Transportation of particles through tubing between the sample inlet and the optics of a particle counter has often been at the forefront of discussion regarding the validity of readings. With the release of the 2008 EU GMP Annex 1 (with an updated draft released in December 2017) the issue of particle loss of large particles has been elevated.
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Most Optical Particle Counters use a light source to illuminate a sample volume, an optical system to collect the particle’s scattered light pulse, a process to convert the scattered light into an electrical signal, and electronics to correlate particle size with the scattered light pulse. While the specific design varies greatly between manufacturers (and while the instruments vary with different sensitivity and flow rates), the fundamental principles of optical particle counter operation are quite similar across the industry.