Automatic siphons allow urinals to be flushed with a pre-determined quantity of water at regular intervals. Siphons are simple devices which contain no moving parts and consequently are extremely reliable. The quantity of water discharged during each flush is determined by 2 factors: the area of the cistern in plan view (i.e. looking down onto the top of the cistern) and the height of the automatic siphon. The interval between flushes is regulated by controlling the flow of water into the cistern via a Petcock or Smart Flush Unit.