## Introduction to Particle Counting

Particle counting may be performed using two different techniques, each with their own limitations and interferences, but one aspect they share in common is that they are both only a quantitative count. Neither method identifies the composition or source of the particles, they only provide a level of particles per ml of sample.

The photodetector can measure the area of the shadow, which it then uses to calculate the size of the particle by assuming what is called spherical equivalency. Essentially, to use a two-dimensional shadow to represent a three-dimensional particle, it must be assumed that unobserved dimension is smaller than either of the observed dimensions, and then it is further assumed that the particle is a sphere. A sphere would cast a circular shadow, so the area is calculated to determine the diameter of a circle of the same area.

Optical Particle Counting does not measure particles by their longest dimension, nor their shortest or average dimension, but rather by assuming every particle is a sphere. As an example of how misleading this might be, consider a rectangular chip that is 10 microns long, and only 1 micron wide or thick; this particle would be assumed to be no larger than 3.6 microns, and therefore would not be included in the reported values (since they begin at 4 microns and larger).

While pore block is not prone to the same inaccuracies as optical particle counting, it is nevertheless not directly measuring each individual particle either. For this reason, particle counting should always be trended for major shifts in the data, primarily looking at the ISO codes, rather than delving into the actual counts.

For specific information on other measured parameters, bookmark this page for future updates. Coming up next: Understanding ISO Codes.