The six components of a dust particle counter

The light source is the key component of the dust particle counter and has a great influence on the performance of the instrument. The light source requires high stability, long life, and no interference.
1. Light source
There are two types of light sources: ordinary light sources and laser light sources. The common light source is iodine tungsten lamp, which is large in size, high in heat generation and short in life. It needs to be preheated after starting up. The laser light source is a laser, which is small in size, high in stability and long in life. It is often integrated with the detection cavity and the photodetector to form a sensor. Common laser light sources include HeNe lasers and laser diodes.
The dust particle counter with ordinary light source has a very low response to the signal of particles below 0.3 μm, and its signal amplitude is almost the same as the noise amplitude of the counter itself, and the signal is difficult to detect from the noise. Although such instruments are marked with a channel of 0.3 μm, they are only suitable for measuring particles larger than 0.3 μm, especially above 0.5 μm.
Due to the good monochromaticity of the laser and the stable light energy concentration, the dust particle counter using the laser light source has a high signal-to-noise ratio of the sensor, and some of these instruments can detect particles of 0.1 μm.
2. Measuring cavity
The measurement chamber is a space for particle observation, and the collected air must 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 photosensitive area, a part of the light energy is scattered, collected by the collecting lens at an angle (90 degrees or 70 degrees) to the incident light, and then projected on the photodetector.
3. Photodetector
Photodetectors are photoelectric conversion devices that convert scattered light energy into electrical signals. Photodetectors commonly used in dust particle counters are photomultiplier tubes and photodiodes.
The photomultiplier tube amplifies the photoelectrons by tens of thousands of times and converts them into electrical signals of several millivolts to tens of millivolts. It has the advantages of good spectral linearity, fast response time, and small dark current. 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 element 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.
4. Traffic monitoring
The sampling flow of the 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 cleanliness in line with Fed-Std-209E .
Sampling with large flow (28.3L/min) can more accurately reflect the cleanness of the air, but it reduces the concentration of large sampling.
5. Air pump and filter
The air pump is located inside the instrument and causes the instrument to generate the sample flow. The air pump requires low noise, low vibration and stable airflow. The filter should be able to filter out particles above 0.3μm, so as to avoid the impact of the air exhausted from the instrument on the clean area.
6. Circuit system
After the particles of different particle sizes are converted by the photoelectric system of the dust particle counter, electrical pulse signals of different amplitudes (voltages) will be 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, there is a one-to-one correspondence between the particle size and the pulse voltage. For example, the conversion sensitivity of a dust particle counter is 0.3μm corresponds to 69mv, 0.5μm corresponds to 531mv, 1.0μm corresponds to 701mv, etc. When a pulse of 100mv is detected, the particle size must be greater than 0.3μm and less than 0.5μm.
The dust particle counter is an instrument that measures the number of particles greater than or equal to a certain particle size, and its 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, to measure the number of particles greater than or equal to 0.3μm in the air, in the circuit is to count the number of pulses greater than or equal to 69mv, and to measure the number of particles greater than or equal to 0.5μm, in the circuit is to count the number of pulses greater than or equal to 531mv number, and so on. Therefore, the measurement of dust particles by the instrument mainly depends on the parameter of conversion sensitivity.

In addition, it should be noted that the conversion sensitivity of each dust particle counter is different. It must be calibrated with standard particles regularly at the factory and thereafter to obtain the best conversion sensitivity value.
The circuit system is the circuit that completes the amplification, discrimination and counting of the pulse signal. In addition, it also includes power supply, control, display, calculation, printing and other circuits.