Abstract: A system for the automatic measurement of the level of material in a container and especially of liquid flowing through a channel is described. An echolocation system operates to detect returns of reflected sonic signals from the surface of the liquid in the channel during successive transmission cycles. A source of pulses is provided having a frequency which provides a certain number of pulses corresponding to the travel time of the sonic signals over a distance equal to a maximum predetermined level or head of the liquid in the channel. The time interval between signal transmission and reception in excess of twice the time for travel of the signals between the transmitter and the maximum level of liquid is a measurement of the liquid level in the channel.