US Patent 7782461 Flow rate measuring device

A flow metering device and method for monitoring flow of a liquid, has a body with an inlet port and an outlet port. A flow chamber is formed therein between a pair of laterally placed windows in the flow chamber. A light emitting source placed in one of the windows and a light receiver placed in a second of the windows. The flow chamber has a flow restricting and light stopping element held therein and movable between a rest position and an operating position.