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The introduction of performance evaluation index of laser particle size analyzer
CLASS: QSI Laser Diode      DATE:2016-3-23    SOURCE:www.longstartech.com.cn

In the past, the particle size analysis method usually adopts the method of screening or settling. Commonly used settlement method has the shortcomings of slow detection speed (especially for small particles), poor repeatability, large error for non spherical particles, not suitable for mixed materials, narrow dynamic range and so on. With the invention of the laser diffraction method, particle size measurement completely overcomes the drawbacks of the settlement method, which greatly reduces the labor intensity and speed up the detection speed of the sample (from half an hour to 1 minutes).

The particle size measurement based on the laser diffraction method is based on the fact that small particles are activated by the scattering angle, and the scattering angle of large particles to the laser is small. By measuring the size of the scattering angle, the particle size can be converted. The optical theory basis for Mie theory and Furlong Hoff theory. Which theory of Fuhuo for many large particles of M theory approximation, the imaginary subset of the Mie theory is ignored, and it is assumed that the opaque particles; and ignore the light scattering and absorption system, that is, set all dispersing agents and quality parameters were 1, so mathematical treatment to much simpler, error of nonferrous materials and small particles is much larger. Similarly, Mie theory is not suitable for the approximation of the emulsion.

In addition, according to the law of Rayleigh scattering, the sixth power of the light scattering intensity and particle diameter is proportional to, inversely proportional to the fourth power of the scattering light and the wavelength of the light source, which means that particle diameter reduced 10 fold, the scattering intensity will be weakened 100 million times. The light source wavelength is shorter, the intensity of scattered light is higher.

Furthermore, due to the small particle scattering angle is large, and the main detector area is limited, generally only received up to 45 degree angle of the scattered light (that is greater than 0.5 micron material). So, how to detect small particles, how to overcome the small particle light scattering of low energy, beyond the scope of the problem the main detector, has become the key to evaluate laser analysis technology.

So, judging the merits of laser particle size analyzer, mainly to see the following:

1 particle size measurement range

Wide size range, suitable for wide application. Not only to see the instruments reported, but look beyond the small particle scattering detection area (< 0.5 m) how to detect. The best way is the full range of direct detection, so as to ensure the consistency of the background subtraction. Hybrid test of different methods, and then the computer fitted a map, certainly will bring the error.

The 2 laser light source

General selection of 2mW laser power is too small, light scattering energy is low, resulting in low sensitivity; in addition, the stability of gas source wave less than solid light source.

3 detector

Because the laser diffraction halo radius is, the weaker light intensity, extremely easy to cause the small particles decrease in signal-to-noise ratio and leakage detection of bolt, so the distribution of small particles can reflect the quality of the instrument. The detector has experienced the development of a circular, semi-circular and fan-shaped stages.

4 channel number

The concept of laser particle size analyzer and the presence of channel, it actually was detected by light area, it has a theoretical and practical optimization value: too few: accept the scattering light is not sufficient, poor accuracy; too much: the sensitivity is too high, resulting in poor reproducibility.

To remedy the defect of sampling rate, some manufacturers use more channels, with the loss of reproducibility and achieve sensitivity, so the measurement time in 20 seconds or more than 1 minutes.

5 whether the use of Mie theory completely

Because of the complexity of the Mie theory, a large amount of data processing, so some manufacturers ignore particle of the refractive index and absorption of optical properties, the approximation of the Mie theory, resulting in limited application range, increase the chances of missing and other issues.

6 correct and repeated index

This index is good and should be detected by NIST standard particle.