Bosanquet’s approximation (Pollard and Present) is an estimation of the effective diffusivity at intermediate Knudsen numbers. As stated in the DiffuDict theoretical background, the formula is  

It computes the effective diffusivity from bulk and Knudsen diffusivity based on the idea that the effect can be described as the sum of two different resistances to the particle movement.

The Bosanquet Approximation command uses the results of the Bulk (Laplace) Diffusion and the Knudsen Diffusion commands to compute the resulting diffusivity . However, both commands have not computed and , respectively, but the corresponding relative diffusivity.

For unary gases, the diffusivities and can be determined from these dimensionless matrices with the help of the mean thermal velocity and the mean free path :

where is the characteristic length as used in the Knudsen Diffusion command. With the definition of the Knudsen number

it becomes obvious that Bosanquets formula delivers for Kn=0 and for and in general the Knudsen number describes which of the two diffusivities is dominant.

For binary gases, equation (176) no longer holds true, but is possible to use equation (166) where describes a measurable quantity:

In this case, Bosanquets formula still holds true, but has to be computed manually, and DiffuDict’s Bosanquet Approximation command cannot be used to compute it.