Different Methods of Particle Size Measurement

Particle size analysis can seem a complex and intricate area. The thought of attempting to measure and draw conclusions about microscopically small particles can seem incredibly confusing and the techniques used to measure particles can perhaps seem daunting. Particle size analysers are of great use in achieving such measurements and techniques such as laser diffraction assist us to understand more about the properties of materials.

However, in order to understand some of the higher level concepts related to this area, it is necessary to have a sound grasp of the concepts that inform results.

Mean, median and mode

These three terms are too often confused and thought to mean the same thing. Far from being one and the same, these terms require definition:

Mean – Mean relates to the arithmetic average of the data.

Median – When concerned with particles, median is the value of the particle size that precisely divides the population into two halves. This means that there is 50% of the population above and 50% of the population below the value.

Mode – Mode defines the most common value in a frequency distribution. This also corresponds to the highest point of the frequency curve.

Common methods of particle size measurement

Because different dimensions of the particle are measured when different techniques are used, different results are obtained. Of the different methods of measurement, each has its own advantages and disadvantages.

Sieves

While this is an old technique, it has the advantage of being cheap and particularly useful for the measurement of large particles. In industries such as mining, this can be particularly useful.

The main disadvantages associated with this technique include: it is not possible to measure sprays or emulsions and measurement of dry powders is also difficult when particles become small. Wet sieving can help to overcome this problem, but it is then very difficult to reproduce results. Materials such as clay, which are cohesive and agglomerated, are also difficult to measure.

As particles tend to orientate themselves through the sieve, operating methods and measurement times need to be standardised if accurate and meaningful results are to be obtained.

Sedimentation

This has been a common method used (historically) in clay and ceramics industries.

There are two main problems with this process: the density of the material is needed and so it is not useful for determining particle size of emulsions where the material does not settle or for dense materials where the material settles quickly. Samples containing components of mixed density can not be accurately resolved. Measurement of small particles is very slow and therefore the process of repeating testing can be tedious.

Electrozone testing

This technique is very good for measuring red blood cells, but for real, industrial materials there are quite a few problems. It is very difficult to measure emulsions and dry powders and it is impossible to measure sprays. It is necessary to measure in an electrolyte and the required calibration standards are expensive.

Laser Diffraction

This is an often favoured technique that is considered to be one of the most accurate and reliable. It has a number of important advantages:

• It is very flexible and can measure all types of particles (powders, emulsions, suspensions and sprays)
• It is very rapid (answers can be produced in less than sixty seconds)
• It offers an absolute method of particle analysis that is grounded in scientific principles and makes it possible for measurements to be taken without the need to calibrate any instrument against the standard
• The technique provides a very wide and dynamic range
• It is possible to measure an entire sample
• The technique is highly repeatable.

Combine technique with instrument

The technique used to measure particle size will depend on the material being analysed, and the instrument used should be one of the highest quality. ATA Scientific is a trusted brand selling a range of scientific instruments suited to measuring particle size. Contact us today to find the right instrument for you.