For example, the selective capture of particles by an aligned, thin-walled circular nozzle is given by Belyaev and Levin as:
where  is particle concentration,  is speed, and the subscript 0 indicates conditions far upstream of the nozzle. The characteristic distance is the diameter of the nozzle.  Here the Stokes number is calculated,
where  is the particles settling velocity,  is the sampling tubes inner diameter, and  is the acceleration of gravity.