Download - Water Measurement
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Water Measurement
Brady S. McElroy, P.E.
USDA-NRCS
Lamar, Colorado
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Objectives
• Why is water measurement important to IWM?
• Explain some of the mathematics of water measurement
• Discuss some of the common measuring devices encountered in NRCS work
• Discuss other opportunities for measurement
• Work some example problems
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Why is water measurement important?
• Difficult to effectively manage irrigation without measurement
• Positive aspects– Maximize use of available water supply– Reduced cost due to leached nutrients– Reduced environmental impact from over-
irrigation
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Why is water measurement important?
• Some measurement may have a negative connotation– Regulatory (mandated by state, etc.)– Billing
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Why is water measurement important?
• Water is one of the most precious resources in the West– Increased competition among water users
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“Whiskey is for drinking. Water is for fighting over.”
Mark Twain
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References
Primary reference for NRCS is Chapter 9 of Part 623 (Irrigation) of the National Engineering Handbook
•States that NRCS’ reference shall be the Bureau of Reclamation’s Water Measurement Manual, 3rd edition, published in 1997
•Available online at http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/
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References
Other useful references
• Other NRCS documents
• Irrigator’s Guides
• Extension publications
• Hydraulic texts– King’s Handbook of Hydraulics
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Definitions
Volume: length3
Flow Rate (Q): volume/time
Velocity: length/time
Area: length2
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DefinitionsHead- measurement of the energy in a fluid. Units are typically length.•Total head at a given point is the sum of three components
– Elevation head, which is equal to the elevation of the point above a datum
– Pressure head, which is the height of a column of static water that can be supported by the static pressure at the point
– Velocity head, which is the height to which the kinetic energy of the liquid is capable of lifting the liquid
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DefinitionsPressure- measurement of the force acting on a surface. Units are force/length2
Often convenient to express in terms of feet of fluid (pressure head)
h=p/γ
(multiply psi x 2.31 for feet of H20)
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Units
• Typically in U.S. Customary units for irrigation work.
• Units vary depending on type of measurement– Q vs. volume– Open channel vs. pipe flow
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Units
Flow rate units expressed in volume/time
• Open channel flow– Cubic feet per second (cfs)
• second-feet
• Pipe flow– Gallons per minute (gpm)
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Units
Handy Conversion Factor
1 cfs = 448.8 gpmor
1 cfs ≈ 450 gpm
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Units
May also vary regionally• Shares• Some canals refer to a head of water as a
delivery unit– Not the same as energy measurement
• Miner’s inches– 38.4 miner’s inches = 1 cfs (Colorado)– 40 miner's inches = 1 cfs (California, et al.)– 50 miner’s inches = 1 cfs (New Mexico, et al.)
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Units
A share is not a share is not a share
Canal Allocation/share (cfs)Bessemer 0.0150
Colorado 0.0125
Rocky Ford Highline 0.180
Oxford 0.0960
Otero 0.050
Holbrook 0.0250
Catlin 0.0180
Rocky Ford 0.140
Fort Lyon 0.0150
Amity 5 cfs at 0.6 hr/share
Lamar 0.0100
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Units
Volume units are often expressed in units of area x depth or depth
Acre-foot = volume of water that would cover 1 acre to a depth of 1 foot
• 12 acre-inches
• 43,560 cubic feet
• 325,851 gallons
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Units
Handy Conversion Factor
1 cfs for 24 hours ≈ 2 acre-feet
or
1 cfs ≈ 1 ac-in/hr
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Water Measurement Mathematics
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Water Measurement Mathematics
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Water Measurement Mathematics
Continuity Equation
Q=vA
Irrigator’s Equation
Qt=Ad
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v1 v2
Qin Qout
A1 A2
Continuity Equation
Q=vA
Q = flow rate
v = velocity
A = area
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Continuity Equation
Q=vA
v=Q/A
A=Q/v
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Continuity Equation
Given: d=12 inches
v=2.5 ft/s
Find: Q in cfs
Qv=2.5 ft/s
12 in.
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Qv=2.5 ft/s
12 in.
Continuity Equation
Solution: Q = vA
4
2dA
4
1 2)ft(A
A = 0.785 ft2
Q = 2.5 ft/s x 0.785 ft2 = 1.96 ft3/s
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Irrigator’s Equation
Qt = Ad
Q = flow rate
t = time
A = area
D = depth
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Irrigator’s Equation
d = Qt/A
Q = Ad/t
t = Ad/Q
A = Qt/d
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Irrigator’s Equation
Given: d = 3 inches
A = 50 acres
Q = 2 cfs
Find: Time required to apply d
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Irrigator’s Equation
Solution: t = dA/Q
1 cfs ≈ 1 ac-in/hr
t = 75 hours
hrinac
)ac)(in(t
2
503
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Irrigator’s Equation
Given: t = 36 hours
A = 20 acres
Q = 2 cfs
Find: Depth of applied water, d
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Irrigator’s Equation
Solution: d = Qt/A
1 cfs ≈ 1 ac-in/hr
d = 3.6 inches
ac
)hr)(hrinac(
d20
362
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Water Measurement Devices
Most water measurement devices either sense or measure velocity, or measure either pressure or head.
Tables, charts, or equations are then used to calculate the corresponding discharge
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Water Measurement Devices
Devices that sample or sense velocity
• Current meters
• Propeller meters
• Vane deflection meters
• Float and stopwatch
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Water Measurement Devices
Devices that measure head or pressure– Open channel devices commonly use h– Pipeline devices may use p
• Flumes• Orifices• Venturi meters• Weirs
– Velocity is computed from h, so weirs are classifed as head measuring devices
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Open Channel Devices
• Weirs
• Flumes
• Submerged Orifices
• Other devices
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Weirs
A weir is an overflow structure installed perpendicular to open channel flow
• Has a unique depth of water at an upstream measuring point for each discharge
• If the water springs clear of downstream face, acts as sharp-crested weir
• A long, raised channel control crest is a broad-crested weir
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Weirs
• Usually named for the shape of the overflow opening– Rectangular– Triangular– Cipolletti
• Lowest elevation on overflow is zero reference elevation for measuring h
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Weirs
Rectangular weirs can be either contracted or suppressed
• Suppressed weirs use side of flow channel for weir ends– No side contraction occurs– Often used in divide boxes
• Canal overshot gates can act as weirs
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Weirs
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Weirs
Cipolletti Weir
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Weirs
Weir Box Turnout with Cipolletti Weir
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Weirs
Compound Weir
90 degree triangular and suppressed rectangular
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Weirs
Advantages• Simple to construct• Fairly good at passing trash• 1 head measurementDisadvantages• High head loss• Susceptible to sedimentation problems• Sensitive to approach and exit conditions
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Weirs
Conditions needed for sharp-crested weirs• Upstream face should be plumb, smooth, normal
to axis of channel• Entire crest should be level for rectangular and
Cipolletti. Bisector of V-notch angles should be plumb for triangular.
• Plate should be thin enough to act as a sharp-crested weir– Chamfer downstream edge if necessary– Upstream edge must be straight and sharp– Thickness should be uniform for entire length
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Weirs
• Maximum downstream elevation should be at least 0.2 ft below crest
• Head measurement should be greater than 0.2 ft for optimal elevation
• Head is measured upstream 4 X maximum head on crest
• Approach must be kept free of sediment deposits
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Weirs
Given: Standard Contracted Rectangular Weir
L = 2 feet
h = 0.40 feet
Find: Q, in cfs
Solution: Refer to Table A7-2 in BoR Water Measurement Manual, 3rd edition
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Weirs
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Weirs
Inspection of Existing Structures• Approach flow• Turbulence• Rough water surface at staff gage• Velocity head• Exit flow conditions• Worn equipment• Poor installation
– Crest must be correctly installed
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Weirs
Poor approach condition
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Weirs
Sediment in approach pool
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Flumes
Flumes are shaped open channel flow sections.• Force flow to accelerate
– Converging sidewalls– Raised bottom– Combination
• Force flow to pass through critical depth– Unique relationship between water surface
profile and discharge
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Flumes
Two basic classes of flumes
• Long throated flumes– Parallel flow lines in control section– Accurately rate with fluid flow analysis
• Short throated flumes– Curvilinear flow in control section– Calibrated with more precise measurement
devices
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Short Throated Flumes
Parshall Flume is most well-known example
of short throated flumes
• Developed by Ralph Parshall at Colorado Agricultural College (now Colorado State University)
• ASAE Historic Landmark
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Parshall FlumesSince the beginning of irrigated agriculture, it has been important to
measure flows of irrigation water. Accuracy of early water measurement methods often suffered because of trash or sediment in
the water, or unusual flow conditions. Ralph L. Parshall saw this problem when he began working for the USDA in 1915, as an irrigation
research engineer. In 1922 he invented the flume now known by his name. When this flume is placed in a channel, flow is uniquely related
to the water depth. By 1953 Parshall had developed the depth-flow relationships for flumes with throat widths from 3 inches to 50 feet. The Parshall flume has had a major influence on the equitable distribution
and proper management of irrigation water. Thousands of flumes have been used to measure irrigation water, as well as industrial and
municipal liquid flows throughout the world. This plaque marks the site of the original Colorado Agricultural College Hydraulics Laboratory,
where Parshall carried out his historic experiments. DEDICATED BY THE AMERICAN SOCIETY OF AGRICULTURAL
ENGINEERS 1985
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Parshall Flumes
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Parshall Flumes
• Designated by throat width– Measure 0.01 cfs with 1 inch flume– Measure 3000 cfs with 50 foot flume
• Dimensions are standardized for each flume– Not geometrically proportionate
• A 12 ft flume is not simply 3x a 4 ft flume
• Relate Ha (or Ha and Hb ) to discharge with rating equation, or consult appropriate chart
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Parshall Flumes
• Flow occurs under two conditions– Free flow
• Downstream water surface does not reduce discharge
• Requires only 1 head reading (Ha)
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Parshall Flumes– Submerged flow
• Downstream flow is high enough to reduce discharge• 2 head readings required
• 50% submergence (Hb/Ha) on 1-3 inch flumes
• 80% submergence (Hb/Ha) ≥8 feet flumes
• After 90% submergence, flume is no longer effective
HaHb
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Parshall Flumes
Advantages
• Relatively low head loss (1/4 of sharp crested weir)
• Handle some trash and sediment
• Well accepted– May be mandated
• Many sizes are commercially available
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Parshall Flumes
Disadvantages
• Complicated geometry for construction
• Tight construction tolerances
• Aren’t amenable to fluid flow analysis
• BoR does not recommend for new construction
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Parshall Flumes
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Parshall Flumes
Given: 1 foot throat Parshall Flume
Free flow
Ha = 0.40 feet
Find: Q, in cfs
Solution: Refer to Table A8-12 in BoR Water Measurement Manual, 3rd
edition
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Parshall Flumes
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Parshall Flumes
Given: 1 foot Parshall Flume
Ha = 1 ft
Hb = 0.8 ft
Find: Q, in cfs
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Parshall Flumes
Solution: Determine if submergence exceeds 70% (Hb/Ha)
0.8/1.0=0.8>0.7
Therefore, must correct for submergence
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Parshall Flumes
Solution: From table A8-12, Q=3.95 cfs
Find correction factor
Use Figure 8-16
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Parshall Flumes
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Parshall Flumes
Correction=0.35 ft3/s
Actual Q =(free flow Q) – (correction)
=3.95 ft3/s – 0.35 ft3/s
=3.6 ft3/s
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Broad-crested Weirs
Long throated flume where only the bottom is raised. No side contractions
• Also called ramp flumes, Replogle flumes
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Broad-crested Weirs
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Broad-crested Weirs
Long throated flume (broad-crested weir) under construction)
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Broad-crested Weirs
Long throated flume (broad-crested weir) Q = 1200 cfs
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Broad-crested Weirs
Advantages
• Easily constructed, especially in existing concrete lined channels
• WinFlume software available to quickly design and rate structures
• Less expensive construction
• Low head loss
• Handle trash and sediment well
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Broad-crested Weirs
Disadvantages
• Some state laws or compacts may preclude use
• Not readily accepted by some water users– Not what they’re used to using
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Other Flumes
Several other types of flumes are used
• H-flumes
• Cutthroat flumes
• Palmer-Bowles
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Other Flumes
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Flumes
Inspection of Existing Structures• Approach flow
– Flumes are in-line structures– Should have smooth flow across width and depth of
cross section– Length of straight approach varies depending on
control width, channel width, and velocity• Turbulence• Level both along and perpendicular to flow• Excessive submergence• Exit flow conditions
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Submerged Orifices
A well defined sharp-edged opening in a wall or bulkhead through which flow occurs• When size and shape of the orifice and the heads acting on it are known, flow measurement is possible• Orifices are typically circular or rectangular in shape• Can be used to regulate and measure water in a turnout structure• Radial gates can act as submerged orifices
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Submerged Orifices
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Submerged Orifices
Advantages• Less head required than for weirs• Used where space limitations prevent weir
or flumeDisadvantages• Sediment and debris accumulation will
prevent accurate measuring• Typically not used if conditions permit
flumes which handle trash better
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Current Meters
Velocity measuring devices
• Sample velocity at one point– Point sample isn’t representative of average
velocity in flow are• Develop relationship between observed and
average velocity, or• Take multiple velocity readings
• Use continuity equation (Q=vA) to compute discharge
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Current Meters
Types of current meters• Anemometer• Propeller• Electromagnetic• Doppler• Optical strobe
Anemometer and propeller are most common for irrigation work
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Current Meters
Anemometer type current meter
![Page 85: Water Measurement](https://reader035.vdocuments.site/reader035/viewer/2022062519/56814d69550346895dbab332/html5/thumbnails/85.jpg)
Other Open Channel Methods
Slope-Area Method• Slope of water surface and average cross-
sectional area used with Manning’s equation• Difficult to estimate “n”• Can only approximate Q
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Float Method
Similar in concept to current meters
• Velocity is estimated by timing how long a floating object takes to travel a pre-determined distance
• Observed velocity is adjusted by some factor to estimate average velocity
• Determine cross-sectional flow area
• Use continuity equation to estimate Q
• Provides only a rough estimate
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Float Method
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Pressurized Conduit Devices
Pipeline devices are usually classified by their basic operation• Calibrated velocity sensing meters • Differential head meters• Positive volume displacement summing meters (municipal water)• Measured proportional or calibrated bypass meters• Acoustic meters
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Differential Head Meters
Include venturi, nozzle, and orifice meters• When properly installed, accuracy ±1%
– Some irrigation operating conditions probably limit accuracy to ±3-5%
• No moving parts– Uses principle of accelerating flow through a
constriction– Resulting pressure difference is related to discharge
using tables or curves, or a suitable coefficient and the proper equation
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Venturi Meter
Common differential head meter• Minimal head loss• Full pipe flow required• Also used to inject chemicals into an
irrigation system– Pressure reduction is used to pull chemicals
into the system
• Examples of venturi meters constructed of standard plastic pipe fittings
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Venturi Meter
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Nozzle Meter
Simplified form of venturi meter
• Gradual downstream expansion of venturi is eliminated
• Higher head loss than venturi
• Full pipe flow required
• Not used extensively in irrigation
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Nozzle Meter
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Orifice Meter
Another differential pressure meter
• Often used for measuring well discharge
• Also used to measure chemical injections– Typically small meters with details provided by
manufacturer
• Requires long straight pipe lengths
• Full pipe flow required
• Limited discharge ratio
![Page 95: Water Measurement](https://reader035.vdocuments.site/reader035/viewer/2022062519/56814d69550346895dbab332/html5/thumbnails/95.jpg)
Orifice Meter
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Elbow Meters
Measure pressure difference between inside and outside of an elbow
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Propeller Meters
Used at end of pipes and in conduits flowing full• Multiple blades that rotate on horizontal
axle
• Must have full pipe flow
• Basically operate on Q=vA principle
• Usually have totalizer plus instantaneous discharge display
• Accuracy can be ±2-5% of actual flow
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Propeller Meters
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Propeller Meters
Saddle type propeller meter
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Propeller Meters
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Propeller Meters
• Should be selected to operate near middle of design discharge range– If system has oversized pipes, some sections
may need replaced with smaller pipes to provide correct velocity and approach
• Must be installed to manufacturer’s specifications for accurate measurement
• Must have full pipe flow
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Propeller Meters
Advantages
• Commercially available
• Totalizing meter
• Can achieve good accuracy
![Page 103: Water Measurement](https://reader035.vdocuments.site/reader035/viewer/2022062519/56814d69550346895dbab332/html5/thumbnails/103.jpg)
Propeller Meters
Disadvantages• Operating conditions different from
manufacturer’s calibration conditions will affect accuracy
• Only tolerate small amount of weeds and debris
• Moving parts operating underwater• Can require a good deal of maintenance
and inspection
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Other Conduit Devices
Pitot Tube Velocity Measurements• Piezometer
– Straight tube attached flush to wall and perpendicular– Senses pressure head in pipe
• Pitot Tube– Right angle bend inserted with horizontal leg pointed
upstream and parallel to flow– Senses both velocity and pressure head
• Velocity head, flow area, and coefficient can then be used to calculate flow rate
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Pitot Tube Velocity
![Page 106: Water Measurement](https://reader035.vdocuments.site/reader035/viewer/2022062519/56814d69550346895dbab332/html5/thumbnails/106.jpg)
Other Conduit Devices
• Magnetic Flowmeters– Use the principle that voltage is induced in an
electrical conductor moving through a magnetic field. Conductor is flowing water
– For a given field strength, the magnitude of the induced voltage is proportional to velocity
• Deflection Meters– Vane or plate projecting into flow and a sensing
element to measure deflection– Calibrated to indicate flow in desired units
• Vortex Flowmeters– Obstructions in flow generate vortex shedding trails
• Properly shaped obstructions create vortices that can be sensed and related to velocity
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Other Conduit Methods
Trajectory Method
• Measure the horizontal and vertical coordinates of a point in the jet of water issuing from the end of a pipe
• Accurate ±15%
• Coordinates can be difficult to accurately measure
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Trajectory Method
• Vertical Pipe• Two kinds of flow occur, depending on how high
water rises– <0.37d, circular weir– Transistional region between– >1.4d, jet flow
• Horizontal Pipe– Pipe must be truly horizontal; slope will skew
results• Vertical component can be difficult to measure
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Trajectory Method
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Trajectory Method
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Trajectory Methods
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Other Conduit Methods
Power Consumption Coefficients• Volume discharged from wells can be estimated
using power consumption records– Wells must be analyzed to determine the energy
needed to pump a certain volume of water– Relationship can then be used to estimate discharge
volume– Only certified well testers can perform the tests and
develop the power consumption coefficient– Must recalibrate every 4 years, or more often
depending on conditions
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Other Conduit Methods
Siphon Tubes• Estimate discharge based on head,
diameter, and length of siphon tubes• Accuracy ±10-15%• Provides an in-field method of estimating
flow• Information also available in irrigator’s
guides and NRCS Engineering Field Manual, Chapter 15
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Siphon Tubes
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Siphon Tubes
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Summary
• Water measurement is an important component of IWM
• BoR Water Measurement Manual• Continuity equation
– Q=vA
• Irrigator’s equation– Qt=dA
• 1 cfs≈450 gpm• 1 cfs≈1 ac-in/hr
![Page 117: Water Measurement](https://reader035.vdocuments.site/reader035/viewer/2022062519/56814d69550346895dbab332/html5/thumbnails/117.jpg)
Summary
• Open channel devices– Flumes– Weirs– Submerged orifices
• Pressurized conduit devices– Propeller meters– Differential head meters
![Page 118: Water Measurement](https://reader035.vdocuments.site/reader035/viewer/2022062519/56814d69550346895dbab332/html5/thumbnails/118.jpg)
Summary
• Installation requirements– Examine existing structures
• Other opportunities for measurement– Canal gates– Float method– Power consumption coefficient– Pipe trajectory– Siphon tubes
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Questions?
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