Vortex flow meters 

Vortex flowmeter is a flowmeter for measuring fluid flow rates in an enclosed conduit. A vortex flow meter is a flow measurement device best suited for flow measurements where the introduction of moving parts presents problems. Vortex flow meters operate under the vortex shedding principle, where an oscillating vortexes occur when a fluid such as water flow past a bluff (as opposed to streamlined) body. The frequency that the vortexes are shed depend on the size and shape of the body. It is ideal for applications where low maintenance costs are important. Industrial size vortex meters are custom built and require appropriate sizing for specific applications.