Principle characteristics and application status of laser particle size analyzer

In recent years, there are many testing techniques and methods related to particle size distribution, and the laser particle size analysis method is generally accepted by people because of its characteristics of fast measurement speed, high accuracy and good accuracy. As the name implies, since the laser particle sizer is a particle sizer, it is of course used to measure particles. It takes advantage of the monochromatic and extremely strong directivity of the laser. The laser particle sizer is recognized as the most advanced and fastest particle testing instrument in the world.

Measuring principle of laser particle size analyzer

Laser particle size analyzer is used to measure the particle size distribution according to the physical phenomenon that particles can scatter laser light. Because of the good monochromatic and strong directivity of the laser, the laser will irradiate to the infinity in the unimpeded infinite space, and there is little divergence in the propagation process.

Michaelis scattering theory shows that when the light beam meets the particle barrier, some of the light will be scattered, and the propagation direction of the scattered light will form an angle with the propagation direction of the main beam θ，θ The size of the angle is related to the size of the particles. The larger the particles, the smaller the scattering light generated θ The smaller the angle; The smaller the particles, the smaller the θ The larger the angle. I.e. small angle（ θ) The scattered light is caused by large particles; Large angle（ θ 1) Of the scattered light is caused by small particles. Further studies show that the intensity of the scattered light represents the number of particles with this particle size. In this way, the particle size distribution of the sample can be obtained by measuring the intensity of scattered light at different angles.

In order to measure the light intensity of scattered light at different angles, the scattered light needs to be processed by optical means. We place a Fourier lens at an appropriate position in the beam and a group of multi-element photodetectors on the rear focal plane of the Fourier lens. When scattered light from different angles is irradiated on the multi-element photodetectors through the Fourier lens, the optical signals will be converted into electrical signals and transmitted to the computer. These signals will be processed by special software to accurately obtain the particle size distribution.

Characteristics of laser particle size analyzer

1. Wide range of measured particle size

The laser particle size analyzer can carry out such a wide range of particle size distribution from nanometer to micron. About: 20nm ~ 2000 μ m. Up to 3500 in some cases μ m； Since there is no need to change the lens and adjust the optical system during the use of the instrument, the stability of the system is improved and the operation process is simplified.

2. Wide application range

Laser particle size analyzer can measure not only solid particles, but also particles in liquid.

3. Good reproducibility

Compared with the traditional method, the laser particle size analysis method is not affected by many factors such as temperature change, medium viscosity, sample density and surface state. As long as the sample to be measured is uniformly displayed in the laser beam, the laser particle size analyzer can give accurate and reliable measurement results.

4. Fast measurement speed

The whole measurement process can be completed in about 2 minutes. Some instruments have realized real-time detection and real-time display, allowing users to observe and monitor samples during the whole measurement process.

5. Simple operation

The laser particle size analyzer can automatically complete the functions of data acquisition, analysis and processing, result saving, printing, etc., with simple operation and high automation.

Application status of laser particle size analyzer

The earliest equipment used to measure the particle size is the standard sieve, but it can only measure the screen balance on one or several particle size points, and cannot give a detailed particle size distribution; And the labor intensity of the test is large and the accuracy is low. Later, it was developed to measure with a sedimentation type particle size meter. Although it can measure the detailed particle size distribution, it is cumbersome, poor repeatability and narrow measurement range. The latest method is laser particle size analyzer. Because of its wide measuring range, good repeatability, fast speed and easy operation, it is very suitable for the use of color glaze industry. The popularization and application of laser particle sizer will obviously play a positive role in promoting the technical progress of China’s color glaze industry. In the process of promotion, there are also some problems, mainly in two aspects:

1. Comparison with traditional screening method

The color glaze industry traditionally uses sieving method to detect the particle size of products. Although the method is backward, it has been widely recognized in the whole industry. Therefore, some users who use the laser particle sizer for the first time always compare the cumulative value of the laser particle sizer at a certain particle size point with the percentage under the screen, and require that the two be consistent. Intuitively, this comparison is natural. However, the particle size measurement is completely different from the general physical and chemical measurement: the test results of the two particle size measurement methods with different principles should be inconsistent, but the consistent ones are special cases or manually adjusted.

Due to the constraints of traditional concepts, thinking habits or traditional production processes, sometimes users do not understand the above differences or strongly require the instrument supplier to modify the test results of the laser particle size analyzer to make them consistent with the traditional results. Since the laser particle sizer uses a computer as a component, it is not difficult to make this modification. The problem is that the modified test results are distorted. Therefore, it is better for the user to gradually accept the results of the laser particle sizer. As we all know, the laser particle size analyzer is a very mature instrument, and it is much more advanced than the screening method.

2. Quantity comparison between different laser particle sizers

Sometimes users will find that the test results given by different laser particle sizers are different. The reasons for this phenomenon include the difference of optical structure of the instrument, the difference of data analysis software, the drift of the working state of the instrument, the artificial result modification (for example, to compare with the screening results), and so on. The effective way to solve this problem is to prepare standard samples suitable for the industry to calibrate all instruments. This requires the cooperation and efforts of the whole industry.

In the actual test, the accuracy is relative and the repeatability is absolute: in the actual particle test, we cannot evaluate the “accuracy” or “inaccuracy” of the results of the laser particle sizer test, because the samples are irregular shaped bodies, and the test results compared with them are only obtained by another instrument. Therefore, we can only say that the test results of instrument a are accurate relative to that of instrument B, However, it can not be said that the test results of instrument a are accurate or inaccurate. Many testers always like to compare them with Marvin’s. as long as they are consistent, they are accurate, and as long as they are not consistent, they are inaccurate. This is actually a misunderstanding. In fact, the standard for considering the quality of the instrument is repeatability. Take samples of a certain type of sample for testing. If the difference between the previous and the next test results is small, it means that the instrument is stable and has good repeatability. If the difference is large, it means that the test effect of the instrument is not good.