# Schmidt number

The **Schmidt number** (**Sc**) is a dimensionless number defined as the ratio of momentum diffusivity to the mass diffusivity, and is used to characterize flow in which there are both simultaneous momentum and mass diffusion convection processes. The Schmidt number physically relates the relative thickness of the hydrodynamic layer and mass-transfer boundary layer. It is defined as

where:

- is the kinematic viscosity (/)
- is the diffusion coefficient
- is the dynamic viscosity of the fluid
- is the density of the fluid.

The heat transfer analog of the Schmidt number is the Prandtl number (Pr). The Schmidt number typically ranges **0.5-2** for gasses and **100-10,000+** for liquids.