A floating siphon made up of concentric tubes is proposed as a reliable and inexpensive device to discharge water from a reservoir tank at a constant rate independent of the water level in the tank, provided the siphon is discharged freely into the atmosphere. By adjusting the opening area in the outlet tube continuously, any constant discharge can be obtained. With simple aids the siphon has been calibrated to work with an accuracy better than 5 ‰. By means of an electric Water Surface Registration System it is proved that the variation in discharge is better than 1 ‰ at a discharge of 0.3–2.4 1/s.