The sedimentation method is based on the principle that the settling of particles in liquid conforms to the Stokes' law, and calculates the particle size distribution based on the final settling velocity of the particles in the liquid. In practical operation, it is difficult to measure the final settling velocity of the tested particles. Therefore, all sedimentation instruments measure other physical parameters related to the final settling velocity, such as pressure, density, weight, concentration or light transmittance, to obtain the particle size distribution.
The sedimentation method can be divided into two types: gravity sedimentation and centrifugal sedimentation. The testing range of gravity sedimentation is usually 0.5-100μm, and the measurable particle size range of centrifugal sedimentation is 0.05-5μm. Currently, most sedimentation particle analyzers generally use a combination of gravity sedimentation and centrifugal sedimentation.
The resistance method, also known as Coulter counting method, is suitable for measuring particles with a relatively narrow size distribution. Its working principle is relatively simple: particles suspended in the electrolyte pass through a small hole made of ruby under negative pressure, and a resistance-type sensor composed of two platinum electrodes inserted on both sides of the small hole detects an increase in electrode resistance when the particles pass through the small hole, producing a voltage pulse. The amplitude of the pulse corresponds to the volume and the corresponding particle size, and the number of pulses corresponds to the number of particles. By counting all the measured pulses and determining their amplitudes, the size of the particles can be obtained, and the particle distribution can be calculated. The resistance method is the highest resolution particle sizing technique currently available.
The laser diffraction method is the most mature and widely used method in scattered laser particle size measurement technology. It uses the distribution of the scattered light energy of particles in a small angular range forward to calculate the particle size and distribution using classical scattering theory and the Fraunhofer theory applicable to large particles. For particles with a larger particle size, the scattering in the forward small angular range is mainly diffraction, so the small forward scattering method is also called diffraction method.
The laser diffraction method for carbon silicon carbide powder such as black silicon carbide powder and green silicon carbide powder has a wide range of particle size measurement, high measurement accuracy, good repeatability, fast measurement speed, requires fewer physical parameters, and can be measured online, so it is widely used. By comparing the test results of the three most commonly used particle size analyzers: laser particle sizer, Coulter counter, and centrifugal particle size analyzer, and analyzing the standard deviation of D50 value, it can be found that the average standard deviation of the laser particle sizer is 0.008, the average standard deviation of the Coulter counter is 0.097, and the average standard deviation of the centrifugal particle size analyzer is 0.219. Therefore, the laser particle sizer has the smallest standard deviation in measuring the DS0 value, the best measurement repeatability and high accuracy, while the centrifugal particle size analyzer has relatively poor measurement repeatability and accuracy.