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A venturi is a point in a pipe that has been narrowed so that the flow is restricted slightly. In the figure below notice that a high-pressure port is provided in front of where the point where the venturi is narrowed down and a low-pressure port is provided directly after the point where the pipe is narrowed down. The high-pressure port is provided to sample the increased fluid pressure where it will increase slightly because of the restriction caused by the venturi, and the low-pressure port is provided to sample the pressure as it drops after it flows past the restriction caused by the venturi. Additional pressure ports are provided on both sides of the restriction in larger venturies to allow an average pressure to be measured. The venturi is widely used because it has no moving parts and the small amount of restriction it produces to induce a pressure drop does not disturb the fluid flow too much. This means the venturi can handle larger volumes of flow than most other types of flow.
The Venturi flowmeter belongs in a category of flowmeters called Velocity Flow Meters. Velocity flow meters use the change in velocity (speed) that occurs when fluid flow changes to measure the amount of flow. Velocity meters can be used in systems that are not adversely affected by changes in viscosity. Examples of velocity flow meters include the paddlewheel flowmeter, the turbine flowmeter, the vortex flowmeter, the electromagnetic flowmeter, and the ultrasonic (Doppler) flow meter.
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