Equivalent spherical diameter

The equivalent spherical diameter of an irregularly shaped object is the diameter of a sphere of equivalent geometric, optical, electrical, aerodynamic or hydrodynamic behavior to that of the particle under investigation.

The particle size of a perfectly smooth, spherical object can be accurately defined by a single parameter, the particle diameter. However, real-life particles are likely to have irregular shapes and surface irregularities, and their size cannot be fully characterized by a single parameter.

The concept of equivalent spherical diameter has been introduced in the field of particle size analysis to enable the representation of the particle size distribution in a simplified, homogenized way. Here, the real-life particle is matched with an imaginary sphere which has the same properties according to a defined principle, enabling the real-life particle to be defined by the diameter of the imaginary sphere.  

The principle used to match the real-life particle and the imaginary sphere vary as a function of the measurement technique used to measure the particle.