The concentration entrance length describes the length needed for the concentration profile in a flow to be fully developed. The concentration entrance length can be determined by relating it to the hydrodynamic entrance length with the Schmidt number or by experimental techniques. The Schmidt number describes the ratio of momentum diffusivity to mass diffusivity.
where
*  is the concentration entrance length,
*  is the pipe inner diameter,
*  is the Reynolds number (based on the pipe diameter), and
*  is the Schmidt number.