The various particle sizing methods all measure different particle dimensions which means that the results between them do not always correlate well. Common methods are:
Sieves are cheap but low resolution and not really suitable for very small particles.
Sedimentation has long measurement times, limited size range and unable to handle mixtures of differing densities.
Electrozone sensing is an excellent technique for blood cells but not suitable for most industrial materials, especially dense or large particles.
Microscopy has the advantage of identifying both the size and shape of the particles, however it is prone to unrepresentative sampling. It’s a useful aide to laser diffraction measurements as a check for aggregation and dispersion.
Laser diffraction has become the preferred particle sizing method in many industries. It is covered by its own ISO standard, ISO13320 and has following advantages:
It’s an absolute method and although it does not require calibration performance can be validated using traceable standards.
A wide dynamic range, 0.2 to 2000 microns, with high resolution of up to 100 size classes.
Flexible sample presentation. Dry powders, aerosols, liquid suspensions and emulsions can all be measured with dedicated dispersion units.
The entire sample is measured giving a volume distribution which is the preferred distribution for chemical engineers.
The method is non-destructive and rapid, producing a result in less than a minute.
Dynamic light scattering (DLS) is a sensitive technique for measuring the size of molecules and particles in the submicron region, including less than 1 nanometre. It is based on analysis of the intensity fluctuations of scattered laser light. Results are expressed as particle hydrodynamic diameter derived by using the Stokes-Einstein relationship.
Particle image analysis. Particles or cells are dispersed on a surface or suspended in a liquid. Microscope optics and CCD camera magnify and capture the image of each particle which is counted and its size and shape are measured. Systems are capable of high resolution sizing from 0.5µm - 1000µm and parameters such as circularity and convexity can quantify subtle differences between samples which are not detectable by size-only. The particle images are stored and can be recalled for visual verification of agglomerates, fines, foreign particles etc.
