Measurement of Very Low Velocities of Water-flow

Abstract

THE need has been felt for some years past for an instrument which will measure low velocities of water-flow over the cross-section of an open channel or a pipe. The use of the Pitot tube, which has proved so valuable in the case of the flow of gases, becomes quite impracticable in the case of slow-moving liquids owing to the difficulty of measuring the extremely small pressure differences involved and also to the need for strict thermal control of the connecting tubes. Rotating-vane flow-meters are likewise impracticable at low flow values, because of uncertain spindle friction. Exposed hot wires accumulate gas bubbles around themselves, and the enclosed type becomes excessively slow in response as the velocity is decreased.