In the realm of fluid system analysis, particle counting is a critical practice, providing insights into the cleanliness and integrity of the system – specifically, counting efficiency. However, a common dilemma arises among users regarding the interpretation of particle counts from non-volumetric particle counters, leading to discussions about the “true” concentration of particles. This blog aims to discuss the science behind these differences, addressing concerns and shedding light on the characteristics of non-volumetric particle counters.
Understanding Particle Counting Efficiency
Counting Efficiency (CE) is a pivotal factor in comprehending particle counts. It represents the percentage of particles detected in the fluid flow compared to the concentration of particles present. Users often grapple with the necessity of dividing particle counts by CE at the lower limit to establish the accurate concentration of particles in the system. This blog aims to demystify this process and elucidate the scientific principles at play.
Instrument Parameters
To navigate the intricacies of particle counting, it’s essential to grasp key definitions:
- 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: Calculated as the instrument sample volume divided by the instrument flow rate, expressed as a percentage.
Distinguishing Volumetric and Non-Volumetric Particle Counters
A fundamental distinction lies in the sample volume percentage, where volumetric particle counters maintain a sample volume percentage of 100%, ensuring a well-defined sample volume. On the other hand, non-volumetric particle counters operate with a sample volume percentage less than 100%, prompting questions about discrepancies in reported values.
The Trade-Off in OPC Design
The design of optical particle counters (OPCs) involves a balance between reducing system-generated noise and amplifying the signal from particles. Noise sources include the light source, optics, window surfaces, sample fluid molecules, and electronic circuits. Particle counters with detection limits beyond 100 nm illuminate sample cell walls while maintaining acceptable signal-to-noise ratios, rendering them volumetric.
Challenges in Non-Volumetric Counters
Non-volumetric particle counters with detection limits below 100 nm face challenges due to elevated noise levels. To mitigate this, lasers must be focused to a size smaller than the sample cell. This creates a region of variable power density. As a result, particles are free to transit the laser anywhere over the cross-sectional area of the beam, introducing complexities in accurate particle counting.
