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Flow measurement
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==Open-channel flow measurement== [[Open channel flow]] describes cases where flowing liquid has a top surface open to the air; the cross-section of the flow is only determined by the shape of the channel on the lower side, and is variable depending on the depth of liquid in the channel. Techniques appropriate for a fixed cross-section of flow in a pipe are not useful in open channels. Measuring flow in waterways is an important open-channel flow application; such installations are known as [[stream gauge]]s. ===Level to flow=== The level of the water is measured at a designated point behind [[weir]] or in [[flume]] using various secondary devices (bubblers, ultrasonic, float, and differential pressure are common methods). This depth is converted to a flow rate according to a theoretical formula of the form <math>Q=KH^X</math> where <math>Q</math> is the flow rate, <math>K</math> is a constant, <math>H</math> is the water level, and <math>X</math> is an exponent which varies with the device used; or it is converted according to empirically derived level/flow data points (a "flow curve"). The flow rate can then be integrated over time into volumetric flow. Level to flow devices are commonly used to measure the flow of surface waters (springs, streams, and rivers), industrial discharges, and sewage. Of these, [[weirs]] are used on flow streams with low solids (typically surface waters), while flumes are used on flows containing low or high solids contents.<ref name=" Hydraulic structures ">{{cite web|url=http://help.openchannelflow.com/customer/portal/articles/949768-hydraulic-structures-primary-devices---flumes-and-weirs|title=Desk.com – Site Not Found (Subdomain Does Not Exist)|website=help.openchannelflow.com|url-status=live|archive-url=https://web.archive.org/web/20150925133652/http://help.openchannelflow.com/customer/portal/articles/949768-hydraulic-structures-primary-devices---flumes-and-weirs|archive-date=25 September 2015}}</ref> ===Area/velocity=== The cross-sectional area of the flow is calculated from a depth measurement and the average velocity of the flow is measured directly (Doppler and propeller methods are common). Velocity times the cross-sectional area yields a flow rate which can be integrated into volumetric flow. There are two types of area velocity flowmeter: (1) wetted; and (2) non-contact. Wetted area velocity sensors have to be typically mounted on the bottom of a channel or river and use Doppler to measure the velocity of the entrained particles. With depth and a programmed cross-section this can then provide discharge flow measurement. Non-contact devices that use laser or radar are mounted above the channel and measure the velocity from above and then use ultrasound to measure the depth of the water from above. Radar devices can only measure surface velocities, whereas laser-based devices can measure velocities sub-surface.<ref>{{cite web|last1=Severn|first1=Richard|title=Environment Agency Field Test Report – TIENet 360 LaserFlow|url=http://www.rshydro.co.uk/PDFs/isco/LaserFlow-EA-field-test-report-17th-Sept-2014.pdf|website=RS Hydro|publisher=RS Hydro-Environment Agency|access-date=3 August 2015|url-status=live|archive-url=https://web.archive.org/web/20150925123359/http://www.rshydro.co.uk/PDFs/isco/LaserFlow-EA-field-test-report-17th-Sept-2014.pdf|archive-date=25 September 2015}}</ref> ===Dye testing=== A known amount of [[dye]] (or [[salt]]) per unit time is added to a flow stream. After complete mixing, the concentration is measured. The dilution rate equals the flow rate. ===Acoustic Doppler velocimetry=== [[Acoustic Doppler velocimetry]] (ADV) is designed to record instantaneous velocity components at a single point with a relatively high frequency. Measurements are performed by measuring the velocity of particles in a remote sampling volume based upon the Doppler shift effect.<ref name="Chanson 2008">{{Cite book |author=Chanson, Hubert |author-link=Hubert Chanson |title=Acoustic Doppler Velocimetry (ADV) in the Field and in Laboratory: Practical Experiences |url=http://espace.library.uq.edu.au/view/UQ:159549 |publisher=in Frédérique Larrarte and Hubert Chanson, Experiences and Challenges in Sewers: Measurements and Hydrodynamics. International Meeting on Measurements and Hydraulics of Sewers IMMHS'08, Summer School GEMCEA/LCPC, Bouguenais, France, 19–21 August 2008, Hydraulic Model Report No. CH70/08, Div. of Civil Engineering, The University of Queensland, Brisbane, Australia, Dec., pp. 49–66 |year=2008 |isbn=978-1-86499-928-0 |url-status=live |archive-url=https://web.archive.org/web/20091028004147/http://espace.library.uq.edu.au/view/UQ:159549 |archive-date=28 October 2009 }}</ref>
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