A particle counter is an instrument that identifies and measures particles. There are three types of particle counters: liquid, aerosol, and solid. Monitoring the present environmental conditions requires the selection of the appropriate monitoring instrument.
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However, in the situation of a clean room, an aerosol particle counter is perfect. The device is designed based on the theory of light scattering by particles. It is real-time equipment generally used to quantify particles with a diameter larger than 0.05 μm.
These considerations are considered when designing equipment to measure particle size distribution based on light scattering concepts. It is a sensitive instrument for aerosol measurement as a single submicrometer particle may give a signal that photodetectors can recognize.
Light scattering is a non-intrusive measuring approach that may provide immediate data for real-time or continuous measurements.
Optical Particle Counters and How they Operate
Optical particle counters feature laser-illuminated optical sensors that permit the sampling of a single particle by capturing the scattered light from each particle using a solid-state sensor.
The electronics amplify the low-level received signal from the photodetector and translate each dispersed light pulse to its appropriate size category (based on the pulse height), which is then stored in a data logger.
Each dispersed pulse correlates to a particle count, which is then added to the corresponding size category to get particle concentration in a specific size particle interval. This monitoring system enables professionals to identify particles as tiny as 2 nm. However, OPCs cannot identify particles of this size.
These hand-held devices are appropriate for a cleanroom since they have a reduced flow rate for spot testing and certification of smaller air volumes. In a sterile environment, a large OPC may be placed to monitor more significant air volumes continuously.
Key Components of Optical Particle Counters
Airflow systems are utilized to separate particles from optical components and limit the particle flow to the laser beam's boundary. The aerosol flow is aerodynamically concentrated through the employment of a sheath airflow. The instrument's inlets reduce aerosol losses.
Components of optical systems, such as lenses and mirrors, are of the finest quality to guarantee minimum signal loss. Particles that traverse the laser beam scattered light is subsequently captured by the optics and sent to the photodiode.
The electric system is made of a photodetector that emits an electric signal which is then preamplified. Pulse height measuring electronics classify the signal into a size category. Each recorded pulse increases the particulate counter by one in the corresponding size bin. The screen system updates each channel's output in real-time as the specimen aerosol flow enters the measurement chamber.
Size Analysis of Particles
The OPC measure particles’ transit time passing between two illuminated parallel surfaces. The transit time is a measure of the particles' aerodynamic diameter. This device measures the aerodynamic diameter in real-time.
An optical signal related to a single particle's flow over an illuminated region can be quantified by analyzing its recorded pulse heights to the height of a calibration curve drawn from homogeneous particles of known diameter.
It is useful to define particle size equivalence to investigate the impact of the optical characteristics and the form of particles on the responsiveness of a light-scattering apparatus. Some measurable link between the aerosol particles’ scattered cross-section and the particle's geometric diameter is dependent on the measurement of spherical shape and refraction index.
A light-scattering diameter relating to equivalent spheres is specified for nonspherical particles, while an optical shape factor represents the particle form.
Consequently, an optical counter measures the cross-section of light-extinction and not the size of the particle itself.
Application of Optical Particle Counter
Several applications for optical particle counters, including monitoring cleanrooms, air pollution, and occupational health. The target surfaces must be covered with an adhesive material to prevent particles from bouncing away.
Recent Development of Optical Particle Counter
A Japanese study looked at optical particle counters in relation to an inkjet aerosol generator (IAG) to create monodisperse testing particles at a consistent rate. The counting effectiveness of optical particle counters was evaluated using polystyrene latex particles.
The counting effectiveness of liquid particles decreased across the micrometer range, suggesting that the particles had accumulated on a surface inside the optical particle counters.
An optical particle counter needs further research because it is often used to analyze and diagnose particle pollution in clean media such as air, water, and chemicals. Optical particle counters have been used to promote clean production procedures in various sectors, including electronic materials and components, pharmaceutical medication products and health devices, and manufacturing technologies.
References and Further Reading
Lida, K., et al. (2021). Using an inkjet aerosol generator to study particle bounce in optical particle counters. Aerosol Science and Technology. https://doi.org/10.1080/02786826.2021.1950910