 Hydraulic head or piezometric head is a specific measurement of liquid pressure above the geodetic datum. It is usually measured as a liquid surface elevation, expressed in units of length, at the entrance or bottom of a piezometer. In an aquifer, it can be calculated from the depth to water in a piezometric well of specialized water well and given information of the piezometer's elevation and screen depth. Hydraulic head can similarly be measured in a column of water using a standpipe piezometer by measuring the height of the water surface in the tube relative to a common datum. The hydraulic head can be used to determine the hydraulic gradient between two or more points. In fluid dynamics, head is a concept that relates the energy in an incompressible fluid to the height of an equivalent static column of that fluid. From Bernoulli's principle, the total energy at a given point in a fluid is the energy associated with the movement of the fluid, plus energy from static pressure in the fluid, plus energy from the height of the fluid relative to an arbitrary datum. The term is expressed in units of height such as meter or feet. The static head of a pump is the maximum height pressure a can deliver. The capability of the pump at a certain RPM can be read from its QH curve flow versus height. A common misconception is that the head equals the fluid's energy per unit weight, while, in fact, the term with pressure does not represent any type of energy in the Bernoulli equation for an incompressibly fluid. This term represents work of pressure forces. Head is useful in specifying centrifugal pumps because their pumping characteristics tend to be independent of the fluid's density.