Because of the way initial values of auxiliary dependent variables are sampled in particle release features, Equation 5-31 will be correct to an extremely high degree of precision even if the number of particles is relatively small. In contrast,
Equation 5-32 will only approximately be respected, with the standard deviation over released particles approaching the specified value
σ as the number of particles
N is increased.
where CMD (SI unit: m) is the count median diameter and
GSD (dimensionless) is the geometric standard deviation. In general
GSD should be greater than 1. By definition, half of the released particles have diameter greater than the count median diameter.
For a lognormal distribution, the mean diameter μ, count median diameter CMD, and Sauter mean diameter
dsm are related by
The equations relating CMD,
μ, and
dsm are special cases of a general class of conversion equations known as the Hatch–Choate Equations, following a general form
where dA is some type of average diameter yet to be specified and
b is a dimensionless constant. The values of
b for some typical average quantities are given in
Table 5-4.