At present, there are more and more users of dust particle counters, which are widely used in industries such as medicine, electronics, precision machinery, color tube manufacturing, microbiology, etc., to monitor the purification effect and cleanliness of workbenches, purification rooms, and purification workshops of various cleanliness levels to ensure the quality of products.

1、 Precautions for dust particle counter

1. Do not start the counter when the inlet pipe is covered or blocked

2. Do not test mixed gases that may cause reactions (such as hydrogen and oxygen). This gas may also be generated within the counter. Measuring these gases requires obtaining more information.

3. Do not sample compressed air without high-pressure pressure reducing equipment (such as high-pressure diffusers), all particle counters are designed to operate at one atmosphere.

4. Water, solution or other liquids cannot enter the sensor through the inlet pipe.

5. The dust particle counter is mainly used to test the clean environment of the purification workshop. When there are loose particle materials, dust sources, and spray in the tested area, it must be kept at least 12 inches away from the pipe. To prevent contamination of sensors and pipelines by the aforementioned particles and liquids.

6. When sampling, the gas emitted from the counter itself or contaminated by the gas emitted from the counter is rarely sampled.

7. When there is a printer and an external printer is connected, the counter needs to be turned off first; When performing printing operations, there must be printing paper on the printer, otherwise it may damage the print head.

8. A particle counter with a sensitivity of 0.1 microns, without special precautions, cannot be used to measure purification workshops larger than level 1000 To avoid damaging its sensors.

2、 The principle, application, and composition of a dust particle counter

1 Introduction

At present, there are more and more users of dust particle counters, which are widely used in industries such as medicine, electronics, precision machinery, color tube manufacturing, microbiology, etc., to monitor the purification effect and cleanliness of workbenches, purification rooms, and purification workshops of various cleanliness levels to ensure the quality of products.

A dust particle counter is an instrument used to measure the number and size of particles in the air, providing a basis for evaluating air cleanliness. The common dust particle counter is the light scattering type (DAPC), which measures particle sizes ranging from 0.1-10 μ m. In addition, there is the condensed nucleus type dust particle counter (CNC), which can measure smaller dust particles. This article will introduce a light scattering dust particle counter.

The working principle of a dust particle counter

Particles in the air scatter when exposed to light, a phenomenon known as light scattering. Light scattering is related to factors such as particle size, wavelength of light waves, refractive index of particles, and absorption characteristics of particles towards light. However, in terms of scattered light intensity and particle size, there is a basic rule that the intensity of scattered light from particles increases with the increase of their surface area. In this way, by measuring the intensity of scattered light, the size of particles can be inferred, which is the basic principle of light scattering particle counters.

In fact, the scattered light intensity generated by each particle is very weak, which is a small light pulse. It needs to be amplified by a photoelectric converter to convert the light pulse into an electrical pulse with a larger signal amplitude, and then further amplified and screened by electronic circuits to complete the counting of a large number of electrical pulses. At this point, the number of electrical pulses corresponds to the number of particles, and the amplitude of the electrical pulses corresponds to the size of the particles.

Figure 1 illustrates the specific working principle of the dust particle counter: light from the light source is focused by lens group 1 into the measurement chamber. When each particle in the air quickly passes through the measurement chamber, the incident light is scattered once, forming a light pulse signal. This optical signal is sent to the photodetector through lens group 2, which is proportionally converted into an electrical pulse signal. After amplification and discrimination by the instrument's electronic circuit, the required signal is picked out and displayed through the counting system.

Composition and functions of three dust particle counters

3.1 Light source

The light source is a key component of the dust particle counter and has a significant impact on the performance of the instrument. The light source requires high stability, long lifespan, and no interference.

There are two types of light sources: ordinary light sources and laser light sources. The ordinary light source is an iodine tungsten lamp, which has a large volume, high heat generation, and short lifespan. It needs to be preheated after starting up. The laser light source is a laser with small size, high stability, and long lifespan. It is often integrated with the detection cavity and photodetector to form a sensor. Common laser sources include HeNe laser and laser diode.

The dust particle counter using a regular light source has a low response to particle signals below 0.3 μ m, and its signal amplitude is almost the same as the noise amplitude of the counter itself, making it difficult to detect the signal from the noise. Although this type of instrument is labeled with a channel of 0.3 μ m, it is only suitable for measuring particles larger than 0.3 μ m, especially those larger than 0.5 μ m.

Due to the good monochromaticity and stable concentration of light energy of lasers, dust particle counters using laser light sources have sensors with high signal-to-noise ratios, and some of these instruments can detect particles as small as 0.1 μ m.

3.2 Measurement chamber

The measurement chamber is a space for particle observation, and the collected air needs to pass through the measurement chamber. The optical system of the instrument directs the light source through a lens and a slit into the measurement chamber, forming a photosensitive area with a volume of approximately several cubic millimeters. When dust in the air passes through the photosensitive area, it scatters a portion of the light energy, which is collected by a light collecting lens at an angle (90 degrees or 70 degrees) to the incident light, and then projected onto the photodetector.

3.3 Light detector

A photodetector is a photoelectric conversion device that converts scattered light energy into electrical signals. The commonly used photodetectors in dust particle counters are photomultiplier tubes and photodiodes.

Photomultiplier tubes amplify photoelectrons tens of thousands of times and convert them into electrical signals ranging from a few millivolts to tens of millivolts. They have the advantages of good spectral linearity, fast response time, and low dark current, but the disadvantage is their large size. When the photomultiplier tube works, a negative high voltage of several hundred volts needs to be applied. There is a corresponding high voltage generation circuit in the instrument, and safety should be taken into account when debugging or calibrating the instrument.

Photodiode is a semiconductor component that can generate electrons when exposed to light. 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 a dust particle counter is generally 2.83L/min or 28.3L/min, and the instrument is often labeled as 0.1cfm (cubic feet per minute) or 1cfm, mainly for the purpose of facilitating the calculation of cleanliness in accordance with Fed-STd-209E.

High flow sampling (28.3L/min) can better reflect the cleanliness of the air, but it reduces the concentration of high sampling.

3.5 Air pump and filter

The air pump is located inside the instrument to generate a sampling flow rate. The air pump requires low noise, minimal vibration, and stable airflow. The filter should be able to filter out particles larger than 0.3 μ m to prevent the air discharged from the instrument from affecting the clean area.

3.6 Circuit System

After being converted by the photoelectric system of the dust particle counter, particles of different sizes will generate electrical pulse signals of different amplitudes (voltages). The larger the particle size, the higher the pulse voltage. The relationship between signal voltage and particle size, also known as conversion sensitivity. For a given dust particle counter, the particle size corresponds one-to-one with the pulse voltage. For example, the conversion sensitivity of a dust particle counter is 0.3 μ m corresponding to 69mv, 0.5 μ m corresponding to 531mv, 1.0 μ m corresponding to 701mv, etc. If the dust particle counter detects a pulse of 100mv, then the particle size must be greater than 0.3 μ m and less than 0.5 μ m.

A dust particle counter is an instrument that measures the number of particles larger than or equal to a certain particle size, and its internal circuit is a circuit that counts the number of pulses larger than or equal to a certain voltage value. For the example in the previous paragraph, measuring the number of particles greater than or equal to 0.3 μ m in the air is equivalent to 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 μ m, counting the number of pulses greater than or equal to 531mv in the circuit, and so on. So the measurement of dust particles by instruments mainly relies on the parameter of conversion sensitivity.

It should also be noted that the conversion sensitivity of each dust particle counter is different, and it must be calibrated regularly with standard particles at the factory and thereafter to obtain the best conversion sensitivity value.

The circuit system is a circuit that amplifies, identifies, and counts pulse signals. In addition, it also includes circuits such as power supply, control, display, calculation, and printing.

Zhejiang Sujing Purification Equipment Co., LtdLocated in Daoxu Town, Shangyu District, Shaoxing City, Zhejiang Province, it is one of the manufacturers in the purification equipment industry in East China. We can provide air purification system engineering design, construction, testing, and technical services for industries such as microelectronics, biomedicine, hospital operating rooms, fiber optic cables, food and beverage, precision instruments, semiconductors, and new material applications in accordance with ISO14644-1 standards, GB50073-2001 national standards, and national GMP specifications.
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