Constant Head Flow
Constant head permeameter is recommended for coarse-grained soils only since for such soils, flow rate is measurable with adequate precision. As water flows through a sample of cross-section area A, steady total head drop h is measured across length L.

Permeability k is obtained from:



Falling Head Flow
Falling head permeameter is recommended for fine-grained soils.

Total head h in standpipe of area a is allowed to fall. Hydraulic gradient varies with time. Heads h₁ and h₂ are measured at times t₁ and t₂. At any time t, flow through the soil sample of cross-sectional area A is

--------------------- (1)

Flow in unit time through the standpipe of cross-sectional area a is

= ----------------- (2)

Equating (1) and (2) ,

or

Integrating between the limits,