional image is seen and the information that is deduced relates to the two dimensional shape. Consequently, there are a number of different diameters that can be reported. If the maximum length of the particle is used, then the result relates to a sphere with diameter of the maximum dimension. Conversely, if the minimum length is used for the calculation, then the result pertains to a sphere with diameter of the minimum dimension.

It is therefore important to note that each technique used to measure a particle will provide a unique result because it measures a different property of a particle.

In light of this, there is no absolute right or wrong answer. All answers are correct for the techniques that are used and the dimension of the particle that they are measuring.

What does this all mean?

For those to whom particle size analysis is important, different techniques for measuring particle size mean that there can be no standard size for particles such as grains of sand. If meaningful comparisons are to be made between different techniques, it must be done with standards containing spherical particles. Further to this, characterisation of particle size standards is only possible when the same technique has been used and this allows comparison between instruments that use the same technique.