1 Introduction
At present, there are more and more users of dust particle counters , which are widely used in medicine, electronics, precision machinery, color tube manufacturing, microbiology and other industries to achieve the purification effect and cleanliness of various clean grade worktables, clean rooms, and purification workshops. Levels are monitored to ensure product quality.
The dust particle counter is an instrument used to measure the amount and size of particles in the air to provide a basis for the assessment of air cleanliness. A common dust particle counter is light-scattering ( DAPC ) with a particle size range of 0.1-10 μm . In addition, a dust-collecting dust particle counter ( CNC ) is available to measure smaller particles of dust. This article will introduce a light scattering dust particle counter.
At present, there are more and more users of dust particle counters , which are widely used in medicine, electronics, precision machinery, color tube manufacturing, microbiology and other industries to achieve the purification effect and cleanliness of various clean grade worktables, clean rooms, and purification workshops. Levels are monitored to ensure product quality.
The dust particle counter is an instrument used to measure the amount and size of particles in the air to provide a basis for the assessment of air cleanliness. A common dust particle counter is light-scattering ( DAPC ) with a particle size range of 0.1-10 μm . In addition, a dust-collecting dust particle counter ( CNC ) is available to measure smaller particles of dust. This article will introduce a light scattering dust particle counter.
2. How the particle counter works
The basic principle is that the detection laser of the optical sensor is scattered by the photosensitive particles after being scattered by the dust particles, and generates a pulse signal, which is outputted and amplified, and then subjected to digital signal processing, and compared with the standard particle signal, the comparison result is different. The parameters are indicated. Particles in the air scatter under the illumination of light. This phenomenon is called light scattering. Light scattering is related to particle size, wavelength of light waves, refractive index of particles, and absorption characteristics of light by particles. However, in terms of scattered light intensity and particle size, there is a basic rule that the intensity of the scattered light of the particles increases as the surface area of ​​the particles increases. In this way, as long as the intensity of the scattered light is measured, the size of the particles can be inferred. In fact, the intensity of the scattered light generated by each particle is very weak, which is a small pulse of light, which needs to be converted by the photoelectric converter to convert the light pulse into The electrical pulse with a large amplitude of signal is then further amplified and screened by the electronic circuit 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 the electric pulses corresponds to the size of the particles. The basic principle of the light scattering particle counter                         Â
The basic principle is that the detection laser of the optical sensor is scattered by the photosensitive particles after being scattered by the dust particles, and generates a pulse signal, which is outputted and amplified, and then subjected to digital signal processing, and compared with the standard particle signal, the comparison result is different. The parameters are indicated. Particles in the air scatter under the illumination of light. This phenomenon is called light scattering. Light scattering is related to particle size, wavelength of light waves, refractive index of particles, and absorption characteristics of light by particles. However, in terms of scattered light intensity and particle size, there is a basic rule that the intensity of the scattered light of the particles increases as the surface area of ​​the particles increases. In this way, as long as the intensity of the scattered light is measured, the size of the particles can be inferred. In fact, the intensity of the scattered light generated by each particle is very weak, which is a small pulse of light, which needs to be converted by the photoelectric converter to convert the light pulse into The electrical pulse with a large amplitude of signal is then further amplified and screened by the electronic circuit 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 the electric pulses corresponds to the size of the particles. The basic principle of the light scattering particle counter                         Â
3.1 light source
The light source is a key component of the dust particle counter and has a great impact on the performance of the instrument. The light source requires high stability, long life and no interference.
The light source has two kinds of ordinary light sources and laser light sources. The common light source is an iodine-tungsten lamp, which has a large volume, high heat generation and short life, and needs to be preheated after starting up. The laser light source is a laser, which has small volume, high stability and long service life. It is often integrated with the detection chamber and the photodetector to form a sensor. Common laser sources are HeNe lasers and laser diodes.
A dust particle counter using a common light source has a low response to a particle signal of 0.3 μm or less, and its signal amplitude is almost the same as the noise amplitude of the counter itself, and the signal is hardly detected from the noise. Such instruments, although labeled with a channel of 0.3 [mu]m , are only suitable for the determination of particles larger than 0.3 [mu]m, especially above 0.5 [mu]m .
Due to the good monochromaticity of the laser and the stable and stable light energy, the dust particle counter using the laser source has a high signal-to-noise ratio (SNR). Some of these instruments can detect 0.1 μm particles.
The light source is a key component of the dust particle counter and has a great impact on the performance of the instrument. The light source requires high stability, long life and no interference.
The light source has two kinds of ordinary light sources and laser light sources. The common light source is an iodine-tungsten lamp, which has a large volume, high heat generation and short life, and needs to be preheated after starting up. The laser light source is a laser, which has small volume, high stability and long service life. It is often integrated with the detection chamber and the photodetector to form a sensor. Common laser sources are HeNe lasers and laser diodes.
A dust particle counter using a common light source has a low response to a particle signal of 0.3 μm or less, and its signal amplitude is almost the same as the noise amplitude of the counter itself, and the signal is hardly detected from the noise. Such instruments, although labeled with a channel of 0.3 [mu]m , are only suitable for the determination of particles larger than 0.3 [mu]m, especially above 0.5 [mu]m .
Due to the good monochromaticity of the laser and the stable and stable light energy, the dust particle counter using the laser source has a high signal-to-noise ratio (SNR). Some of these instruments can detect 0.1 μm particles.
3.2 measuring chamber
The measuring chamber is the space for the observation of the particles, and the collected air passes through the measuring chamber. The optical system of the instrument causes the light source to illuminate the measuring cavity through the lens and the slit to form a photosensitive region of a volume of about several cubic millimeters. When the dust in the air passes through the light sensitive area, a part of the light energy is scattered, collected by the collecting lens at an angle ( 90 degrees or 70 degrees) with the incident light , and then projected onto the photodetector.
The measuring chamber is the space for the observation of the particles, and the collected air passes through the measuring chamber. The optical system of the instrument causes the light source to illuminate the measuring cavity through the lens and the slit to form a photosensitive region of a volume of about several cubic millimeters. When the dust in the air passes through the light sensitive area, a part of the light energy is scattered, collected by the collecting lens at an angle ( 90 degrees or 70 degrees) with the incident light , and then projected onto the photodetector.
3.3 photodetector
A photodetector is a photoelectric conversion device that converts scattered light energy into an electrical signal. The photodetectors commonly used in the dust particle counter are photomultiplier tubes and photodiodes.
The photomultiplier tube converts photoelectrons into tens of thousands of times and converts them into electric signals of several millivolts to several tens of millivolts. It has the advantages of good spectral linearity, fast response time and small dark current. The disadvantage is that it is bulky. When the photomultiplier tube is working, it needs to add a negative high voltage of several hundred volts. There is a corresponding high voltage generating circuit in the instrument. Pay attention to safety when debugging or calibrating the instrument.
A photodiode is a semiconductor component that can generate electrons after being illuminated. It has the characteristics of small size and simple peripheral circuit, and is often integrated with the detection cavity.
A photodetector is a photoelectric conversion device that converts scattered light energy into an electrical signal. The photodetectors commonly used in the dust particle counter are photomultiplier tubes and photodiodes.
The photomultiplier tube converts photoelectrons into tens of thousands of times and converts them into electric signals of several millivolts to several tens of millivolts. It has the advantages of good spectral linearity, fast response time and small dark current. The disadvantage is that it is bulky. When the photomultiplier tube is working, it needs to add a negative high voltage of several hundred volts. There is a corresponding high voltage generating circuit in the instrument. Pay attention to safety when debugging or calibrating the instrument.
A photodiode is a semiconductor component that can generate electrons after being illuminated. It has the characteristics of small size and simple peripheral circuit, and is often integrated with the detection cavity.
3.4 Traffic Monitoring
The sampling flow rate of the dust particle counter is generally 2.83L/min or 28.3L/min . The imported instrument is often marked as 0.1cfm (cubic feet per minute) or 1cfm , mainly for the convenience of Fed-Std-209E . .
Large flow sampling ( 28.3 L/min ) more accurately reflects the cleanliness of the air, but reduces the large sampling concentration of zui .
The sampling flow rate of the dust particle counter is generally 2.83L/min or 28.3L/min . The imported instrument is often marked as 0.1cfm (cubic feet per minute) or 1cfm , mainly for the convenience of Fed-Std-209E . .
Large flow sampling ( 28.3 L/min ) more accurately reflects the cleanliness of the air, but reduces the large sampling concentration of zui .
3.5 air pump and filter
The air pump is located inside the instrument to allow the instrument to generate sample flow. The air pump requires low noise, low vibration, and stable airflow. The filter should be able to filter out particles larger than 0.3μm to prevent the air discharged from the instrument from affecting the clean area.
The air pump is located inside the instrument to allow the instrument to generate sample flow. The air pump requires low noise, low vibration, and stable airflow. The filter should be able to filter out particles larger than 0.3μm to prevent the air discharged from the instrument from affecting the clean area.
3.6 circuit system
Particles of different particle sizes are converted by the photoelectric system of the dust particle counter, and electric pulse signals of different amplitudes (voltages) are generated. The larger the particle size, the higher the pulse voltage. The relationship between signal voltage and particle size is also called conversion sensitivity. For a given dust particle counter, particle size and the pulse voltage is one to one, for example, the sensitivity of a conversion table for dust particle counter corresponding to 0.3μm 69mv, 0.5μm corresponding to 531mv, 1.0μm and the like corresponding to 701mv if the airborne particle counter When a pulse of 100 mv is detected , the size of this particle must be greater than 0.3 μm and less than 0.5 μm .
The dust particle counter is an instrument that measures the number of particles larger than or equal to a certain particle size, and the internal circuit is a circuit that counts the number of pulses greater than or equal to a certain voltage value. For the example in the previous paragraph, measure the number of particles larger than or equal to 0.3μm in the air . In the circuit, count the number of pulses greater than or equal to 69mv , measure the number of particles larger than or equal to 0.5μm , and count the pulse larger than or equal to 531mv in the circuit. The number, and so on. Therefore, the instrument's measurement of dust particles depends mainly on the conversion sensitivity parameter.
In addition, it should be noted that the conversion sensitivity of each dust particle counter is different, and it is necessary to periodically calibrate with standard particles at the time of shipment and afterwards to obtain a good conversion sensitivity value.
The circuit system is a circuit that completes the amplification, discrimination, and counting of the pulse signal. It also includes circuits for power, control, display, calculation, and printing.
Particles of different particle sizes are converted by the photoelectric system of the dust particle counter, and electric pulse signals of different amplitudes (voltages) are generated. The larger the particle size, the higher the pulse voltage. The relationship between signal voltage and particle size is also called conversion sensitivity. For a given dust particle counter, particle size and the pulse voltage is one to one, for example, the sensitivity of a conversion table for dust particle counter corresponding to 0.3μm 69mv, 0.5μm corresponding to 531mv, 1.0μm and the like corresponding to 701mv if the airborne particle counter When a pulse of 100 mv is detected , the size of this particle must be greater than 0.3 μm and less than 0.5 μm .
The dust particle counter is an instrument that measures the number of particles larger than or equal to a certain particle size, and the internal circuit is a circuit that counts the number of pulses greater than or equal to a certain voltage value. For the example in the previous paragraph, measure the number of particles larger than or equal to 0.3μm in the air . In the circuit, count the number of pulses greater than or equal to 69mv , measure the number of particles larger than or equal to 0.5μm , and count the pulse larger than or equal to 531mv in the circuit. The number, and so on. Therefore, the instrument's measurement of dust particles depends mainly on the conversion sensitivity parameter.
In addition, it should be noted that the conversion sensitivity of each dust particle counter is different, and it is necessary to periodically calibrate with standard particles at the time of shipment and afterwards to obtain a good conversion sensitivity value.
The circuit system is a circuit that completes the amplification, discrimination, and counting of the pulse signal. It also includes circuits for power, control, display, calculation, and printing.
Second, the use of particle counters .
1. Do not start the counter when the inlet tube is covered or blocked.
2 , the particle counter should be used in a clean environment to prevent damage to the laser sensor
3. Do not measure mixed gases (such as hydrogen and oxygen) that may react. This gas may also explode inside the counter. These gases need to be measured with the manufacturer for more information.
4. No high pressure decompression equipment (such as high pressure diffuser) should not sample compressed air. All particle counters are designed to operate at one atmosphere.
5. Water, solution or other liquids cannot enter the sensor from the inlet tube.
6. The particle counter is mainly used to test the clean environment of the purification workshop. When there is loose particle material, dust source and spray place, it should be kept at least twelve inches away from the inlet pipe. In order to avoid contamination of the sensor and piping with the above particles and liquids.
7. When sampling, the gas that is discharged from the counter itself or contaminated by the gas from the counter is sampled.
8. When connecting an external printer or connecting an external temperature and humidity sensor, first turn off the counter; when performing the printing operation, the printer must have printing paper, otherwise the print head will be damaged.
1. Do not start the counter when the inlet tube is covered or blocked.
2 , the particle counter should be used in a clean environment to prevent damage to the laser sensor
3. Do not measure mixed gases (such as hydrogen and oxygen) that may react. This gas may also explode inside the counter. These gases need to be measured with the manufacturer for more information.
4. No high pressure decompression equipment (such as high pressure diffuser) should not sample compressed air. All particle counters are designed to operate at one atmosphere.
5. Water, solution or other liquids cannot enter the sensor from the inlet tube.
6. The particle counter is mainly used to test the clean environment of the purification workshop. When there is loose particle material, dust source and spray place, it should be kept at least twelve inches away from the inlet pipe. In order to avoid contamination of the sensor and piping with the above particles and liquids.
7. When sampling, the gas that is discharged from the counter itself or contaminated by the gas from the counter is sampled.
8. When connecting an external printer or connecting an external temperature and humidity sensor, first turn off the counter; when performing the printing operation, the printer must have printing paper, otherwise the print head will be damaged.
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