In granulometry, the particle-size distribution (PSD) of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size.

Particle size distribution can greatly affect the efficiency of any collection device.

Settling chambers will normally only collect very large particles, those that can be separated using sieve trays.

In these systems, the scrubbing liquid (usually water) comes into contact with a gas stream containing dust particles.

The greater the contact of the gas and liquid streams, the higher the dust removal efficiency.

A simple treatment assumes the particles are spheres that will just pass through a square hole in a "sieve".

The material to be analysed must be carefully blended, and the sample withdrawn using techniques that avoid size segregation, for example using a rotary divider[3]p. 5.

Alternatively, the sample may be washed through with a non-reacting liquid (usually water) or blown through with an air current.

Disadvantages: many PSDs are concerned with particles too small for separation by sieving to be practical.

Over-energetic sieving causes attrition of the particles and thus changes the PSD, while insufficient energy fails to break down loose agglomerates.

Material may be separated by means of air elutriation, which employs an apparatus with a vertical tube through which fluid is passed at a controlled velocity.

This technique determines particle size as a function of settling velocity in an air stream (as opposed to water, or some other liquid).

Photoanalysis equipment and software is currently being used in mining, forestry and agricultural industries worldwide.

This is impossibly arduous when done manually, but automated analysis of electron micrographs is now commercially available.

The results are only related to the projected cross-sectional area that a particle displaces as it passes through an orifice.

This is a physical diameter, not really related to mathematical descriptions of particles (e.g. terminal settling velocity).

These are based upon study of the terminal velocity acquired by particles suspended in a viscous liquid.

Typical apparatus disperses the sample in liquid, then measures the density of the column at timed intervals.

Other techniques determine the optical density of successive layers using visible light or x-rays.

Advantages: this technique determines particle size as a function of settling velocity.

These depend upon analysis of the "halo" of diffracted light produced when a laser beam passes through a dispersion of particles in air or in a liquid.

Advances in sophisticated data processing and automation have allowed this to become the dominant method used in industrial PSD determination.

A focused laser beam rotates in a constant frequency and interacts with particles within the sample medium.

Instead of light, this method employs ultrasound for collecting information on the particles that are dispersed in fluid.

[5] There have been hundreds of papers studying ultrasound propagation through fluid particulates in the 20th century.

The resulting ultrasound attenuation frequency spectra are the raw data for calculating particle size distribution.

However, as concentration increases and the particle sizes approach the nanoscale, conventional modelling gives way to the necessity to include shear-wave re-conversion effects in order for the models to accurately reflect the real attenuation spectra.