How does a auto flush work on a urinal cistern?
Its a simple vertical outlet pipe inside the cistern covered partly by a ‘bell’ which has an inch or so gap at the base. As the water level rises eventually the vacuum inside the bell is compromised, the water enters the bell and flows up and over into the outlet pipe and starts a siphon which empties the water in the tank until the level reaches the bottom of the bell and the siphon is ceased. The tank slowly refills and the process is repeated. The time between flushes depends on the incoming water flow rate.
