In a weightless or almost weightless state (e. g. in a satellite station), the head (H) of a centrifugal pump tends towards infinity because g ≈ 0, as a result of its definition. In the case of any centrifugal pump installed a long distance away from the earth, the finite specific energy (Y) should be adopted instead of the head. It is the useful mechanical energy transmitted by the pump onto the fluid, per unit mass of fluid handled (the latter being independent of the acceleration due to gravity). The same applies in accelerated or retarded systems (e. g. in a rocket). The following correlation exists between specific energy (Y) and the head (H):

Y = g • H 

Acceleration due to gravity is the acceleration (which varies according to geographic location) caused by the earth's gravitational field which a body experiences irrespective of friction when in freefall to the earth's surface.

The symbol is g. The exact individual values are dependent on the centrifugal force resulting from the earth's rotation, the earth's oblateness and the elevation profile, and vary from one another by less than 7 ‰. The average value is 9.81 m/s2, as is also used in centrifugal pump technology. It is also used in accordance with ISO 9906 and EN 12723 in many definition equations.