Comparing Laser Diffraction and Sieve Results

Laser diffraction and sieving can provide similar results when characterising spherical or semi-spherical particles. However, significant differences can be observed for non-spherical particles due to the different particle properties measured by each technique.

The reason for the difference between the two techniques can be shown by assuming that both are used to measure a rectangular particle with dimensions of 100µm by 100µm by 200µm. During the sieve analysis a 110µm sieve is used. The particle will fall through this sieve because the particle is classified according to its 2nd largest dimension (100µm). Laser diffraction will, however, report a spherical equivalent size relating to the volume of the particle. In this case the diameter of the sphere that has the same volume as the particle being measured is 156µm.

Volume of particle is now 2x106 µm³
Diameter of sphere with same volume = 156µm

As a general rule, laser diffraction will always report a larger particle size compared to sieving, with the magnitude of the difference being related to the aspect ratio of the particle. For those applications where sieve-like results are required based on laser diffraction measurements, it is possible to emulate the performance of a sieve by developing a set of emulation factors which allow for the change in results caused by the particle shape.

Typical comparison between laser diffraction and sieving showing how the different properties measured by each technique changes the reported size distribution.