In this test, water is forced by a known constant pressure through a soil specimen of known dimensions and the rate of flow is determined. This test is used primarily to determine the suitability of sands and gravels for drainage purposes, and is made only on remolded samples 

A typical hydraulic control system comprises a hydraulic ram/cylinder coupled to the lift car, which is supplied with hydraulic fluid through a two-position control valve (fast and slow speed) by an electrically-driven pump submerged in an oil tank