In scuba diving

Scuba diving

Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

, a freeflow occurs when the diving regulator

Diving regulator

A diving regulator is a pressure regulator used in scuba or surface supplied diving equipment that reduces pressurized breathing gas to ambient pressure and delivers it to the diver. The gas may be air or one of a variety of specially blended breathing gases...

 continues to supply air instead of cutting off the supply when the diver is not breathing in. This may be caused by very cold water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 freezing the second stage valve open, a malfunction of either the first or second stages, or by the venturi effect

Venturi effect

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...

 on very sensitive demand valves keeping the internal pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

low enough for the valve to stay open. If the freeflow is caused by freezing, then switching to an alternative air supply briefly should allow water that fills the second stage to unfreeze it. Otherwise, simply replacing the valve in the mouth and attempting to breathe normally will usually resolve the problem.