A sail switch, vane

Vane

Vane may refer to:* Cooper vane* Vane anemometer* Weather vaneA vane is also each of the parts besides the shaft in a pennaceous feather...

 switch or flow switch is a mechanical

Machine

A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

 switch that is switched on or off in response to the flow or non-flow of a fluid

Fluid

In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 such as air or water. A sail switch typically operates through the use of a paddle

Paddle

A paddle is a tool used for pushing against liquids, either as a form of propulsion in a boat or as an implement for mixing.-Materials and designs:...

 which gets displaced due to the force of fluid moving past it. Sail switches find application in the detection of fluid flow and measurement of fan speeds. A sail switch might be used to protect a central heating system electric heating element from being energized before the air flow from the blower is established. Sail switches might also be used to alarm if a ventilation fan in a hazardous location fails and air flow has stopped.