How does a float valve stop a tank filling?

A float valve links a tank's water level to its incoming flow. The float displaces water, which pushes upward on it; it settles at the surface where that upward support balances its weight. When more water raises the surface, the float rises too. A mechanical arm passes that movement to a valve that controls the inlet.

In the close-on-rise arrangement shown, the inlet admits water while the float is below its closing position. More incoming water raises the surface, and the float rises with it. The arm then moves the valve toward closure. At the marked level, the ideal valve stops the incoming flow.

This is feedback because the result of filling changes the mechanism controlling the filling. A rising level acts against continued inflow rather than encouraging more of it. The float both responds to the measured condition and transmits a mechanical signal; no electrical sensor or computer is needed in this illustration.

When water is drawn from a tank using this kind of valve, the falling surface lets the float descend and the linkage can reopen the inlet. The device responds again as the level rises. This simple repeated action joins a measurement and a correction in the same moving parts. Real float valves may gradually reduce the incoming flow as they approach closure rather than switching abruptly.