The design of subsurface drainage systems depends on the saturated hydraulic conductivity (or permeability) of the soil.

The graphs produced by the permeameter allow an overall interpretation of the infiltration tests, from the beginning during the phase of saturation. 

K versus time for a slightly clayey sandy soil
K versus time for a slightly clayey sandy soil

The graph of K versus time will usually show a fast decrease in the K value, which tend to stabilize with time, when the water infiltration-flow becomes steady.

The K value at saturation is called the Ksat or Kfs.

The time of saturation of the soil will depend of the type of soil, of the initial water content, and water temperature during the test.

A graph of the time necessary to attain a steady state flow which we call saturation phase, shows that most of the silty and sandy soils have a period of saturation not exceeding 4 hours. The sandy clays need 2 to 4 hours, when the sandy silts do not exceed 2 hours.

The clayey soils tend to have longer saturation period, more than 4 hours is not uncommon at all. The maximum time recorded in the region of Armorican massif in France, has been ...12 hours for a clayey silt.