Why is laser diffraction size results different from sieve analysis ?

Probably the most frequently asked question we encounter is why is laser diffraction results different from sieve analysis. 

 The answer lies in the nature of a particle and what aspect of the particle each sizing technique measures.

When we measure perfectly spherical particles all sizing techniques will generate very similiar (if not exactly the same) result. In the real world most samples measured contains irregular shaped particles. In the case of laser diffraction we measure the average particle size of the sample (in random orientation) assuming the aspect ratio (length to width) is less than 3 to 1. A sieve will measure the 2nd smallest diameter of a particle.

It is clear that the 2nd smallest diameter of particle and the average diameter of a particle is always different and therefor laser diffraction results tend to be different from sieve results. Typically laser diffraction results is larger or coarser than sieve results.

The follow up question is how different are the two techniques ?

The answer is specific to each sample measured as the overal shape of each sample is unique. It is possible to correlate the two techniques but it is important to remember that the strength of the correlation is sample specific and might change over time if the shape of the sample changes.

We therefore recommend that if your intention is to replace sieving with laser diffraction to keep the comments above in mind.