A thin plate that is mounted inside a pipe and has a sharp-edged aperture through which the fluid in the pipe is accelerated. The acceleration causes the local static pressure to decrease. The flow rate is sensed by taking one pressure reading upstream and one downstream of the orifice.
The orifice plate is commonly used as an instrument to meter or control the rate of flow of the most common or newtonian fluids. These comprise all gases, including air and natural gas, and many liquids, such as water, and most hydrocarbons. With no moving parts and a simple design, the orifice is easily machined, and thus has been a popular flow-measuring device. However, its pressure loss is large compared to more expensive devices such as the venturi tube.
The metering orifice is one of a class of differential-pressure-sensing devices that are used to indicate flow rate. Others in this category include flow nozzles, venturis, elbow meters, target meters, and wedge meters. In open-channel flow, such as occurs in streams and canals, the weir serves the same purpose as the orifice; however, because the flow occurs at constant pressure, the height of the fluid over the weir, rather than the pressure, is used to sense the flow rate. See Nozzle, Pitot tube, Venturi tube