Abstract
Some users believe it is necessary to divide the particle counts of non-volumetric particle counters by the counting efficiency (CE) at the lower limit to establish the “true” concentration of particles in the fluid system. Other users do not understand why two, similar-sized channels of particle counters with different limits of detection do not report the same value. This paper will discuss the science behind these differences and alleviate concerns that come from the characteristics of non-volumetric particle counters.
Definitions
Counting Efficiency: The percentage of particles detected in the fluid flow compared to the concentration of particles that are present.
Instrument Sample Volume: The volume per unit time of fluid being inspected for particles.
Instrument Flow Rate: The volume per unit time of fluid passing through the instrument.
Sample Volume Percentage: The instrument sample volume divided by the instrument flow rate expressed as a percentage.
Volumetric Particle Counter: A particle counter where the sample volume percentage is 100%
Non-Volumetric Particle Counter: Any particle counter where the sample volume percentage is less than 100%
Background
The process of designing optical particle counters (OPCs) is a trade-off between reducing noise generated by the system and increasing the signal coming from the particles. The sources of noise can be from the light source itself, the optics used to shape the light, window surfaces/capillary walls, the molecules of the sample fluid, and the electronic circuits used to collect and process the scattered light. Particle counters with detection limits greater than 100 nm are able to illuminate the sample cell walls while maintaining acceptable signal to noise (s:n) ratios. The ability to illuminate the entire sample cell with the uniform portion of the light source (typically a laser) makes these instruments volumetric and provides a well-defined sample volume for the particles of interest.
Illumination of the sample cell walls in particle counters with detection limits less than 100 nm, however, will produce noise levels that exceed the amount of light collected from the particles of interest. To overcome this issue, the laser must be focused to a size smaller than the sample cell, and only the region in the center of the cell is inspected for particles. The focusing of the beam creates a region of variable power density. The result of a beam smaller than the sample cell is that particles are free to transit the laser anywhere over the cross-sectional area of the beam.
