Average Particle Size Definition: F.S.S.S, D50, APS & Mesh

The average particle size refers to the diameter of a particle swarm composed of powder particles of different sizes and shapes. Compared to a hypothetical particle swarm composed of uniform spherical particles, if the total length of the two particles is the same, the diameter of the spherical particles is called the average particle size of the actual particle swarm.

Definition of Different Average Particle Sizes

Linear Average Particle Size

Linear average particle size refers to adding the diameters of all particles in a sample and dividing by the total number of particles. The average diameter of the particles in the sphere is equal to the arithmetic mean of the diameters of all particles.

Area Average Particle Size

The area average particle size refers to the average diameter of a particle whose surface is equal to the average surface area of all particles in the particle. The surface area of one particle with this diameter is exactly equal to the average surface area of all particles.

Volume Average Particle Size

The volume average diameter refers to the volume of one particle with this diameter, which is exactly equal to the volume average of all particles, and is called the volume average particle size.

Weight Average Particle Size

Assuming that the density of all particles is equal (which is a reasonable assumption in most cases), the weight average particle size refers to the weight of one particle with this diameter, which is exactly equal to the average weight of all particles.

Specific Surface Area Average Particle Size

The average particle size refers to the specific surface area (surface area per unit volume of particles) of a particle with this diameter, which is exactly equal to the average specific surface area of all particles.

Other Average Particle Sizes

In addition, there are many other average diameters, such as resistance diameter (based on the resistance of particles in the fluid), free fall diameter (based on the velocity of particles falling freely in the fluid), perimeter diameter (based on the projected perimeter of particles), sieve diameter, etc.

Measurement Method for Average Particle Size

There are many methods for powder particle size testing. There are five commonly used methods: settlement method, laser method, screening method, image method, and resistance method. In addition, there are several testing methods commonly used in specific industries and fields. Specifically, as follows:

Laser Method

The laser method is used to test the particle size distribution by observing the phenomenon of diffraction or scattering caused by laser irradiation on particles. The commonly used measuring instrument is a laser particle size analyzer. The average particle size is represented by D50 value.

Screening Method

The sieving method is one of the most traditional particle size testing methods. It tests particle size by passing particles through sieve holes of different sizes. A single sieve can be used to control the pass rate of a single particle size, or multiple sieves can be stacked together to simultaneously measure the pass rate of multiple particle sizes and calculate the percentage. The commonly used measuring instrument is a sieve. The average particle size is expressed in mesh size.

Microscopic Imaging Method

Microscopic imaging method consists of a microscope, CCD camera (or digital camera), graphics acquisition card, computer, and other components. Its basic working principle is to transmit the magnified particle images of the microscope to the computer through a CCD camera and a graphics acquisition card, and the computer performs edge recognition and other processing on these images to calculate the projection area of each particle. Based on the principle of equivalent projection area, the particle size of each particle is obtained, and then the number of particles in the set particle size range is counted to obtain the particle size distribution. The commonly used equipment is SEM electron microscope or microscope. The average particle size is represented by APS.

Fisher Method

Fisher particle size is a powder particle size value, and the basic testing method is the steady flow air permeation method, which measures the specific surface area and average particle size under constant air flow velocity and pressure. The Fisher method is a relative measurement method that cannot accurately determine the true particle size of powder and is only used to control the process and product quality. The average particle size is measured using F S. S.S represents.

Comparison of Different Average Particle Sizes

There are many methods to test the average particle size, but the results of different testing methods are significantly different from the actual values. Below is a comparison of the most common average particle sizes in powder. It can be used as a reference:

Item	F.S.S.S	D50	APS	Mesh
Definition	Fisher Particle Size	Laser Average Particle Size	Average Particle Size	Mesh
Instrument	Fisher Sub-Sive Sizer	Laser Particle Size Analyzer	SEM	Screen Mesh
Characteristic	Relative Measurement	Close to the True Value	Close to the True Value	Rough Measurement
Accuracy	Relatively large	Close to the True Value	Close to the True Value	Rough Distribution
Applicable	Irregular Powders	All Shapes of Powders	All Shapes of Powders	All Shapes of Powders
Grade	Fine & Coarse	Ultrafine, Fine & Coarse	Submicron & Nanopowders	Fine & Coarse