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A particle counter is used to monitor and diagnose particulate contamination in certain cleaning media, including air, water, and chemicals. Particle counters are used in a variety of applications to support clean manufacturing practices, industries including: electronic components and assemblies, pharmaceutical drug products and medical devices, and industrial technologies such as oil and gas.
Particle counters work primarily using the principles of light scattering, although other technologies may also be employed. Light scattering by particles uses instruments consisting of a high-intensity light source (a laser), a controlled media flow (air, gas or liquid) and a highly sensitive light-gathering detector (a photodetector).
Laser optical particle counters employ five main systems:
Lasers and optics: A laser operates at a single wavelength, so the light source is consistent with constant power output to illuminate the particle sample region.
Controlled flow: The viewing volume is a small chamber illuminated by a laser. The sample medium (air, liquid or gas) is drawn into the viewing volume, the laser passes through the medium, the particles scatter (reflect) light, and a photodetector collects the scattered light sources (particles).
Photodetector: A photodetector is an electrical device that is sensitive to light, and when particles scatter light, the photodetector observes the flash of light and converts it into an electrical signal or pulse. An amplifier converts the pulses into a proportional control voltage.
Pulse height analyzer (PHA): The pulses from the photodetector are sent to a pulse height analyzer (PHA). The PHA checks the amplitude of the pulse and stores its value in an appropriately sized channel, called a bin. Bins contain data about each pulse and this data is related to particle size.
Black Box: The black box, or support circuitry, looks at the number of pulses in each bin and converts the information into particle data.
Light obscuration by particles works on the principle that the presence of particles blocks some of the light from the photodetector, usually through absorption or light scattering. The photodetector records the opacity of light and converts it into an electrical signal, this signal being related to the above scattering details using PHA with particles of a certain size.
Direct imaging particle counting uses a high-resolution camera and a light source to detect particles. A vision-based particle size unit receives two-dimensional images that are analyzed by computer software to measure particle size, the images can be retained and replayed for additional analysis.
Using the laser diffraction principle that the diffraction angle increases as particle size decreases, this method is used for sizes between 0.1 and 3,000μm. Laser scattering measures the particle size distribution as a percentage or concentration by mass of the sample of scattered particles.
A coulter counter is an instrument for counting and sizing particles suspended in an electrolyte. It is commonly used for cellular particles. The Coulter principle, and the Coulter counter based on it, is the commercial term for the technique known as resistive pulse sensing or electric zone sensing.
