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Principle of laser dust particle counter application

principle, application and composition of laser dust particle counter 1 introduction at present, there are more and more users of laser dust particle counter. Laser dust particle counter is widely used in medicine, electronics, precision machinery, color tube manufacturing, microbiology and other industries to monitor the purification effect and cleanliness level of worktables, purification rooms and purification workshops of various cleanliness levels, To ensure the quality of products. Laser dust particle counter is an instrument used to measure the number and size distribution of dust particles in the air, so as to provide a basis for the evaluation of air cleanliness. The common laser dust particle counter is light scattering type (DAPC), measuring the particle size range of 0 m. In addition, there is a condensed core laser dust particle counter (CNC), which can measure smaller dust particles. This paper will introduce the light scattering laser dust particle counter. 2. Working principle of the laser dust particle counter the basic principle of the laser dust particle counter is the detection of the optical sensor. The laser dust is received by the photosensitive element and generates a pulse signal after being scattered by the particles of the flexible packaging material, which is mainly high molecular polymer or its coherent material. The pulse signal is output and amplified, and then digital signal processing is carried out. By comparing with the standard particle signal, The comparison results are expressed with different parameters. Particles in the air will scatter under the irradiation of light. This phenomenon is called light scattering. Light scattering is related to particle size, light wavelength, particle refractive index and light absorption characteristics of particles. However, in terms of scattered light intensity and particle size, there is a basic law, that is, the intensity of scattered light from particles increases with the increase of particle surface area. In this way, the size of particles can be inferred as long as the intensity of scattered light is measured. In fact, the scattering generated by each particle has the function of automatic correction; The light intensity is very weak, which is a very small light pulse. It needs to be amplified by the photoelectric converter to convert the light pulse into an electrical pulse with a large signal amplitude, and then further amplified and screened by the electronic circuit, so as to complete the counting of a large number of electrical pulses. At this time, the number of electric pulses corresponds to the number of particles, and the amplitude of electric pulses corresponds to the size of particles. This is the basic principle of light scattering laser dust particle counter. 3.1 light source light source is the key component of laser dust particle counter, which has a great impact on the performance of the instrument. The light source requires high stability, long service life and no interference. The light source of laser dust particle counter includes ordinary light source and laser light source. The ordinary light source is iodine tungsten lamp, which has large volume, high calorific value and short service life. It needs to be preheated after startup. The laser light source is a laser, which has small volume, high stability and long service life. It is often integrated with the detection cavity and optical detector to form a sensor. Common laser sources include HeNe laser and laser diode. The laser dust particle counter with ordinary light source has a low response to the particle signal below 0.3 m, and its signal amplitude is almost the same as the noise amplitude of the counter itself, so the signal is difficult to detect from the noise. Although this kind of instrument is marked with the channel of 0.3 m, it is only suitable for the determination of particles larger than 0.3 m, especially above 0.5 m. Due to the good monochromaticity of the laser and the stable concentration of light energy, the sensor of the laser dust particle counter using the laser light source has a high signal-to-noise ratio. Some of these instruments can detect particles of 0.1 M. 3.2 measurement chamber the measurement chamber is the space for particle observation, and the collected air should pass through the measurement chamber. The optical system of the instrument makes the light source irradiate into the measuring cavity through the lens and slit, forming a light sensitive area with a volume of about several cubic millimeters. When the dust in the air passes through the light sensitive area, it will scatter some light energy, which will be collected by the light collecting lens at an angle (90 degrees or 70 degrees) with the incident light, and then projected onto the optical detector. 3.3 photodetector photodetector is a photoelectric converter that converts scattered light energy into electrical signals. The most commonly used photodetectors in laser dust particle counters are photomultiplier tubes and photodiodes. Photomultiplier tubes amplify photoelectrons by tens of thousands of times and convert them into electrical signals of several millivolts to tens of millivolts. They have the advantages of good spectral linearity, fast response time and small dark current, but their disadvantages are large size. When the photomultiplier tube works, it needs to add a negative high voltage of hundreds of volts. There is a corresponding high voltage generation circuit in the instrument. Pay attention to safety when debugging or calibrating the instrument. Photodiode is a semiconductor element that can generate electrons after being illuminated. It has the characteristics of small volume and simple peripheral circuit. It is often integrated with the detection cavity. 3.4 flow monitoring the sampling flow of the laser dust particle counter is generally 2.83l/min or 28.3l/min, and the imported instruments are often marked as 0.1cfm (cubic feet per minute) or 1cfm, mainly to facilitate the calculation of the cleanliness in accordance with fed-std-209e. Large flow sampling (28.3l/min) can more accurately reflect the clean condition of the air, but reduce the maximum sampling concentration. 3.5 the air pump and filter air pump are located inside the laser dust particle counter, and the air pump enables the instrument to generate sampling flow. The air pump requires low noise, low vibration and stable air flow. The filter should be able to filter out particles more than 0.3 m, so as to avoid the impact of the air discharged from the instrument on the clean area. 3.6 the particles with different particle sizes in the circuit system will generate electric pulse signals with different amplitudes (voltages) after being converted by the photoelectric system of the laser dust particle counter. The larger the particle size is, the higher the pulse voltage is. The relationship between signal voltage and particle size is also called conversion sensitivity. For a given laser dust particle counter, the particle size is one-to-one corresponding to the pulse voltage. For example, the conversion sensitivity of a laser dust particle counter is 0.3 m, corresponding to 69mv, 0.5 m, corresponding to 531m, and the high molecular chain naturally curls under the action of no external force V, 1.0 m, corresponding to 701mv, etc. if the laser dust particle counter detects a pulse of 100mV, the particle size must be greater than 0.3 m and less than 0.5 m. Laser dust particle counter is an instrument that measures the number of particles greater than or equal to a certain particle size. Its internal circuit is the circuit that counts the number of pulses greater than or equal to a certain voltage value. For the example in the above paragraph, measuring the number of particles greater than or equal to 0.3 m in the air means counting the number of pulses greater than or equal to 69mv in the circuit, measuring the number of particles greater than or equal to 0.5 m, counting the number of pulses greater than or equal to 531mv in the circuit, and so on. Therefore, the instrument's measurement of dust particles mainly depends on the parameter of conversion sensitivity. In addition, it should be noted that the conversion sensitivity of each laser dust particle counter is different, and it must be calibrated with standard particles regularly before and after leaving the factory to obtain the best conversion sensitivity value. The circuit system is a circuit that amplifies, discriminates and counts the pulse signal. In addition, it also includes power supply, control, display, calculation, printing and other circuits. 2、 Precautions for using laser dust particle counter 1. When the inlet pipe is covered or blocked, do not start the counter. 2. The laser dust particle counter should be used in a clean environment to prevent damage to the laser sensor. 3. Do not detect mixed gases that may react (such as hydrogen and oxygen). This gas may also explode in the counter. To measure these gases, you need to contact the manufacturer for more information. 4. Do not sample compressed air without high-pressure decompression equipment (such as high-pressure diffuser). All particle counters are designed to operate at one atmospheric pressure. 5. Water, solution or other liquids cannot enter the sensor from the inlet pipe. 6. The particle counter is mainly used to test the clean environment of the purification workshop. When there are loose particle materials, dust sources, and spray, it must be kept at least 12 inches away from the inlet pipe. To prevent the above particles and liquids from polluting the sensor and pipeline. 7. When sampling, avoid sampling the gas discharged from the counter itself or polluted by the gas from the counter. 8. When connecting an external printer or an external temperature and humidity sensor, turn off the counter first; When performing printing operation, there must be printing paper on the printer, otherwise the print head will be damaged. Leon spring is the head of the business department of Lighthouse worldwide solutions. He is responsible for the marketing of the company in the Chinese market and is responsible for a number of large-scale projects

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