Optical particle counters (OPCs) work by maintaining the air quality of a cleanroom environment. To learn more about it, this article looks at the efficiency and resolution of OPC counting. Additionally, it answers the key questions you should consider when thinking about selecting and using OPCs.
The basics of OPC counting efficiency
OPC counting efficiency refers to the probability of accurately detecting the size ranges of particles passing through a sample volume. In the language of ISO 14644 we speak of light scattering air particle counters (LSAPCs) rather than OPCs. However, for better readability, we will stick to the OPC language in this article. Thus, OPC counting efficiency describes the percentage of the number of particles counted at or above a certain size. The reliability of these results depends on the flow rate detected.
A crucial aspect here is the counting efficiency of 50% at the most sensitive size threshold. This efficiency is important because it represents the balance between detecting smaller particles and avoiding false positives. The counting efficiency is asymmetric due to the exponential relationship between particle size and scattered light signal.
Understanding OPC resolution
The resolution of an OPC is its ability to distinguish between small differences in particle size. Several factors affect resolution, including the amount of light. The light source influences the uniformity of illumination within the sample volume. The quality of the optical system and electronics in the pulse height analyzer module also influences the resolution. Therefore, it will be able to distinguish between small particles and particles that are larger than expected.
High resolution in OPCs is crucial as it allows for more accurate particle size determination. This is crucial in environments where even the smallest particles can impact overall cleanliness.
The influence of OPC counting efficiency and resolution on cleanroom data
Controlling airborne particles is critical to maintaining cleanroom standards. This includes product quality and compliance with industry standards. Therefore, the accuracy of particle count data is extremely important. The counting efficiency and resolution of OPCs are crucial factors that directly influence this data.
Particle measurements and OPC counting efficiency
The efficiency of OPC counting plays a crucial role in determining the sensitivity of particle measurement. This efficiency is particularly critical at the OPC’s most sensitive size threshold.
Smaller but significant particles could be missed if the counting efficiency is too low. This could potentially lead to contamination levels being underestimated.
Conversely, too high an efficiency can lead to overcounting, including the occurrence of false positives. This could lead to unnecessary alarms and disruptions in cleanroom operations. Therefore, achieving a balanced counting efficiency, typically 50% at the most sensitive threshold, is critical for reliable particle measurement.
Particle sizes and OPC resolution
The efficiency and resolution of the OPC counting are equally important. Resolution refers to the ability of the instrument to distinguish between particles of different sizes. By ensuring high resolution, it is easier to determine the particle size distribution with the highest precision. 
This is extremely important in environments where certain particle sizes can have different effects, such as in semiconductor manufacturing. Different sizes of smaller and larger particles can cause damage in the production process.
A high resolution OPC can accurately identify and quantify these specific particle sizes. This can enable more targeted and effective cleanroom management. However, challenges arise in transferring the resolution observed with monodisperse calibration particles such as polystyrene latex spheres (PSLs) to real particle distributions.
The challenges of OPC resolution
Ideally, an OPC would respond to particles of different sizes. This would be a perfect Gaussian distribution to indicate excellent resolution. But in real conditions, factors such as the particle’s composition, shape and refractive index can influence this distribution. Internal factors of the OPC, such as the intensity of light scattering and sensor noise, can also lead to changes.
Insufficient resolution can therefore lead to a broadening or flattening of the particle size distribution curve. This change can lead to inaccuracies in understanding the true nature of the particle population in the cleanroom.
These inaccuracies can have greater impact if left unchecked. They could lead to an incorrect assessment of the cleanroom conditions. Additionally, they could impact critical decisions such as batch releases or contamination investigations and lead to a product recall or regulatory non-compliance.
Why is the efficiency and resolution of OPC counting important?
It is important to understand and optimize the efficiency and resolution of OPC counting. Regular calibration, maintenance, and understanding the specific particle characteristics of the cleanroom environment can help produce more accurate and reliable particle count data. This ensures that the cleanroom meets its intended environmental standards, thereby ensuring the quality and safety of the products manufactured therein.
Sensor resolution and its factors
The sensor resolution of an OPC, determined by factors such as the uniformity of illumination and the quality of the optical and electronic systems, determines the minimum possible width of the OPC size channels. This basic resolution sets the limit on the number of size channels an OPC can have.
Therefore, an OPC with good resolution may have more size channels than one with poor resolution. This difference is crucial in applications that require high precision in particle size differentiation.
Calibration and data integrity
Calibration and data integrity in OPCs are critical for accurate particle counting. To ensure data is complete, accurate and consistent, OPCs must adhere to standards such as the ALCOA standard and 21CFR11.
OPCs must have integrated audit trails for data traceability. You must also adhere to user access control and data verification policies.
Are you ready to find the perfect particle counter?
OPCs are crucial for monitoring air quality in cleanrooms. The efficiency and resolution of OPC counting play a crucial role in the accuracy of particle counting from different air volumes. Understanding these aspects helps select the right OPC for specific cleanroom environments. This way you can ensure the reliability of the data they produce.
Advanced OPCs with high-resolution capabilities and sensor condition monitoring provide additional benefits for precise particle counting in the cleanroom to achieve cleanroom certification. There are even portable particle counters available.
To find out more about it Particle counterParticle Measurement Systems would like to invite you to examine the available devices. There you will find the particle counter that best suits your project, be it a general device for detecting contaminants in cleanrooms or an aerosol particle counter.
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