Types and principles of particle counters

　　Optical Particle Counter

　　Optical particle counters use the Tyndall Effect to detect particles. The Tyndall Effect is named after John Tyndall and is usually caused by the scattering of light by particles in colloids. When a bright beam of light shines on dust in the air or fog, the scattering produced is the Tyndall Effect.

　　When the refractive index changes, light will scatter. This means that in a liquid, bubbles scatter light in the same way as solid particles. Mie Theory describes the scattering of light by particles.

　　The Lorenz-Mie-Debye theory, first proposed by Gustav Mie, describes how light is scattered in many different directions. The exact scattering depends on the refractive index of the medium, the scattering effect of the particle on the light, the size of the particle, and the wavelength of the light. Examining the details of Mie's theory is beyond the scope of this article; however, there are many public domain applications that can be used to verify how light is scattered.

　　The scattering of light varies with the size of the particle. In particle counters, the most important results of the Mie theory and its predictions about light scattering are related to it. When the particle size is much smaller than the wavelength of light, the light scatters mainly in the forward direction. When the particle size is much larger than the wavelength of light, the light scatters mainly in the right angle and backward directions.

　　Light can be viewed as a wave that oscillates perpendicularly to the direction of propagation. This oscillation direction is called polarization. The polarization of the incident light is very important. In the previous examples, the scattering of light was measured in the plane of polarization of the incident light.

　　The scattering behavior is similar for particles up to 5 μm; the scattering behavior is very different for particles up to 0.3 μm, which have polarization. Since the intensity of the scattered light does not change with frequency for less than a factor of ten, the logarithmic representation does not show a change: shorter wavelength means more scattering. All else being equal, blue light scatters about 10 times more intensely than red light. Most particle counters use near-infrared or red lasers; until recently, this was the most cost-effective option. Blue gas and semiconductor lasers are expensive; and semiconductor lasers have a short lifespan.

　　Air Particle Counter

　　There is a vacuum device at the outlet of the sensor to draw air through the sensor. The particles in the air scatter the laser. The scattered light is focused by the condenser lens at the back onto the optical detector, which then converts the light into a voltage signal, amplifies and filters it. After that, the signal is converted from analog to digital and classified by the microprocessor. The microprocessor connects the counter to the control data collection system through an interface.

　　Laser Particle Counter

　　The gas laser was invented in 1960, and the semiconductor laser in 1962. These lasers were expensive at first, but as they became economical, gas lasers replaced white light in particle counters, and by the late 1980s, cheaper semiconductor lasers had replaced gas lasers in most applications.

　　There are two types of lasers used for particle counting: one is a gas laser, such as helium-neon (HeNe) lasers and argon-ion (arg-ion) lasers; the other is a semiconductor laser. Gas lasers are able to produce strong monochromatic light, sometimes even polarized light. Gas lasers produce collimated Gaussian beams, while semiconductor lasers produce a small divergent point source, usually with two different axes of divergence, and always in multiple modes. Due to the multi-axis nature of the divergent light, semiconductor lasers usually have an elliptical output, which brings certain challenges and also certain advantages. Scattered light on different axes means either to accept this elliptical output reluctantly or to design a set of complex and expensive optical mirrors to compensate. On the other hand, elliptical beams are well suited for certain applications, and by using the long axis, better coverage can be obtained.

　　In short, the output of the helium-neon laser is "directly available without the need to add any optical components. To produce a beam similar to that of a helium-neon laser, the light from a semiconductor laser must be focused by a lens, which results in a loss of light energy. However, semiconductor lasers are the best choice for particle counters due to their low cost, small size, low operating voltage, and low power consumption.

　　In applications where high sensitivity is required, HeNe lasers can be used in open cavity mode to generate very high powers. This type of laser is not suitable because the sample is passed through an optical cavity resonator and the laser is interrupted (the "Q factor" cannot be maintained) when the particle concentration is high.

　　Particle Counter Inlet Nozzle Type

　　The inlet sample entering the particle counter plays a vital role in the resolution of the counter. There are two types of inlets: one is flat (10mm wide and 0.1mm high) and the other is round with an inner diameter of 2-3mm.

　　When the inlet nozzle is flat, the laser beam is usually a narrow line coaxial with the nozzle. The airflow velocity coming out of the flat nozzle is quite uniform, and it passes through the strongest and most uniform part of the laser beam, so the accuracy is the highest. However, the small cross-section of the flat nozzle means that the vacuum degree required is higher than that of the round nozzle, which increases energy consumption (this is very important, especially when using battery power). Flat nozzles are more complicated to manufacture and more expensive, and their coordination with the laser is also a problem.

　　When the inlet nozzle is round, the laser beam is usually at approximately right angles to the axis of the inlet. The particle passes through a very narrow, high-intensity laser face. A round nozzle is simpler because it has a larger cross section and requires less vacuum for the same airflow velocity, so less energy is consumed when air is drawn in. The lower airflow velocity relative to a flat nozzle means more light is scattered per particle. The disadvantage of a round nozzle is that it reduces the uniformity of the airflow and the laser beam power is not uniform; the beam is coarser and therefore less accurate.

　　Development History of Particle Counters

An air particle counter is a special instrument　　for testing the particle size and distribution of air dust particles. It has developed from a microscope and has gone through the process of microscope, sedimentation tube, sedimentation instrument, centrifugal sedimentation instrument, particle counter, laser air particle counter, and PCS nano laser air particle counter. Among them, the laser air particle counter has become the mainstream product in many industries in recent years due to its advantages such as fast testing speed, wide dynamic distribution, and no human influence.