# Topics - Column Permeability

**permeability**, when referred to a packed bed, is the volume flow of fluid per unit time per unit area per unit pressure gradient. The expression for the volume flow of fluid through a packed bed is given by D’Arcy’s law. D’Arcy’s law states that the volume flow of fluid through a packed bed is

*directly proportional*to the differential pressure across the bed and the fourth power of the particle diameter and

*inversely proportional*to the product of the fluid viscosity and the column length. In liquid chromatography, there is a maximum pressure that is practical to use, and so, if small diameter particles are to be employed to obtain short plate heights, then the column must have a limited practical length and solvents of the lowest viscosity should be used.