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A brief analysis of the basic concept of particle size test of PM2.5 sensor

Update: 2019-12-23 16:58 Source: LUFTMY Reading: ↻ News

PM2.5 particles are very harmful to human body. Particles enter human body through human respiratory system and directly affect lungs. Moreover, small particles can even enter blood and other organs directly. The average person breathes about 10000 liters of air every day. Once there are pollutants in the air, human health will be harmed. Let's understand the basic concept of PM2.5 sensor granularity test.

What are particles?

Particle is a small object with a certain size and shape, which is the basic unit of powder. It's very small at the macro level, but it contains a lot of molecules and atoms at the micro level.

What is granularity?

The particle size is called the particle size.

What is particle size?

The diameter of a particle is called particle size, which is usually expressed in micrometers or nanometers.

What is the equivalent particle size?

When a particle has the same or similar physical properties as a homogeneous spherical particle, we use the diameter of the spherical particle to represent the diameter of the actual particle. The particle size measured by laser method is generally considered as equivalent volume size, that is, the diameter of homogeneous spherical particles with the same volume as the measured particles.

Why use the concept of equivalent particle size?

Because the actual shape of particles is usually non spherical, it is difficult to directly use the particle size value to express its size, and the diameter is the simplest quantity to describe the size of a geometry, so the concept of equivalent particle size is adopted.In short, the particle size is the particle diameter. The laser particle sizer uses particles to make an equivalent comparison of the scattering characteristics of the laser. The equivalent particle size measured is the equivalent scattering particle size, that is, the diameter of the spherical particles with the same scattering effect as the actual particles is used to represent the size of the actual particles. When the measured particle is spherical, its equivalent particle size is its actual diameter.

Average diameter: data indicating the average particle size. There are many different averaging algorithms, for example, d [4,3] is the volume or mass momentum average.

What is the physical meaning of D [4,3]?

For the average value in the general sense, it is the ratio between an accumulated value and a quantity, which is called the arithmetic average value. If we use this method to calculate the average particle size, we need to know the number of particles. D [4,3], also known as volume average diameter, can be obtained by laser method.

What is the accuracy of granularity testing and how is it calculated?

The accuracy of particle size test refers to the error between the measurement result of a certain instrument for the particle size standard sample and the nominal value of the standard sample. The algorithm is as follows:

Here: X is the average value of multiple measurement results D50;

D is the nominal value of the standard sample;

Δ is accuracy error

What is the resolution of the particle sizer?

The resolution of the particle sizer refers to the minimum difference between different particle sizes of two samples that can be distinguished, or the minimum difference between two peaks that can be distinguished for bimodal samples.

How to verify the resolution of laser particle sizer?

Test two samples with small particle size difference to see whether the results can correctly reflect the small difference of samples.

Test a bimodal sample to see if the results can clearly distinguish the bimodal phenomenon. If baseline separation is achieved, the resolution is better.

Find two samples with narrow particle size distribution, test one first, and then slowly add another sample. If the result shows double peaks, it means that the resolution of the instrument is good. If only the particle size data changes without double peaks, it means that the resolution is poor.

What factors affect the resolution of laser particle sizer?

The number and structure of detectors: the more the number of detectors, the more reasonable the structure, the higher the resolution of the system.

Inversion algorithm: the quality of inversion algorithm determines the resolution.

Optical path system: the resolution of the system is determined by the small difference of the Fourier lens and the parallelism of the parallel light.

Dispersion system: sufficient dispersion, uniform mixing and smooth circulation are conducive to ensuring the resolution of the system.

What is the complex scattering phenomenon in laser particle size measurement?

Laser particle size measurement is realized by receiving and recognizing the scattered light caused by particles. The complex scattering phenomenon is the phenomenon that the scattered light meets other particles in the process of propagation and is re scattered.

What is the effect of complex scattering on the results of laser particle size measurement?

According to Mie's scattering theory, particles with a certain particle size generate scattering light with a fixed angle. Directly receiving and recognizing these scattering light will get the accurate particle diameter corresponding to them. If the received and recognized complex scattered light signals do not conform to the law of Mie scattering theory, the wrong results will be obtained and the resolution of the system will be reduced.

Therefore, in order to prevent people from breathing and inhaling too many harmful particles, PM2.5 sensor is needed to monitor PM2.5 in the air. PM2.5 sensor is also called dust sensor. PM2.5 sensor can be used to detect the dust concentration in the air around us, that is, the value of PM2.5.

LD16 Laser dust sensor
LD16 Laser dust sensor

The working principle of PM2.5 sensor is developed according to the light scattering principle. Particles and molecules will scatter light and absorb part of the energy of light. When a beam of parallel monochromatic light is incident on the particle field to be measured, it is affected by the scattering and absorption around the particle, and the light intensity is attenuated.

In this way, the relative decay rate of the incident light passing through the concentration field to be measured can be obtained. The relative decay rate basically reflects the relative concentration of dust to be measured. The intensity of light is proportional to the intensity of the electrical signal converted by photoelectricity. By measuring the electric signal, the relative attenuation rate can be obtained, and then the dust concentration in the field to be measured can be determined.

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