Abstract: Pump-off control apparatus for preventing cavitation of a sub-surface pump which provides artificial lift for fluid produced from a fluid bearing strata located downhole in a borehole. A sound wave is focused downhole and travels down the casing annulus where it strikes the liquid surface and rebounds to a receiver located near the well head. The time lapse of the traveling wave is used to measure the relative location of the liquid surface relative to the pump inlet, thereby enabling the control apparatus to shut-in a well before it can encounter a pump-off condition which can cause fluid pounding.