A float valve is a device which can be used to control the level of a fluid in a tank by opening and closing a valve in response to changes in the fluid level. One of the most famous examples of a float valve is found in the ballcock found in toilets around the world, but these valves are used in a number of applications. Many hardware stores stock float valves of various sizes and designs, and enterprising people can also make their own with objects lying around the house.
The basic float valve includes a balloon float, an air-filled float which is sealed so that water cannot enter, attached to an arm. When the water level is low, the float hangs, keeping the valve open. As the water level rises, the float starts to float on the surface as a result of buoyancy, causing the arm to move, and eventually pulling the valve closed so that water will stop flowing. This process is entirely mechanical, requiring no human input.
In toilets, the float valve is used when the tank fills. When someone flushes, a flap opens to dump the water in the tank into the bowl, forcing the contents of the toilet into the plumbing under the toilet. The flap drops shut after the flush, and the tank begins to fill from an inlet valve. As the tank fills, the float valve rises, eventually shutting the flow of water off when the water reaches the desired level. This automates the process of filling the toilet tank.
Float valves are also often used in automated waterers for animals, along with water tanks for plant irrigation systems. In the case of animal watering devices, when the animals drink a lot of water or evaporation occurs, the float valve sinks, opening the inlet and allowing the tank to fill back up to the prior level. It will still be necessary to periodically clean the water supply for safety, but people can rest assured that their animals will always have a steady supply of water.
Autofillers for aquariums and other types of water tanks are also commonly controlled with float valves. In all cases, the float valve can be adjusted to achieve a customized water level. In a toilet, for example, people who want to use less water with each flush can adjust the valve with a small screw so that the inlet valve shuts off sooner. Adjustments made by hand can be changed if necessary to respond to changing needs and conditions.