**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 **h**_{1} and **h**_{2} are measured at times **t**_{1} and **t**_{2}.
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,