Editing Flow measurement (section)

===Current meter===
{{Main|Current meter}}
[[File:Dumas Neyrpic Current Meter.JPG|thumb|right|alt=Spiral propeller connected to a streamlined housing, held by a hand. Wire leads at the right.|A propeller-type current meter as used for hydroelectric turbine testing]]

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 [[International Electrotechnical Commission|IEC]] standard 41. Such flow measurements are often commercially important when testing the efficiency of large turbines.