The components of a dust particle counter include:a measuring chamber, a light source, an air pump filter, a flow monitor, a light detector, and necessary circuit systems.Follow Esky Purify to understand the definition and function of the main components of a dust particle counter!

1. Measuring chamber

The measuring chamber is a space for particle observation, and the collected air must pass through the measuring chamber. The optical system of the instrument allows the light source to irradiate the measuring chamber through the lens and slit, forming a light-sensitive area with a volume of about several cubic millimeters. When dust in the air passes through the photosensitive area, it will scatter a part of the light energy, which is collected by the light-collecting lens at an angle (90 degrees or 70 degrees) to the incident light and then projected onto the photodetector.

2. 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.

There are two types of light sources:ordinary light source and laser light source. Ordinary light source is iodine tungsten lamp, which is large in size, high in heat, short in life, and needs to be preheated after startup. 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 light detector to form a sensor. Common laser light sources include HeNe laser and laser diode.

The dust particle counter using ordinary light source has a very low response to the particle signal below 0.3μm, and its signal amplitude is almost the same as the noise amplitude of the counter itself, so it is difficult to detect the signal from the noise. Although this type of instrument is marked with a 0.3μm channel, it is only suitable for measuring particles larger than 0.3μm, especially those above 0.5μm. Due to the good monochromaticity of the laser and the concentrated and stable light energy, the dust particle counter using laser light source has a higher signal-to-noise ratio. Some of these instruments can detect particles of 0.1μm.

3. Air pump filter

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

4. Flow monitoring

The sampling flow of the dust particle counter is generally 2.83L/min or 28.3L/min. Imported instruments are often marked as 0.1cfm (cubic feet per minute) or 1cfm, mainly to facilitate the calculation of cleanliness in accordance with Fed-Std-209E. Sampling with a large flow rate (28.3L/min) can more accurately reflect the cleanliness of the air, but it reduces the maximum sampling concentration.

5. Photodetector

Photodetector is a photoelectric conversion device that converts scattered light energy into electrical signals. The most 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 of several millivolts to tens of millivolts. They have the advantages of good spectral linearity, fast response time, and low dark current, but the disadvantage is that they are large in size. When working, photomultiplier tubes need to be added with negative high voltage of several hundred 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 exposed to light. It has the characteristics of small size and simple peripheral circuit, and is often integrated with the detection cavity.

VI. Circuit System

After particles of different 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, the particle size and pulse voltage are one-to-one corresponding. For example, the conversion sensitivity of a dust particle counter is 0.3 μm corresponding to 69 mv, 0.5 μm corresponding to 531 mv, 1.0 μm corresponding to 701 mv, etc. If the dust particle counter detects a pulse of 100 mv, the size of the particle must be greater than 0.3 μm and less than 0.5 μm.