The permeability test spans multiple standards, soil or rock types and alternate methods or hydraulic systems for the measurement of hydraulic conductivity of water-saturated porous materials. Water is used as the standard permeant liquid, but other liquids (such as chemical wastes) can be employed. The common methods used are: Constant Head, Falling Head (constant tailwater), Falling Head (rising tailwater), Constant Rate of Flow, Constant Volume-Constant Head and Constant Volume-Falling Head (rising tailwater). The hydraulic conductivity generally decreases with increasing amount of air in the pores, and it will also vary with changing void ratio. The test is repeated at different effective stresses when the relationship between hydraulic conductivity and void ratio needs to be determined.