In an open channel in which liquid flows, there are provided a first flow velocity measuring device and a second liquid level or height measuring device. At flows above a predetermined level the first and second devices are coupled to give a flow quantity readout. Below the predetermined flow level the height measuring device is automatically uncoupled from the velocity measuring device and is coupled to a computer in which the output of the height measuring device is substituted in a known formula relating height with flow quantity to give a flow quantity readout, the readouts for all levels being substantially continuous.

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