Abstract: A supply pipe having its inlet end adapted for communication with a source of a liquid under pressure in excess of that of the ambient medium and its outlet end communicating via a nozzle with a shroud adapted to communicate with a vessel containing therein a fluid, at a level being monitored. The variation of the hydrodynamic condition of the flow of the liquid through the nozzle is registered dependent on the mixing of this liquid flow with the fluid being monitored. The shroud has at least one cross-section of which the ratio of the maximum size of the internal contour to the spacing of this cross-section from the outlet section of the nozzle is less than of 0.35:1. The inlet portion of the shroud is a structure wherein the internal contour of the cross-section of this portion encompasses therein the contour of the outlet section of the nozzle.