Particle Measurement What is particle size and how is it measured?

Different particle measurement methods for the characterization of particle size distributions in granulates, bulk materials, powders and suspensions exist. These include laser diffraction, image analysis, dynamic light scattering as well as sieve analysis.

Particle measurement with these different methods leads to different results, because the "size" of particles can be interpreted quite differently: Size is unambiguously defined only for spherical particles (diameter = particle size). In all possible measuring directions, the same result is obtained.

For non-spherical particles, however, the result of the particle measurement depends both on the orientation of the particles during the measurement process and on the peculiarities of the method used. Since the result of a particle measurement depends on how "size" is defined, there is often confusion in the interpretation of the measurement results.

With an extensive understanding of the strengths and weaknesses of each method, Microtrac offers an unrivalled product range of technologies for particle measurement. Our experts will be happy to assist finding the right solution for your application.

Particle measurement with static or dynamic image analysis

Imaging techniques for particle measurement offer a number of advantages. A distinction is made between particle measurement with Dynamic Image Analysis and Static Image Analysis.

With the static method, the particles are at rest during the measurement (as with a microscope); with dynamic image analysis, moving particles are analyzed, either in a liquid, in an air stream or in free fall. By analyzing individual particle images, both shape and size are measured. Feret diameters, for example, can be specified to describe the various dimensions.

These are determined as one would with a caliper: by measuring the distance between parallel tangents. The largest distance would be the Feret length (X_{Fe max}), the smallest the Feret width (X_{Fe min}). Alternatives would be chord dimensions (e.g. smallest inner diameter, X_{c min}) or Martin diameter (area bisector). Furthermore, the diameter of an equal area circle can be defined as the size of the particle projection. Depending on the problem, a suitable size definition is used for the particle measurement.

Different size definitions in image analysis. X_{c min} (particle width, red), X_area (diameter of the equal area circle, green) and X_{Fe max} (particle length, blue). Depending on the selected size definition, a different measurement result is obtained (cumulative curves on the right)

Particle measurement with Laser Diffraction

There are some fundamental differences in particle measurement by laser diffraction compared to image analysis.

While in imaging techniques each recorded particle represents a measurement event and is included in the overall result, scattered light or diffraction analysis are so-called ensemble measurement techniques. This means that the measurement signal is generated simultaneously by many particles of different sizes.

It is therefore a superposition of angle-dependent scattered light intensities, from which the contributions of the different particle sizes must be calculated. This is done either via the Mie theory, for which the refractive index of the particles must be known, or via the Fraunhofer approximation, which, however, is only usefully applicable for larger particles.

Particle measurement by laser diffraction cannot distinguish between length and width. All scattered light data are referred to a spherical model, they are so-called equivalent diameters. For non-spherical particles, this usually results in a wider distribution being output than in image analysis.

Comparability of particle measurement with different methods

Image Analysis and sieve analysis: very good comparability when image analysis considers particle width during image evaluation. 3D analysis improves the comparability. Particle measurement by image analysis can completely replace sieving!

Image Analysis and Laser Diffraction: Good comparability. Laser diffraction often shows wider distributions, especially for strongly irregular shaped particles. For image analysis, the definition xarea should be used.

Sieve analysis and Laser Diffraction: poorly comparable, laser diffraction tends to give a larger result.

Laser Diffraction and Dynamic Light Scattering: compares well, for small particles (< 100nm) DLS is better, for large particles (>1µm) laser diffraction is superior.