Pressure regulator is a valve that automatically cuts off the flow of a liquid or gas at a certain pressure, usually for the purpose of preventing damage to plumbing. They allow sources of water to be used which come at a higher pressure than can be tolerated. Pressure regulators are not only used to stop water, they react to the pressure on their output side, and close when the pressure in the plumbing reaches the designated level. Also when the pressure come down, the valve opens and allows flow until the plumbing pressure goes back up.