The standard process to measure the number of particles is currently automatic particle counting on an opto-electronic basis (e.g. HIAC), but this does not specify the size of the particles.
In contrast, microscopic particle counting also allows particles to be sorted into 5 size ranges. The process is highly accurate, but also requires a high level of effort.
In the case of gravimetric contamination, the solid particles are weighed and stated as a percentage by mass. It does not allow a statement of the number and/or size of the individual particles.
A prerequisite for such a contamination measurement is ensuring that the samples taken have a representative distribution of particles. This is to be ensured through a suitable location and method of sampling. In addition, the sampling and measurement devices must be effectively contamination-free.