<P> Flow through a large penstock such as used at a hydroelectric power plant can be measured by averaging the flow velocity over the entire area . Propeller - type current meters (similar to the purely mechanical Ekman current meter, but now with electronic data acquisition) can be traversed over the area of the penstock and velocities averaged to calculate total flow . This may be on the order of hundreds of cubic meters per second . The flow must be kept steady during the traverse of the current meters . Methods for testing hydroelectric turbines are given in IEC standard 41 . Such flow measurements are often commercially important when testing the efficiency of large turbines . </P> <P> There are several types of flow meter that rely on Bernoulli's principle, either by measuring the differential pressure within a constriction, or by measuring static and stagnation pressures to derive the dynamic pressure . </P> <P> A Venturi meter constricts the flow in some fashion, and pressure sensors measure the differential pressure before and within the constriction . This method is widely used to measure flow rate in the transmission of gas through pipelines, and has been used since Roman Empire times . The coefficient of discharge of Venturi meter ranges from 0.93 to 0.97 . The first large - scale Venturi meters to measure liquid flows were developed by Clemens Herschel who used them to measure small and large flows of water and wastewater beginning at the end of the 19th century . </P> <P> An orifice plate is a plate with a hole through it, placed perpendicular to the flow; it constricts the flow, and measuring the pressure differential across the constriction gives the flow rate . It is basically a crude form of Venturi meter, but with higher energy losses . There are three type of orifice: concentric, eccentric, and segmental . </P>

How can you measure the flow in pipelines
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