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IRRIGATIONMANAGEMENTS E R I E S
Irrigation WaterMeasurement
Danny H. RogersExtension Agricultural Engineer
Richard D. BlackExtension Agricultural Engineers
Cooperative Extension ServiceKansas State University
Irrigation is the largest water use inKansas. There are about 3 million irri-gated acres, of which nearly 95 percentis supplied by groundwater. Unfortu-nately, groundwater levels have beendeclining in most of the major irrigatedareas due to overpumping, causing con-cern over the long term water availabil-ity from these aquifers.
Improved irrigation water manage-ment is needed, but without water mea-surements it is impossible to determinecurrent usage and what managementmust accomplish. Measuring water useis a first step. Several groundwatermanagement districts have installed amandatory plan or are discussing howto accomplish metering of all irrigationwater.
Water measurements provide thedata necessary for:
1. Determining irrigation efficiency;
2. Improving water management;
3. Monitoring pumping plantperformance;
4. Detecting well problems; and
5. Completing annual water use reports.
Just as there are a number of benefitsfor measuring irrigation water, there area number of ways of doing the job. Themore common methods are listed below.
PROPELLER METERSPropeller meters are one of the most
common, convenient and accuratedevices available for measuring irriga-tion water. Propeller meters are in-linedevices that relate flow velocity andpipe cross section area to give a flowrate, volume, or both.
Flow velocity is measured with apropeller or impeller mounted on a shaftin the pipe. The meters are normallyinstalled so the propeller is centered inthe pipe with its centerline running onor parallel to the pipe axis. The propel-ler is connected to an indicating headwhich registers flow rate, volume, orboth.
Propeller meters can be installed inany position - vertical, horizontal or atan angle. But, no matter how they areinstalled, the pipe must always be flow-ing full for accurate measurement. Themeter also must be installed at an ade-quate distance from valves, elbows orother fittings which may disturb theflow. For more discussion of propeller
water meters, see “Guidelines for theUse of Propeller-type Irrigation WaterMeters,” Cooperative Extension Servicepublication L-869.
PITOT TUBEA pitot tube can be used to determine
the velocity of flow in a pipe. If thevelocity is measured at proper pointsacross the pipe diameter an averagemay be computed and the flow ratecomputed from the cross-sectional area.
The simplest pitot tubes are smallL-shaped tubes (Figure 1). Water risesin the tube when it’s placed in a pipewith the open end facing the flow. Theheight of the water above the open endof the tube is a measure of the velocityof flow at that point, plus the pressure.By attaching another tube to the sidewall of the pipe, one can measure thepressure component. The difference isthe velocity of the flow.
A pitot tube normally indicatesvelocity at only one point in the pipe,but velocity in a pipe varies. It is nor-mally highest at the center of the pipeand decreases toward the pipe walls.Because of this, an average velocity isusually computed by taking severalreadings at different locations in thepipe. A series of readings from near thepipe wall to the center of the pipe is suf-ficient if the flow is uniform and undis-turbed. A series of measurementsacross the pipe is normally conducted,if possible, to determine if the flow isuniform. Some instruments use thisprinciple but make only a single mea-surement at the centerline of the pipe.Obviously, if the flow is not uniformsuch instruments give a faulty reading.
Another popular pitot tube device isthe “Collins meter,” a probe that is acylindrical tube with two holes near thecenter. A hole is drilled through both
Figure 1. Simple pitot tubes.
Outdated Publication, for historical use. CAUTION: Recommendations in this publication may be obsolete.
IRRIGATIONMANAGEMENTS E R I E S
Irrigation WaterMeasurementDanny H. RogersExtension Agricultural Engineer
Richard D. BlackExtension Agricultural Engineers
Cooperative Extension ServiceKansas State University
Irrigation is the largest water use inKansas. There are about 3 million irri-gated acres, of which nearly 95 percentis supplied by groundwater. Unfortu-nately, groundwater levels have beendeclining in most of the major irrigatedareas due to overpumping, causing con-cern over the long term water availabil-ity from these aquifers.
Improved irrigation water manage-ment is needed, but without water mea-surements it is impossible to determinecurrent usage and what managementmust accomplish. Measuring water useis a first step. Several groundwatermanagement districts have installed amandatory plan or are discussing howto accomplish metering of all irrigationwater.
Water measurements provide thedata necessary for:1. Determining irrigation efficiency;2. Improving water management;3. Monitoring pumping plant
performance;4. Detecting well problems; and5. Completing annual water use reports.
Just as there are a number of benefitsfor measuring irrigation water, there area number of ways of doing the job. Themore common methods are listed below.
PROPELLER METERSPropeller meters are one of the most
common, convenient and accuratedevices available for measuring irriga-tion water. Propeller meters are in-linedevices that relate flow velocity andpipe cross section area to give a flowrate, volume, or both.
Flow velocity is measured with apropeller or impeller mounted on a shaftin the pipe. The meters are normallyinstalled so the propeller is centered inthe pipe with its centerline running onor parallel to the pipe axis. The propel-ler is connected to an indicating headwhich registers flow rate, volume, orboth.
Propeller meters can be installed inany position - vertical, horizontal or atan angle. But, no matter how they areinstalled, the pipe must always be flow-ing full for accurate measurement. Themeter also must be installed at an ade-quate distance from valves, elbows orother fittings which may disturb theflow. For more discussion of propeller
water meters, see “Guidelines for theUse of Propeller-type Irrigation WaterMeters,” Cooperative Extension Servicepublication L-869.
PITOT TUBEA pitot tube can be used to determine
the velocity of flow in a pipe. If thevelocity is measured at proper pointsacross the pipe diameter an averagemay be computed and the flow ratecomputed from the cross-sectional area.
The simplest pitot tubes are smallL-shaped tubes (Figure 1). Water risesin the tube when it’s placed in a pipewith the open end facing the flow. Theheight of the water above the open endof the tube is a measure of the velocityof flow at that point, plus the pressure.By attaching another tube to the sidewall of the pipe, one can measure thepressure component. The difference isthe velocity of the flow.
A pitot tube normally indicatesvelocity at only one point in the pipe,but velocity in a pipe varies. It is nor-mally highest at the center of the pipeand decreases toward the pipe walls.Because of this, an average velocity isusually computed by taking severalreadings at different locations in thepipe. A series of readings from near thepipe wall to the center of the pipe is suf-ficient if the flow is uniform and undis-turbed. A series of measurementsacross the pipe is normally conducted,if possible, to determine if the flow isuniform. Some instruments use thisprinciple but make only a single mea-surement at the centerline of the pipe.Obviously, if the flow is not uniformsuch instruments give a faulty reading.
Another popular pitot tube device isthe “Collins meter,” a probe that is acylindrical tube with two holes near thecenter. A hole is drilled through both
Figure 1. Simple pitot tubes.
Outdated Publication, for historical use. CAUTION: Recommendations in this publication may be obsolete.
Figure 2. Diagram of a venturi meter.
Figure 3. Diagram of flow nozzle.
Pump discharge pipe
Figure 4. Orifice meter.
walls of the pipe, and packing glandsare secured into the holes to sealbetween the probe and the pipe, so nowater will leak from the test pipe. Withone hole in the probe facing directlyupstream and the other facing down-stream, a series of velocity measure-ment are taken as described above andthe flow rate determined.
The other common pitot device is the“Anubar,” a probe with two openingsupstream and one down. It is installedin the pipe through a single hole using apacking gland to seal against leakage,but it is not moved. For each size ofpipe there is an appropriate probe whichaverages the velocity; corrections maybe made for slight differences in thepipe diameter. These devices are nor-mally used for measuring flows insprinkler lines where a sprinkler may beremoved to install the meter.
CONVERGENCE METERSConvergence meters use differences
in pressure or head to measure the flowrate. The flow of water through a con-striction in a pipe lowers the pressure atthe constriction. How much the head or
pressure drops between the uncon-stricted flow and the constricted flow isa function of the flow rate.
Three common types of convergencemeters are the venturi meter (Figure 2),the nozzle (Figure 3), and the orificeplate (Figure 4).
All of these devices work on the sameprinciple. The major difference is theamount of loss that occurs due to flowthrough the device. The venturi meterhas the least pressure loss by far, so it isthe preferred method where energylosses are a concern. The nozzle issomewhat better than the orifice plate,but the difference is much smaller thanthe difference between the venturi meterand the nozzle or orifice plate.
All of these devices convert the pres-sure in the pipe upstream from anobstruction into increased speed of thewater at the obstruction, which lowersthe pressure where the speed isincreased. Through a given amount ofobstruction, this lowering of pressure isa constant for each rate of flow. Theflow is determined by measuring thepressure change. The temperature of thewater and the shape of the obstructionmake a difference, but they are not con-sidered in this discussion. Water temper-atures in the field do not vary greatly,and each device generally has a ratingtable or characteristic curve which isused to determine flow from the pressuremeasurements.
All of these devices may be usedwithin a pipe flowing full of water or afull pipe discharging freely into theatmosphere, but in the latter case, thereis no advantage for a venturi meter ornozzle so orifice plates are used almostexclusively.
With full pipe flow, the difference inpressure is normally not more than a fewfeet of water if the meter has been prop-erly sized and the range of flows is notgreat. As a consequence, normal pres-sure gauges are not suitable for the mea-surements. A foot of water pressure isless than 0.5 pound per square inch. Somanometers, explained below, and otherdevices are used to measure pressuredifferences.
Convergence meters normally onlymeasure the rate of flow, but it is possi-ble to measure volumes. One venturimeter which does this uses a shunt line.The two pressure measuring points areconnected with a pipe and, because of
the difference in pressure, water willflow through this pipe. In the shunt-linemeter, this is measured with a householdwater meter. A restricting device isinserted into the shunt line, so the flow isproportional to the flow through the ven-turi. This type of system could be usedfor either the nozzle or the orifice, butthe equipment has not been developed.
Pressure differences usually are mea-sured with a manometer, which is sim-ply a glass or plastic tube in which onecan see the level to which water willrise at a given pressure. If the differencein pressure between two points is re-quired, an inverted U-shaped tube maybe used to measure the difference with asingle reading. Manometer measure-ments are generally more accurate thanpressure gauge readings, because1 pound per square inch pressure willcause a rise in water level of about2.31 feet. Thus, even a crude measure-ment to the nearest inch reveals thepressure to the 0.04 psi, which is muchbetter than most pressure gauges.
Properly installed orifice plates onopen pump discharges can provide afairly inexpensive and accurate way tomeasure flow rate. For simplicity, thepipe should be horizontal, with noelbows, valves or other fittings closerthan 10 pipe diameters upstream fromthe orifice plate. The ratio of the orificediameter to the pipe diameter should begreat enough to cause an easily measur-able pressure rise and the ratio must beknown to select the proper dischargecoefficient. A restriction of about0.50 is a common starting point.
Using the manometer readings, thedischarge can then be read from a table(see Table l), or calculated:
Q=Ca 2gh,where:Q = discharge, gpm
C = discharge coefficient, see Figure 5.a = cross-sectional area of the orifice
in square inchesg = acceleration due to gravity
(32.2 feet per second squared)h = head on the orifice in inches
measured from the center of theopening
Outdated Publication, for historical use. CAUTION: Recommendations in this publication may be obsolete.
Figure 5. Ratio of orifice coefficients.
TRAJECTORYMEASUREMENT
The trajectory or Perdue method esti-mates the flow rate from the end of a hor-izontal pipe using the horizontal and ver-tical components of the top of the dis-charging stream (Figure 6). A straightedge and plumb bob or a carpenter’ssquare can be used for the measurements.The flow rate from a full horizontal pipecan be calculated from the equation:
where:Q = 3.61 AX/Y
A = cross sectional area of dischargepipe in square inches.
X = horizontal distance in inchesY = vertical distance in inches
Tables frequently give the flow froma given size of pipe based on the dis-tance out to a point where the top of thejet has fallen 12 inches or, conversely,based on the amount of fall at some setdistance from the end of the pipe.
With some modifications, thismethod can be used for pipes that arenot flowing full and for vertical pipes.
ULTRASONIC FLOW METERSA recent development in water mea-
surement devices are ultrasonic metersthat are used primarily for survey andfield testing of other meters. One majoradvantage is that they can measure flow
in a non-intrusive manner—the systemdoes not have to be shut down to inserta probe or other equipment—but theyare expensive.
Ultrasonic meters use one of threedistinctly different principles: the Dop-pler effect, transit time, and eddy-corre-lation principle. Various models ofmeters using these principles are avail-able. They may be important tools forthe irrigation industry when used forlarge scale survey work and in-fieldmeter accuracy verification, but are notlikely to be used by individual irrigators.
OTHER MEASUREMENTMETHODS
Turbine meters are somewhat likepropeller meters. One type uses a smallpaddle wheel that is inserted into the
side of the pipe and may be extendedinto the flow by various amountsdepending upon the size of the pipeand the manufacturer’s recommenda-tions. The other is a simple propeller-type blade, which is mounted in thecenter of the pipe and rotates througha substantial portion of the pipe area.
The problem with the first type isthat it essentially measures the velocityat a single point within the flow. If themeasuring section is within a longstraight length of pipe, it may work wellwith a standard rating. But, if anythingdisturbs the normal velocity distributionwithin the pipe, it must be calibrated inplace, using another measuring device;if the flow is variable, it may need a cal-ibration for each flow rate.
The second type is essentially a pro-peller meter with the same restrictionsand benefits listed for propeller meters.
The other methods listed below arenot commonly used in Kansas. However,there may be occasions when they canbe used.
Injection-Velocity Methods. Saltvelocity, color velocity and radioisotopemethods are based on the injection of asolution or material into the stream flow.Time measurements taken from sensingelements downstream are then used toestimate the average time of travel,which should be closely related to theaverage velocity of flow. The amount ofdilution can also be measured to obtain avolume-time relationship.
Magnetic Flow Meters. Their oper-ation relies on the principle that a volt-age is induced in an electrical conductormoving through a magnetic field. In thiscase, the conductor is the flowing waterbeing measured. For a given fieldstrength, the magnitude of the inducedvoltage is proportional to the velocity ofthe conductor.
Figure 6. Required measurements to obtain flow from horizontal pipes.
Outdated Publication, for historical use. CAUTION: Recommendations in this publication may be obsolete.
Proportional meters. Proportionalmeters divert and measure a small,known percentage of the total flow, usu-ally with a propeller-type sensor or pos-itive displacement type of meter. (Seethe section on venturi meters).
Deflection meters. Flow rate alsocan be determined by measuring theforce that moving water exerts onobjects placed in the flow. Vanes orplates with sensing elements are placedin the stream to measure the deflectioncaused by the flowing water. Thedeflection is calibrated with flow rate togive a flow rate reading.
SUMMARYIncreasing energy costs and decreas-
ing water supplies point out the need forbetter water management, which requiresknowing flow rates and volumes. Watermeasurement is one of the first steps in atotal water management program.
In most cases, metering devices willpay for themselves in water savings,optimum yields and lowered energycosts.
The most common water measure-ment device is the propeller meter whichcan give both flow rate and flow volume.Other methods include pitot tubes, ori-fice plates, nozzles, venturi meters, andtrajectory methods.
Additional information on flow mea-surement methods is available throughExtension irrigation engineers, yourcounty Extension agricultural agent, theSoil Conservation Service and irrigationequipment dealers.
UNITS OF WATER MEASUREMENTThe units below are common water measurement terms, conversions andrelationships.VOLUME:Acre-inch (ac.in.): The volume of water required to cover an acre 1 inch deep.Acre-foot (ac.ft): The volume of water required to cover an acre 1 foot deep.1 cubic foot = 7.48 gallons = 62.4 pounds1 acre-inch = 3,630 cubic feet - 27,154 gallons1 acre-foot = 12 acre inches = 43,560 cubic feet = 325,851 gallons1 cubic meter = 1,000 liters = 264.18 gallonsFLOW RATE:1 cubic foot per second (cfs) = approximately 450 gallons per minute (gpm)1 acre-inch/hour = approximately 450 gpm = 1 cfs
3-in 4-in. 5-in. 6-in. 7-in. 8-in.
Head orifice orifice orifice orifice orifice orifice
(inches) 4-in. 6-in. 6-in. 8-in. 6-in. 8-in. 8-in. 10-in. 10-in. 10-in.Pipe Pipe Pipe Pipe Pipe Pipe Pipe Pipe Pipe Pipe
6 108 82 160 150 305 240 408 3458 122 94 185 170 350 280 458 395
10 133 104 205 190 393 316 508 44512 146 114 225 208 430 346 556 49014 157 123 243 224 465 376 599 53016 167 132 257 238 495 402 636 56818 178 140 271 252 524 426 672 60420 187 148 285 266 548 449 708 63622 197 156 299 279 572 470 744 66424 205 164 310 291 596 488 776 69226 214 171 323 303 620 504 805 72028 222 177 335 314 644 520 831 74730 230 183 346 325 668 536 857 77332 239 189 357 335 692 552 882 79934 246 195 369 345 715 568 907 82436 254 200 380 354 737 584 931 84738 260 205 390 363 759 600 935 86740 266 210 401 371 781 616 979 88742 272 214 411 380 800 631 1001 90644 278 219 420 388 820 645 1023 92546 284 224 429 396 837 659 1045 94448 290 229 440 405 855 672 1067 96350 296 234 448 413 872 686 1089 982
G.p.m. G.p.m. G.p.m. G.p.m. G.p.m. G.p.m G.p.m. G.p.m. G.p.m. G.p.m.
600666728785838887933979
10221064110411431181121812511281131113411371140114311461
52 302 238 457 421 888 700 1110 1000 149154 307 243 465 429 904 714 1130 1018 152056 313 248 472 437 919 727 1150 1036 154858 317 252 480 445 934 739 1170 1052 157460 323 257 489 453 948 751 1190 1068 159862 328 262 496 461 961 763 1209 108464 333 266 504 469 974 775 1227 109966 338 271 513 475 988 787 1245 111368 343 275 520 483 1002 799 1263 112770 349 280 525 491 1016 811 1280 1140
93510401120119412661336140414711529158516411697175318091865
Table 1. Discharge from circular pipe orifices with free discharge.
COOPERATIVE EXTENSION SERVICE, MANHATTAN, KANSASL-877 June 1993Issued in furtherance of Cooperative Extension Work, acts of May 8 and June 30, 1914, as amended. Kansas State University, County ExtensionCouncils, and United States Department of Agriculture Cooperating, Richard D. Wootton, Associate Director. All educational programs andmaterials available without discrimination on the basis of race, color, national origin, sex, age, or disability. 6-93—3M
File: Engineering 4-3 (Irrigation)
Outdated Publication, for historical use. CAUTION: Recommendations in this publication may be obsolete.
Figure 2. Diagram of a venturi meter.
Figure 3. Diagram of flow nozzle.
Pump discharge pipe
Figure 4. Orifice meter.
walls of the pipe, and packing glandsare secured into the holes to sealbetween the probe and the pipe, so nowater will leak from the test pipe. Withone hole in the probe facing directlyupstream and the other facing down-stream, a series of velocity measure-ment are taken as described above andthe flow rate determined.
The other common pitot device is the“Anubar,” a probe with two openingsupstream and one down. It is installedin the pipe through a single hole using apacking gland to seal against leakage,but it is not moved. For each size ofpipe there is an appropriate probe whichaverages the velocity; corrections maybe made for slight differences in thepipe diameter. These devices are nor-mally used for measuring flows insprinkler lines where a sprinkler may beremoved to install the meter.
CONVERGENCE METERSConvergence meters use differences
in pressure or head to measure the flowrate. The flow of water through a con-striction in a pipe lowers the pressure atthe constriction. How much the head or
pressure drops between the uncon-stricted flow and the constricted flow isa function of the flow rate.
Three common types of convergencemeters are the venturi meter (Figure 2),the nozzle (Figure 3), and the orificeplate (Figure 4).
All of these devices work on the sameprinciple. The major difference is theamount of loss that occurs due to flowthrough the device. The venturi meterhas the least pressure loss by far, so it isthe preferred method where energylosses are a concern. The nozzle issomewhat better than the orifice plate,but the difference is much smaller thanthe difference between the venturi meterand the nozzle or orifice plate.
All of these devices convert the pres-sure in the pipe upstream from anobstruction into increased speed of thewater at the obstruction, which lowersthe pressure where the speed isincreased. Through a given amount ofobstruction, this lowering of pressure isa constant for each rate of flow. Theflow is determined by measuring thepressure change. The temperature of thewater and the shape of the obstructionmake a difference, but they are not con-sidered in this discussion. Water temper-atures in the field do not vary greatly,and each device generally has a ratingtable or characteristic curve which isused to determine flow from the pressuremeasurements.
All of these devices may be usedwithin a pipe flowing full of water or afull pipe discharging freely into theatmosphere, but in the latter case, thereis no advantage for a venturi meter ornozzle so orifice plates are used almostexclusively.
With full pipe flow, the difference inpressure is normally not more than a fewfeet of water if the meter has been prop-erly sized and the range of flows is notgreat. As a consequence, normal pres-sure gauges are not suitable for the mea-surements. A foot of water pressure isless than 0.5 pound per square inch. Somanometers, explained below, and otherdevices are used to measure pressuredifferences.
Convergence meters normally onlymeasure the rate of flow, but it is possi-ble to measure volumes. One venturimeter which does this uses a shunt line.The two pressure measuring points areconnected with a pipe and, because of
the difference in pressure, water willflow through this pipe. In the shunt-linemeter, this is measured with a householdwater meter. A restricting device isinserted into the shunt line, so the flow isproportional to the flow through the ven-turi. This type of system could be usedfor either the nozzle or the orifice, butthe equipment has not been developed.
Pressure differences usually are mea-sured with a manometer, which is sim-ply a glass or plastic tube in which onecan see the level to which water willrise at a given pressure. If the differencein pressure between two points is re-quired, an inverted U-shaped tube maybe used to measure the difference with asingle reading. Manometer measure-ments are generally more accurate thanpressure gauge readings, because1 pound per square inch pressure willcause a rise in water level of about2.31 feet. Thus, even a crude measure-ment to the nearest inch reveals thepressure to the 0.04 psi, which is muchbetter than most pressure gauges.
Properly installed orifice plates onopen pump discharges can provide afairly inexpensive and accurate way tomeasure flow rate. For simplicity, thepipe should be horizontal, with noelbows, valves or other fittings closerthan 10 pipe diameters upstream fromthe orifice plate. The ratio of the orificediameter to the pipe diameter should begreat enough to cause an easily measur-able pressure rise and the ratio must beknown to select the proper dischargecoefficient. A restriction of about0.50 is a common starting point.
Using the manometer readings, thedischarge can then be read from a table(see Table l), or calculated:
Q=Ca 2gh,where:Q = discharge, gpmC = discharge coefficient, see Figure 5.a = cross-sectional area of the orifice
in square inchesg = acceleration due to gravity
(32.2 feet per second squared)h = head on the orifice in inches
measured from the center of theopening
Outdated Publication, for historical use. CAUTION: Recommendations in this publication may be obsolete.
Figure 5. Ratio of orifice coefficients.
TRAJECTORYMEASUREMENT
The trajectory or Perdue method esti-mates the flow rate from the end of a hor-izontal pipe using the horizontal and ver-tical components of the top of the dis-charging stream (Figure 6). A straightedge and plumb bob or a carpenter’ssquare can be used for the measurements.The flow rate from a full horizontal pipecan be calculated from the equation:
where:Q = 3.61 AX/Y
A = cross sectional area of dischargepipe in square inches.
X = horizontal distance in inchesY = vertical distance in inches
Tables frequently give the flow froma given size of pipe based on the dis-tance out to a point where the top of thejet has fallen 12 inches or, conversely,based on the amount of fall at some setdistance from the end of the pipe.
With some modifications, thismethod can be used for pipes that arenot flowing full and for vertical pipes.
ULTRASONIC FLOW METERSA recent development in water mea-
surement devices are ultrasonic metersthat are used primarily for survey andfield testing of other meters. One majoradvantage is that they can measure flow
in a non-intrusive manner—the systemdoes not have to be shut down to inserta probe or other equipment—but theyare expensive.
Ultrasonic meters use one of threedistinctly different principles: the Dop-pler effect, transit time, and eddy-corre-lation principle. Various models ofmeters using these principles are avail-able. They may be important tools forthe irrigation industry when used forlarge scale survey work and in-fieldmeter accuracy verification, but are notlikely to be used by individual irrigators.
OTHER MEASUREMENTMETHODS
Turbine meters are somewhat likepropeller meters. One type uses a smallpaddle wheel that is inserted into the
side of the pipe and may be extendedinto the flow by various amountsdepending upon the size of the pipeand the manufacturer’s recommenda-tions. The other is a simple propeller-type blade, which is mounted in thecenter of the pipe and rotates througha substantial portion of the pipe area.
The problem with the first type isthat it essentially measures the velocityat a single point within the flow. If themeasuring section is within a longstraight length of pipe, it may work wellwith a standard rating. But, if anythingdisturbs the normal velocity distributionwithin the pipe, it must be calibrated inplace, using another measuring device;if the flow is variable, it may need a cal-ibration for each flow rate.
The second type is essentially a pro-peller meter with the same restrictionsand benefits listed for propeller meters.
The other methods listed below arenot commonly used in Kansas. However,there may be occasions when they canbe used.
Injection-Velocity Methods. Saltvelocity, color velocity and radioisotopemethods are based on the injection of asolution or material into the stream flow.Time measurements taken from sensingelements downstream are then used toestimate the average time of travel,which should be closely related to theaverage velocity of flow. The amount ofdilution can also be measured to obtain avolume-time relationship.
Magnetic Flow Meters. Their oper-ation relies on the principle that a volt-age is induced in an electrical conductormoving through a magnetic field. In thiscase, the conductor is the flowing waterbeing measured. For a given fieldstrength, the magnitude of the inducedvoltage is proportional to the velocity ofthe conductor.
Figure 6. Required measurements to obtain flow from horizontal pipes.
Outdated Publication, for historical use. CAUTION: Recommendations in this publication may be obsolete.
Proportional meters. Proportionalmeters divert and measure a small,known percentage of the total flow, usu-ally with a propeller-type sensor or pos-itive displacement type of meter. (Seethe section on venturi meters).
Deflection meters. Flow rate alsocan be determined by measuring theforce that moving water exerts onobjects placed in the flow. Vanes orplates with sensing elements are placedin the stream to measure the deflectioncaused by the flowing water. Thedeflection is calibrated with flow rate togive a flow rate reading.
SUMMARYIncreasing energy costs and decreas-
ing water supplies point out the need forbetter water management, which requiresknowing flow rates and volumes. Watermeasurement is one of the first steps in atotal water management program.
In most cases, metering devices willpay for themselves in water savings,optimum yields and lowered energycosts.
The most common water measure-ment device is the propeller meter whichcan give both flow rate and flow volume.Other methods include pitot tubes, ori-fice plates, nozzles, venturi meters, andtrajectory methods.
Additional information on flow mea-surement methods is available throughExtension irrigation engineers, yourcounty Extension agricultural agent, theSoil Conservation Service and irrigationequipment dealers.
UNITS OF WATER MEASUREMENTThe units below are common water measurement terms, conversions andrelationships.VOLUME:Acre-inch (ac.in.): The volume of water required to cover an acre 1 inch deep.Acre-foot (ac.ft): The volume of water required to cover an acre 1 foot deep.1 cubic foot = 7.48 gallons = 62.4 pounds1 acre-inch = 3,630 cubic feet - 27,154 gallons1 acre-foot = 12 acre inches = 43,560 cubic feet = 325,851 gallons1 cubic meter = 1,000 liters = 264.18 gallonsFLOW RATE:1 cubic foot per second (cfs) = approximately 450 gallons per minute (gpm)1 acre-inch/hour = approximately 450 gpm = 1 cfs
3-in 4-in. 5-in. 6-in. 7-in. 8-in.Head orifice orifice orifice orifice orifice orifice
(inches) 4-in. 6-in. 6-in. 8-in. 6-in. 8-in. 8-in. 10-in. 10-in. 10-in.Pipe Pipe Pipe Pipe Pipe Pipe Pipe Pipe Pipe Pipe
6 108 82 160 150 305 240 408 3458 122 94 185 170 350 280 458 395
10 133 104 205 190 393 316 508 44512 146 114 225 208 430 346 556 49014 157 123 243 224 465 376 599 53016 167 132 257 238 495 402 636 56818 178 140 271 252 524 426 672 60420 187 148 285 266 548 449 708 63622 197 156 299 279 572 470 744 66424 205 164 310 291 596 488 776 69226 214 171 323 303 620 504 805 72028 222 177 335 314 644 520 831 74730 230 183 346 325 668 536 857 77332 239 189 357 335 692 552 882 79934 246 195 369 345 715 568 907 82436 254 200 380 354 737 584 931 84738 260 205 390 363 759 600 935 86740 266 210 401 371 781 616 979 88742 272 214 411 380 800 631 1001 90644 278 219 420 388 820 645 1023 92546 284 224 429 396 837 659 1045 94448 290 229 440 405 855 672 1067 96350 296 234 448 413 872 686 1089 982
G.p.m. G.p.m. G.p.m. G.p.m. G.p.m. G.p.m G.p.m. G.p.m. G.p.m. G.p.m.
600666728785838887933979
10221064110411431181121812511281131113411371140114311461
52 302 238 457 421 888 700 1110 1000 149154 307 243 465 429 904 714 1130 1018 152056 313 248 472 437 919 727 1150 1036 154858 317 252 480 445 934 739 1170 1052 157460 323 257 489 453 948 751 1190 1068 159862 328 262 496 461 961 763 1209 108464 333 266 504 469 974 775 1227 109966 338 271 513 475 988 787 1245 111368 343 275 520 483 1002 799 1263 112770 349 280 525 491 1016 811 1280 1140
93510401120119412661336140414711529158516411697175318091865
Table 1. Discharge from circular pipe orifices with free discharge.
COOPERATIVE EXTENSION SERVICE, MANHATTAN, KANSASL-877 June 1993Issued in furtherance of Cooperative Extension Work, acts of May 8 and June 30, 1914, as amended. Kansas State University, County ExtensionCouncils, and United States Department of Agriculture Cooperating, Richard D. Wootton, Associate Director. All educational programs andmaterials available without discrimination on the basis of race, color, national origin, sex, age, or disability. 6-93—3M
File: Engineering 4-3 (Irrigation)
Outdated Publication, for historical use. CAUTION: Recommendations in this publication may be obsolete.
