1

Ultrasonic Flowmeter SystemsBackground information

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TABLE OF CONTENTS

1 INTRODUCTION 4

2 APPLICATIONS 5

3 THEORETICAL FOUNDATIONS 6

3.1 OVERVIEW OF THE MEASUREMENT PRINCIPLES 63.2 SING-AROUND PRINCIPLE 63.3 ULTRASONIC REPRODUCTION PRINCIPLE 63.4 TRAVEL-TIME PRINCIPLE 63.5 TRAVEL-TIME MEASUREMENT 7

4 QUANTUM´S FLOWMETER SYSTEM 8

4.1 SYSTEM DESCRIPTION 84.2 LATEST METHODS IN DIGITAL PROCESSING 84.3 INSTALLATION 84.4 CONFIGURATION POSSIBIITIES 8

4.4.1 SINGLE PATH SYSTEM 84.4.2 CROSSED PATH SYSTEM 84.4.3 RESPONDER SYSTEM 94.4.4 MULTIPLE LEVEL MEASUREMENT 9

5 CALIBRATION 10

5.1 VELOCITY COEFFICIENT 1k 10

5.1.1 EMPIRICAL 105.1.2 LOGARITHMIC VELOCITY DISTRIBUTION 105.1.3 HYDRO-NUMERIC MODELING 11

5.2 CALIBRATION FACTOR 2k 11

6 MEASUREMENT UNCERTAINTIES DUE TO PHYSICAL FACTORS 13

6.1 SUSPENDED PARTICLES 136.2 AIR BUBBES 136.3 TEMPERATURE AND SALT CONTENT 14

7 PROBLEM SOLVING OR ERROE DETECTION 16

7.1 RANDOM ERRORS 167.2 SYSTEMATIC ERRORS 16

8 ENVIRONMENTAL COMPATIBILITY 17

9 INSTALLATION LOCATION EVALUATION 18

9.1 HYDRAULIC CONDITIONS 189.2 MORPHOLOGICAL CONDITIONS 189.3 PHYSICAL CONDITIONS 18

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9.4 PRACTICAL AND LOGISTICAL CONDITIONS 199.5 INSTALLATION CONFIGURATIONS 20

10 DESIGN AND CONSTRUCTION 21

10.1 PROJECT PLAN 2110.2 BUILDING SPECIFICATIONS 2210.3 SYSTEM INSPECTIONS AND WARRANTY 22

11 THE MANAGEMENT AND MAINTENANCE OFTHE ULTRASONIC MEASUREMENT SYSTEM 22

11.1 TRANSDUCER 2211.2 SIGNAL CABLE 2211.3 MEASUREMENT ELECTRONIC 2211.4 HOUSING 2311.5 SOFTWARE 2311.6 WATER AND SITE MAINTENANCE 23

12 WORK SAFETY 23

13 LITERATURE 24

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1. Introduction

Quantum Hydrometrie GmbH is an innovative and dynamic company located in Berlin,Germany, which develops and markets state-of-the-art ultrasonic flowmeter systems. As aspecialist in integrated hydrometric system solutions, Quantum is a reliable partner in watermanagement projects.

Quantum Hydrometrie was founded in 1994 by a group of people associated with theUniversity of Technology, Berlin. Since then, Quantum Hydrometrie has grown quickly andestablished itself within Europe as a successful company; with over 200 projects completed,the Quantum product family has become the de-facto standard instrumentation used todayby hydrologists to improve their understanding of water flow in rivers and channels. Quantumhas also developed a discharge measurement system for pipes that has been extremely wellreceived based on its reliability and efficiency.

This hand book is concerned with explaining the fundamental, theoretical foundations ofultrasonic discharge measurement, as well as possible applications and installation modelsof our flowmeter system. Ultrasonic discharge installations are rapidly replacing less efficientand less accurate traditional measurement methods and are increasingly being used in themeasure of discharge in rivers, channels, and pipes. Because ultrasonic measurement is themost dynamic and cost effective measurement to date, it is important to introduce the newpossibilities afforded by this method. We hope that you find this handbook both helpful andinformative and are able to benefit from our excitement about ultrasonic measurement. Ifthere are any questions, please contact us at:

Quantum Hydrometrie GmbHGeneststr. 510829, BerlinGermany

Tel: (0 30) 69 81 10-0Fax: (0 30) 69 81 10-99

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2. Applications

Discharge measurement is an importanthydrographic and hydrometric concern.Here are some of the many possibleapplications:

• appropriate management of watersupplies

• protection from high waters• management of water projects (dams,

pump works, reservoirs or storagebasins, siphon constructions, etc.)

• calibration and validation of hydraulicand hydrometric calculations

• generation of statistics and records forconsulting and other purposes

• bestowing of water rights

There exist different methods to determinethe water velocity within a determinedcross- section and thereby deriving thedischarge measurement. Because of thevarious difficulties previously encounteredwith mechanical propeller type systems,the trend has been towards thedevelopment of modern electronic systems;for example, measurement with the help ofultrasonic technologies has led to thedevelopment of state-of-the-art flowmeters.

Traditional propeller systems measured thewater velocity by the mechanicalmovement of propellers. Thedisadvantages to this method are asfollows: propeller systems are bothexpensive and time consuming becausethey require more manual labor both toinstall and to operate; the measurementpossibilities afforded by traditional propellersystems are also limited because acontinuous discharge measurement is onlypossible in watercourses in which a stabledischarge curve exists, i.e. the directrelationship between the water level anddischarge is explicit and continuous.

Picture 1: An ultrasonic flowmeter in a water course withmoving current

In opposition to mechanical methods, it ispossible with ultrasonic dischargemeasurement to carry out continuos, fullyautomatic discharge measurements overextended periods of time and without anymanual assistance. Not only does acontinuous measurement provide morereliable data, but due to the dramaticreduction of required labor time, theultrasonic flowmeter systems are more costeffective. Also, in water storage areas andbi-directional water flow areas where adirect relationship between the water leveland the discharge does not exist, acontinuous discharge measurement is notonly possible with the help of ultrasound,but also more accurate and cost effectivethan mechanical methods of measurement.Another advantage to ultrasonicmeasurement is that the current of the riveris in no way influenced by the measuringequipment, allowing for a more accuratemeasure.

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3. Theoretical foundations

3.1 Overview of the measurementmethodsUltrasonic discharge measurement is anindirect measurement method, meaningthat the discharge is calculated by meansof an equation in a continuos measure ofthe water velocity and the water level.

For the velocity measurement withultrasound, there are three possiblemethods:

• the sing-around method• the ultrasonic-reproduction method• the ultrasonic travel-time method

3.2 Sing-around methodOn a defined path, a short ultrasonic signalis sent from one transducer to another sothat the receiving transducer, with the helpof a returning signal device, is triggered tosend a new signal.

Diagram 2: Principle of the acoustic sing-around-method

With this model, it is possible to “capture”and “observe” an ultrasonic signal withinshort, measured distances over as long aperiod as needed. When the number ofcycles that the ultrasonic signal hascompleted within a determined time periodare counted, the frequency is obtained andwith this particular frequency, the signal´svelocity is determined and thereby thevelocity of the current.

3.3 Ultrasonic-reproduction methodWith the Ultrasonic-reproduction principle,the ultrasonic transducer receives thereflected signal with a different frequencythan was measured when the signal wasfirst sent. Once the difference in the

frequencies is determined, it is postulatedhow fast the current moves.

Diagram 3: Principle of the reproduction measurement

The frequency difference is marked by theshift in the reproduction frequency, which isproportional to the velocity of the reflectedparticles. In addition to the frequency shift,the travel time of the returning “reproduced”signals are measured. From this measure,the reduction of the particles found in the“reproduced” signals are determined,thereby enabling the determination of thevelocity distribution along the measuredpath.

The reproduction principle is useful in thefollowing areas:

• ADCP-measurement systems fordischarge measurement in rivers andchannels;

• ADCP-measurement systems forcurrent measurement in oceanographyultrasonic-reproduction-detectors forthe discharge measurement in wastewater canals and small coagulates.

3.4 Travel-time methodThe measuring principle of the travel-timemethod is based upon the directmeasurement of the running time of theacoustic signals travelling between twoultrasonic transducers.

This method is described as follows: atleast two ultrasonic transducers areinstalled so that the acoustic signalstransmitted between them are diagonal tothe water current. The acoustic signalrunning against the current has a longertravel time than the acoustic signal runningwith the current. The difference in traveltimes is directly proportional to the flowvelocity and, when the cross section is

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known, directly proportional to thedischarge.

The travel-time measurement is realizedwith two methods: the frequency methodand the impulse method. With thefrequency method, a sequence of definedfrequency signals are sent through thewatercourse between a sending and areceiving transducer, the travel-time ofthese signals being the object ofmeasurement. With the impulse method,the travel-time of a short acoustic impulseis measured with a determined frequency.

The travel-time principle, configured withthe impulse method, has the advantagethat a continuous measurement is possible.It is because of this advantage that thisparticular travel-time principle has beenapplied with great success to the ultrasonicmeasurement of discharge in rivers,channels, and pipes. This is the methodused by Quantum and accepted by today´shydrologists as the method most worthy ofthe investment of capital.

3.5 Travel-time MeasurementFor the determination of the travel time, anultrasonic transducer receives an acousticsignal in the form of a square wave signal(sine wave signal). The transducer thentransforms these signals, which were sentthrough the watercourse, in an acousticwave packet. The travel-time of this wavepacket is then measured.

Diagram 4: A travel-time schematic diagram

The travel times 21−t and 12−t aredetermined with the direct travel-timemethod, whereby the zero crossing thatwas previously determined by themanufacturer with the help of an analogueor a digital signal recognizer is calculated.

The travel-time of the path traveling withthe current is calculated with:

21

2121

−

−− +

=vc

Lt

and the travel-time of the path 12−t

21

2112

−

−− −

=vc

Lt

whereby:21−L The length of the acoustic paths

between transducers 1 and 221−V The mean value between

transducer 1 and 2 of the localcurrent velocities along the acousticpath.

The travel-time difference t∆ is thenapproximated with the acceptance c >> v.

221212

cvLt −−=∆

Example:The velocity of the acoustic signal in waterdepends on the salt content and the watertemperature ca. 1400 until 1500 m/s.

For example, with a signal velocity of c =1450 m/s, a path length of L = 10 m, and acurrent velocity of 121 =−v mm/s (0,001m/s), the measured time difference onlyamounts to ∆ t ≅ 9,5 × 10 9− S.

With the travel-time method, the timemeasurement must be taken in nanoseconds in order for the smaller currentstream velocities to be resolved with therequired exactness.

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4. Quantum´s flowmeter system

Our ultrasonic flowmeter systems, whichutilize the travel-time method, are knownfor their high quality and wide range offunctions. Along with hydrometricconsultations, Quantum offers completeand ready to use system installations whichare known for their stability and wide rangeof capabilities.

4.1 System descriptionOur systems are known for their user-friendliness and accessibility. The softwareis easy to install and direct access to datais possible by modem through a PC orlaptop. While remote data transmissionsenable efficient access to the system froma remote computer, including the capabilityto store collected data, remote diagnosisfunctions enable easy and reliablemaintenance. Because these diagnosisfunctions allow full access to the measuringsystem, expensive on-site repairs can belargely avoided, just another advantage toour ultrasonic flowmeter systems.

A variety of display and output optionsare available as well, ensuring that ourultrasonic flowmeters are efficient,compact, and require little maintenance.

4.2 Latest methods in digital processingBecause of our strong emphasis on thedevelopment of the latest methods in digitalsignal processing, our systems are able todeal with the most challenging hydrometricmeasurements facing hydrologists today; inparticular, the dampening of the acousticsignal sent by the ultrasonic transducers bysuspended solids or its disturbance bysignificant amounts of air bubbles can bealmost eliminated by our ultrasonicflowmeter system. These topics arediscussed in more detail later in thehandbook.

4.3 InstallationEven within the most challenging rivers, amore accurate measurement can beachieved with the help of Quantum´s costeffective software. One advantage of oursystem is that the use of costly equipmentto align the transducer heads duringinstallation can be avoided: due to the

latest signal recognition technology, thegeometry of the signal paths is determinedby functions within the software and not bythe physical positioning of the transducers,meaning that not only is the installation lesscostly, but the resulting measurement moreaccurate.

4.4 Configuration possibilitiesThe arrangement of the transducersdepend on the flow conditions found in theriver at the place of measurement.

4.4.1 Single Path SystemIf the current runs parallel to the banks, asingle path system can be used. Thisconfiguration consists of two transducers,which both send and receive the acousticsignal, being installed on one level anddiagonally to the direction of flow.

The single path system is best suited tochannels, canals and other constructedwaterways with easily definable cross-sections.

Diagram 5: Single path system

The acoustic signal travels alternativelybetween 1-2 (with the current) and 2-1(against the current).

4.4.2 Crossed Path SystemRarely does the main current run parallel tothe bank. Bends in the river, the profilegeometry, and differing depths influencethe water flow. In these cases, a crossedpath system is recommended because asecond “crossed” path, installed on thesame level and crossed to the first path,enables the calculation of the anglebetween the main current and the riverbank, providing a more accurate measure.

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Diagram 6: Crossed path system

The running time of the acoustic signalsbetween transducers 1-2 and3-4 are simultaneously measured. Thereversed measure is then taken betweentransducers 2-1 and 4-3.

4.4.3 Responder systemBoth the single and crossed path systemsrequire that signal cables cross the river. Insituations where this is inappropriate, aresponder system is recommended. Thisconfiguration consists of the installation ofa responder on one side of the river,opposite to the transducer(s) located on thesame side of the river as the measuringsystem.

Diagram 7: Responder system

In a Responder configuration, the acousticsignal is first sent against the current, alongthe path defined by transducers 4 and 3.With minimal delay, the signal is guided bythe responder and once again sent againstthe current, along the path defined bytransducers 2 and 1. Next, the measure ofthe running time when the acoustic signaltravels with the current is taken when thesignal travels in the opposite direction,along the paths defined by 1-2 and 3-4.

4.4.4 Multiple level measurementAll three of the above systems are alsoavailable as multiple level measurements.This configuration is recommended at siteswith an irregular profile or a large variationin water level; the installation of two ormore levels enables a better measure ofirregular profiles and changing water levels.

Diagram 8: Multiple level measurement

Each individual level can be configuredwith a single or crossed path system.

Even though all of the configurations haveapplication possibilities, they do differ inreliability. For example, the dischargemeasurement is most accurate with amultilevel, crossed path system; while themeasure of the main current direction ismost accurate with the crossed pathsystem, the profile of the measured area isbest measured with a multiple levelmeasurement.

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5. Calibration

A continued understanding of ourmeasuring principle is possible with adiscussion of calibration. What we mean bycalibration is as follows: the deviation of themeasured values are determined from adetermined reference value.

The hydrometric calibration of an ultrasonicflowmeter installation is based on theestablished relationship between theunknown velocities found in the determinedprofile and the mean velocity of the current.

Diagram 9: Typical example of a velocity profile in adetermined cross-section

This relationship can be determinedthrough a theoretical deduction,mathematical calculation, or a mechanicalmeasurement in the natural environment.

To differentiate between the theoreticallygrounded velocity coefficient and a specificcalibration factor from a measurementinstallation, it makes sense to split the k-Factor equation into two parts.

21 kkk ⋅=

With:

1k Theoretically derived velocitycoefficient

2k Calibration factor for a specificmeasurement installation

It is then possible to calculate thedischarge with the following equation:

gvAkkQ ⋅⋅⋅= 21

With the two measurements v and A, is itpossible to calculate the discharge Q andto determine the calibration factors 1k and

2k in accordance to a determinedsignification.

5.1 Velocity coefficient 1k

5.1.1 EmpiricalFor wide natural watercourses, the velocitydistribution is mostly a function of the waterdepth. According to ISO 6416 (1992), thetheoretical velocity coefficient 1k is valid fornatural watercourses, and is dependant onthe height of the installed measurementpaths. The following table has the resultingnumerical values.

Table 1 Allocation of 1k -factors accordingto ISO 6416

Z/h 0,1 0,2 0,3 0,4 0,5K1

0,846 0,863 0,882 0,908 0,937

Z/h 0,6 0,7 0,8 0,9K1

0,979 1,039 1,154 1,424

With:

z/h Transducer installation level/waterdepth

These values were ascertained from 15measurements in 7 different measurementsites (1,94 m< h m <2,20m).

5.1.2 Logarithmic velocity distributionIt is also possible to calculate the meancurrent velocity in natural watercourses byan approximation based on the velocitydistribution. Best suited to water courses inwhich the banks exceed a 90 degreeangle(b>10h), this method requires that asingle path measurement be installed 40%of the water depth above the watercoursefloor. From this measurement, the meancurrent velocity corresponds to the totalmeasured current velocity in the cross-section. With the calculation of thecalibration factor, the relationship betweenthe current velocity gV and the mean

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current velocity mV finds an acceptance of1.

Based on an ultrasonic flowmeterinstallation, the mean current velocity isdirectly measured by an acoustic signalwhich is installed 40% of the water depthabove the watercourse.

Any time the water depth in a watercoursechanges, or the watercourse floor becomesrougher so that secondary currents arecreated, there is an impact on themeasurement of the velocity distribution.The secondary currents interact with eachother and cause the slower water particleslocated closer to the banks to betransported into the main current area. Theresult is the reduction of the water velocityand the increased loss of friction, whichrenders the velocity distribution logarithminvalid.

5.1.3 Hydro-numeric modelingFor concrete canals with rectangular ortrapazoidal cross-sections, the influence ofoffset margins on the velocity profile cannotbe neglected.

In these instances, it is possible with thehelp of hydrometric numeric models (forexample SIMK-modeling, Koelling, 1994) tocalculate the velocity distribution in cross-sections with a constant current, making itpossible to ascertain the size of the velocitycoefficient 1k for every arbitrary altitudealong the measurement path (See diagram10).

Through the repetition of the simulationcalculation, with the help of the Finite-element network, and the analysis of thedifferent water levels, the completecalibration for the measurement installationas function )(1 hk is obtained in relation tothe water level.

Diagram 10: Determination of the 1k value in themeasurement path with the help of the SIMK-methods(Koelling, 1994)

5.2 Calibration factor 2kIn natural watercourses where a selectedcross-section is influenced by tidal watersor changing floor levels (i.e. variation in thewatercourse geometry), the deviation of thetheoretical velocity profile can also becalculated.

Further impacts on the velocity profileresults from bends in the water course,wind on the water surface, ground or watersurface currents located before drainagepipes or spill way gates. Diagram 11 showsthe different velocity profiles in relation tothe profile shape of the water course floor.

Diagram 11: Different forms of the velocity profile

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The specific deviations found in thetheoretical velocity coefficients 1k inspecific measurement installations arecorrected with the discharge measure of asecond velocity coefficient factor 2k .

In these cases, it is essential that amanager carry out the hydrometriccalibration as a result of the unforeseeablenature of the current behavior in themeasurement installation area.

Respective to the water level anddischarge, different calibration factors couldbe given so that for the calibration factor

2k , the following general correlation mustfind acceptance.

2k = f(water level w, current angle φ )with A = f(w) and Q = f(φ )

2k = f(cross-section A, discharge Q)

From the reference measurements derivedfrom the hydrometric calibration referred toearlier, it is possible to calculate functionalcorrelation for the calibration 2k that takethe form of a calibration matrix.

For a multiple-point measurement, ahydrometric propeller type system isemployed to carry out a point measurementof the current velocity throughout the entirecross-section. The total discharge isdetermined through the vertical andhorizontal integration of the individual pointmeasurements.

Diagram 12: Discharge cross-section according to thevelocity area method (Pegelvorschrift, 1992)

In large watercourses where a multiple-point measurement is possible only with asubstantial cost, there exists anotherpossibility to measure the velocity profile;the ADCP (Acoustic transducer currentprofiler) is a measurement method thatoperates on the transducer-effect model toascertain the velocity of the current. Thatmeasurement system is mounted on aspecial boat or float. It is uncommon,however, that such a measurement isrequired.

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6. Measurement uncertainties due tophysical factors

6.1 Suspended particlesWhen an acoustic signal is sent throughwater, the signal loses some energy. As aresult, the amplitude of the acoustic signalcontinually decreases and the signal isdampened. This dampening means that theintensity of the received signal is less thanthe intensity of the original sent signal.While the amplitude decreases as theacoustic signal travels through water, otherparameters remain unchanged, forexample the frequency, etc.

The damping of the acoustic signal is theresult of two factors:

FrictionSome of the energy of the acoustic signalis converted into frictional heat as the resultof the water viscosity.

DispersionWhen the acoustic signal collides withsuspended particles in water, part of theenergy contained in the signal is dispersed.

Diagram 13: An example of the dispersion that happenswhen the acoustic signal encounters suspended particles inwater

Small particles have the tendency tovacillate with the same frequency of theacoustic signal, causing a relatively largeamount of acoustic energy to be lostthrough the friction factor. Larger particleswith a diameter from 0,1 to 1 mm have thetendency to cause the dispersion of theacoustic energy (see diagram 14).

Diagram 14: Dampening of an acoustic signal in relation tothe particle diameter and signal frequency (Laenen, Smith,1982)

It is clear from diagram 13 that the amountof dampening that takes place at similarconcentrations and sizes of suspendedparticles is primarily dependent on thefrequency of the acoustic signal.So a signal with a frequency of 30 kHz atfactor 10 (diameter of suspended particlesin mm) will be dampened less than a signalwith a frequency of 200 kHz.

Because there are limits to how much thepiezoelectronic crystal can be enhanced,the maximum path lengths under the sameconditions are primarily determined throughthe acoustic frequency.

Note: in general, the reliability of theflowmeter system increases with the use oflower frequencies. Because low frequencytransducers cost more, however, it isimportant to avoid extra cost in particularmeasurement installations where higherfrequency transducers are both costeffective and reliable.

6.2 Air bubblesAir bubbles that develop for example whenwater descends into a watercourse from agate of a dam or when air bubbles rise fromoxygen producing plants on the floor of awater course cause the dampening of theacoustic signal. The physical effect of airbubbles is similar to that of suspendedparticles, including the problem of friction(the conversion of the signal´s energy into

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heat) and dispersion. In opposition tosuspended particles in water, however, airbubbles are easily compressible andthereby have a further effect on theacoustic velocity.

In measurement sites with a largepresence of oxygen either created bybiological processes or trapped in thewater, it is typical that during the day themeasurement is disturbed or completelystopped. After sunset, the acousticconditions improve because the biologicalprocesses decrease and the flowmeter isable to function again.

Similarly, the air bubbles created by shipsalso cause the dispersion of the acousticsignal. This interference, however, usuallydoesn´t cause any loss of data, but rather areduced quantity of measurement values inthe average measurement time.

6.3 Temperature and salt contentThe ultrasonic signal´s velocity in water isalso influenced by different distinctions intemperature and salt content. When astrong temperature difference existsbetween the air and water, an energytransfer takes place between these twoelements. This leads to the development ofa temperature gradient in the water thatdiverts the acoustic signal from its normalhorizontal path. The diversion can be sosevere, that the signal fails to meet thereceiving transducer. In this case, whenthere exists no acoustic connectionbetween the sender and the receiver, ameasurement is of course impossible. Saltcontent gradients have a similar influence.

These physical conditions have to beaccounted for by the proper selection of themeasurement site location and suitablecomponents.

Diagram 15: No vertical density differentiation

Temperature changes of 1°C per meterappear predominately in slow flowing water

courses and are usually 0,5 m deep. In theunderlying water layers, the temperaturechanges are less.

Diagram 16: Significant vertical density differentiation as theresult of temperature gradients caused by sunlight

Diagram17: Extreme vertical density differentiation as theresult of salt content

Temperature changes are also prevalent inthe following instances: in a cool waterintake in which a substantial amount ofwarm water enters into the watercourse;near power plants where cool waterreenters the river and rejoins the naturalcurrent; where water from a dead channelflows together with the main current of thecross-section; where deepwater mixes withthe natural discharge; or in the case wherea sea opening and a river with differingtemperatures meet each other.

Diagram 18: No horizontal density differentiation

Diagram 19: Horizontal density differentiation as the resultof the introduction of warm water

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In diagram 20, the diversion of the signalas the result of temperature differences isillustrated. The temperature gradientaverages 0,1°C per meter with a distanceof 200m between the transducers. Theresult is as follows: the most concentratedpart of the signal, because of a shift thattakes place when it passes through thetemperature gradients, ends up 4m fromthe ideal course, meaning the signalreaches the bank 4m from the receivingtransducer. The sent signal is spread outduring propagation by the beam angleestablished in the sending transducer; forexample, when a beam angle of 5° isconfigured with a path length of 500 m, thediversion of the signal amounts to 24 munder the same temperature gradient. Abeam angle of 5° is in this case not enoughfor the signal to be detected on theopposite bank.

Diagram 20: An estimation of the signal´s diversion in alinear temperature gradient with different path lengths

Since this problematic effect occurs only ininstances of intensive solarization, inextremely slow current velocity in watercourses or bi-directional tidal areas, in alarge introduction of salt water intowatercourses, installations on normalinland water courses are usually noteffected by this disturbance.

The calculation of the beam angle takesplace with the formula from VIGOREUX(1979). A formula to calculate the signal´svelocity is provided by DEL GROSSO(1974). For the velocity of the signal c:

STSTSS

TTTPc

⋅⋅⋅+⋅⋅

−⋅⋅+⋅

+⋅⋅+⋅

−⋅+⋅+=

−

−

−

23

23

332

10097,00128,01013,0330,1

102215,005509,0011,5156,039,1402

With:

c Signal velocity in m/sT Water temperature in °CS Salt content in %oP Depth of the measurement layer

from the water surface in m

The application of this formula is restrictedto 0°C < T < 40° and 0%o < S < 40 %o .Within these limits, the exactness of theformula exceeds 0,5%o and its use is notrecommended.

For the radius of the curvature of theacoustic signal in water:

nc

cR ∂∂−= 11

with:

c Signal velocity in m/sR Bending radius in mn Normal arrangement of the signal

propagation

For the approximation of the radius of thecurvature of a horizontal signal in water of10°C:

1

13,163,31450−

∂∂+

∂∂−=

nS

PTR

In summer, strong solarization has agreater effect on the water temperature inthe upper layers than the lower layers sothat a negative temperature water gradientappears. In winter, with temperaturebeneath 4°C, this effect is reversed.

In instances of different salt contentgradients, the larger amount of salt contentis always located in the lower layers,thereby creating always a positive gradient.

In general, a negative temperature gradientin summer causes the signal to diverge intothe lower, deeper layers of the watercourse. A positive salt content gradientcauses the signal to diverge into the upperlayers.

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7. Problem solving or Error detection

There is a distinction between random andsystematic errors.

7.1 Random errorsAccording to LÄNDERARBEITS-GEMAINSCHAFT WASSER (1992), theinsecurity of ultrasonic dischargemeasurement systems is mainly attributedto the following four points:

1. Selection of the measurement siteShifting current directions, air bubbles,suspended particles, temperaturestratification, the presence of too muchoxygen in the water, and a watervelocity that is too slow are allproblematic for discharge measurementwith ultrasound.

2. Installation and calibration of themeasurement systemSystem positioning, inexact or incorrectgeometric indications, inexact orincorrect reference measurements(dependant on the chosen method)

3. Location of the measurement systemFaulty or defected measurementsystem (electronic error), choice ofintegration time that is too large (>10min) or too small (<1 min)

4. Analysis of the measurement dataInaccurate evaluation of thetroublesome points, insufficientattention to reference guides pertainingto the various surrounding conditions.

According to RACK (1982), the accuracy ofa discharge measurement with a singlepath system is quoted at .gesf =5%; therandom errors in the determination of thecalculated sizes (longitudinal, angle, andtime measurement) are accounted for in sofar as the uncertainty of the correlationbetween the measured value gv and thedischarge Q is also accounted for.

For an estimation of how big theanticipated random measurement errorsare, the individual errors are calculated by

the measurement of the current velocity inmeasurement path ( )vf , the cross-sectionarea ( )Af , and the calibration factor ( )kf .With the help of a quadratic law of errorpropagation, the total error of thedischarge measurement ( )Qf is thefollowing:

Equation:

222kAvQ ffff ++=

7.2 Systematic errors

Accessing systematic failures is moredifficult; i.e. disturbance within themeasured cross-section, data mistakes,system defects, etc. These failures canrange from an insignificant but constantinfluence to a complete system breakdown.

An evaluation of whether or not anirregularity exists can be known only whenthe troublesome influence is found and itseffect on the measurement valueseliminated.

In a single path system, when the actualcurrent direction deviates from theassumed current direction, a velocityperpendicular to the measurement pathcan be calculated with the followingequations that doesn´t correspond to theactual velocity:

21

21

cos −

−=φ

vvg

with:

21−φ The angle between themeasurement path and the currentdirection.

−=

−−−

−

122121

21 11cos2 ttLvg φ

The relationship between the calculateddischarge and the actual discharge is

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dependant on the path angle φ and thesize of the angle deviation α between theassumed and actual current direction.

As shown in picture 27, the larger theangle, the larger the deviations appear inthe assumed mean current direction.

When the current direction is constant,these systematic mistakes are corrected bythe calibration measurement, a correctionof the ultrasonic discharge measurementsystem which indicates too big or too smallof a velocity measurement respective to thedischarge measurement.

Diagram 21: Relative error in a discharge calculation isdependant on the angle calibration between assumed andactual current directions (single path system)

8. Environmental compatibility

On principle, our ultrasonic measuringsystems are installed to minimize anynegative impact on local ecologicalfunction. The installations are constructedin existing river banks, meaning that boththe transducers and the accompanyingcables are mostly unobservable. Usuallythe cabinet that contains the measuringsystem is the only observable part of theinstallation. In areas where even a smallcabinet is obtrusive, such as wildlifepreserves or city installations, specialbuilding elements can be used to concealthe equipment as much as possible.

According to a conclusive study by theFederal office for the Environment and

Nature, ultrasonic signals have no negativeaffects on animal life found near ourmeasuring sites. Between 1999 and 2001,biologists from the Rostock Universitystudied the effects of a crossed pathultrasonic system on a natural riverenvironment. The influence of the acousticsignals on the movement of fish was thefocus of the study. By employing ameasuring system that could be switchedon and off, and installing video camerasthat could observe the movements of fishat various depths, it was found that thatacoustic signals had no effect on themovement of the fish. (Rothbarth &Winkler, 2001)

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9. Installation location evaluation

9.1 Hydraulic conditionsTo determine whether or not a cross-section is suitable for an ultrasonicdischarge measurement system, it isimportant to evaluate the hydraulicconditions present at the desiredinstallation site. Table 2 contains the mostimportant criteria:

Table 2 Hydraulic conditionsCriteria Conditions Test

possibilitiesIncomingflow

Optimal: The watercourse must be 5 to10 times the watercourse width abovethe measurementinstallation and 1 to2 times thewatercourse widthbelow theinstallation.

Alternative:Crossed pathsystem

On siteinspection

Access tosite plan

Dammedareas

Without problem On siteinspection

Water level Manufacturerwarranty isdependant on asufficient waterlevel.

Consultation ofthe manufacturer.

Consider-ation of thehydraulicstatistics

Velocitydistribution

No cross-sectionswith predominateback streamingzones allowed

On siteinspection

Hydraulicprobemeasure-ment inpossiblecross-sections

Windinfluence

Negligible

9.2 Morphological conditionsThe morphological conditions areconcerned with the physical condition ofthe installation site itself. Table 3 containsthe most important criteria:

Table 3 Morphological conditionsCriteria Conditions Test

possibilitiesWatercoursefloor

Stableconstruction, nosediment and noerosion

Comparison ofthe translatedcross-sectionmeasurements

Geographicalsurveys

Bank Definite form,stableconstruction,banks withshallow slopesmust have a run-off formation

On siteinspection

Measure-ment path

No stones orother hindrancesin measurementpath, novegetationblockingmeasurementpath

On siteinspection

A profile viewalong themeasurementpath with areference point.

Locationabove abarrage

Measurementcross-sectionshould belocated from thebarrage 3 to 4times its depth.

On siteinspection

In the incidence of an instable river floor,the changes caused by the instability mustbe quickly ascertainable because the areaof the cross-section plays an important rolein the calculation of discharge.

Similarly, installations with largeforegrounds must be avoided. In theinstance of large foregrounds, even if theriver floods a little, the accuracy of themeasurement is largely impacted and themean velocity measure becomes volatile.Therefore, in installations where there isslight flooding or even partial flooding, it isimpossible to carry out a trustworthymeasurement with any of the methodsdescribed.

9.3 Physical conditionsFor the proper function of the ultrasonicmeasurement system, compliance to thefollowing physical conditions listed in thetable below is necessary.

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Table 4: Physical conditionsCriteria Conditions Test

possibilitiesTemper-aturegradient

Measurement sitesshould not beinstalled belowcool water intakesor stagnate water(dead channel)

With a temperaturedifference of >3until 5 °C in water

Consultation ofthe manufacturer

Temperatureprofile of thearea

Saltgradient

Normallyproblematic only incoastalwatercourses

Consultation ofthe manufacturer

Measurementof the salinity

Air bubbles Measurementpoints are to beinstalled 5 to 10times the width ofthe water waybelow the dam.

With possible entryof smallmicroscopic airbubbles as theresult of pumpsinstalled againstthe water current.

Consultation ofthe manufacturer

On siteinspection

Site plan

Suspendedparticles

Until 10 g/l

Consultmanufacturerabout therecommendedtransducerfrequency

Suspendedparticlemeasurement

9.4 Practical and logistical conditionsIn general, the measurement system has tobe accessible on every bank becauseinstallation requires the use of heavy andbulky machinery. For example, theinstallation and mounting of thetransducers in the banks is only possiblewith heavy machinery suited for thatpurpose.

Despite the small amount of power inputrequired for a trustworthy and stableoperation of the flowmeter, a bus bar is stillrecommended. In fact, our system can alsobe run with solar power, even though aconnection to a local electrical system isoften more economical due to the lowoperating expenses and maintenance costsof electric power. To prevent unwantedinterruption of the measurement due totemporary power failures, the installation ofa battery is also recommended.

Because it is important from time to time tocheck the accuracy of a measurementsystem, the installation site should not beexclusively suitable for ultrasonicmeasurement. Rather, there should be thepossibility either at the site itself or nearbyto install another type of dischargemeasurement system to carry outcontrolled comparisons when they arerequired.

Table 5 contains the most importantpractical conditions For ultrasonicmeasurement.

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Table 5: Logistical practical conditionsCriteria Conditions Test

possibil-ities

Access tomeasure-mentinstallation

Accessibility also forheavy machinery

On site

Energysupply

Optimal: energysupply with 220 V

Alternative: solarenergy

On site

Data trans-mission

Optimal: conventionaltelephone network

Alternative: wirelessmodem

On site

Cablearrange-ment

Contained in aprotective pipe duginto the bank.

Watercourse cablecrossing: availabilityof a bridge or thelaying of the cablealong the water-course floor;alternative also existsto tunnel through thewatercourse bottom.

When it is notpossible for cables tocross thewatercourse:

Responderinstallation-placement possibilityfor the responder onthe opposite bankwhich eliminates theneed for a cablecrossing

On site

Entranceforpropellertypesystems

Possibility ofcomparablemeasurements withhydrometric propellertype systems ortransducers:

Bridge nearby?

With ADCP-measurements:

Is it possible to leavea measurement bootin the water?

On site

9.5 Installation configurationsIf the current runs parallel to the banks andthe profile is regular so that the current isnot strongly influenced, a single pathsystem can be used. This configurationconsists of two transducers, which bothsend and receive the acoustic signal, beinginstalled on one level and diagonal to thedirection of flow. This is the most basicconfiguration with ultrasonic measurement.

Diagram 21: Single path system, parallel current

One disadvantage to the single pathsystem is that if a problem arises with thesystem, for example damage due to highwater or ship traffic, there is no secondpath to continue the dischargemeasurement.

For hydrometric measurement withultrasonic transducers, it is important toknow the exact angle between the maincurrent and the acoustic signal whichcrosses it.

If the direction of the current is influencedby physical factors, such as meanderingcurrents, changes in direction because ofdiffering water depths or curves in thewatercourse, or the presence of aconverging stream, a crossed path systemis recommended; with the installation of asecond “crossed” path on the same levelas the first path, a more exact calculation ofthe angle between the main current and theriver bank is possible and thereby a moreaccurate measure.In natural watercourses, the crossedpath system is the best option to obtaina discharge measurement with thehighest possible accuracy.

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Diagram 22: Crossed path system

Although only a few years ago it wasthought important to install multiple levelsystems, people in the field of dischargemeasurement are now more reserved intheir evaluations. Of course, the initialcalibration costs are avoided with a multiplelevel system, because the calibrationrequired until the varying water levels andcurrent velocities and resulting k-worthvalues for the entire measured area arealready known. New methods are nowavailable which are based on calibration;because the K-worth value is dependant onthe discharge and water depth of the river,and therefore can be numerically derived,these new methods offer an alternative tothe higher costs of a multiple levelmeasurement. With a multiple levelmeasurement, there still exists theadvantage that if the dischargemeasurement fails on one of themeasurement levels, it remains possible tocontinue to collect data with the remainingmeasurements. Multiple level systemsare still preferred in cross-sections withlarge fluctuations in the water level.

The maximal range of the acoustic signal ina watercourse is determined by its acousticfrequency and transmitter power, thetransmitting power being limited by themost constructive setting for thepiezoelectronic crystal. Finally, it isimportant to know that in watercourses thatcontain a lot of suspended matter, lowerfrequency signals are more effective thanhigh frequency signals which tend to bedampened to a larger extent by suspendedmatter. Therefore, in the case of highwaters or flooding, the installation of lowfrequency transducers substantiallyincreases the reliability of themeasurement.

For large measurements or watercoursesthat contain a lot of suspended matter, lowfrequency transducers are employed, themaximal watercourse width being 1500 m.For your knowledge, these low frequencytransducers are bigger than the highfrequency transducers. High frequencytransducers are employed for smallermeasurements, the minimal water widthbeing 0,5m, and in instances ofcomparatively clear water.

10. Design and construction

10.1 Project planThe realization of an installation requiresseveral steps. First, there is a preliminaryinquiry into the best location for theinstallation of the ultrasonic flowmetersystem. Next follows a preliminary draft ofthe installation which is the foundation forboth the specifications required by stateregulations and the award of contract. Thesubsequent project outline is issued if thepermission of the property owner is givenand the realization of the project isforeseeable.

The project outline contains the following:

• Technical drawings of the installationand mounting of the transducers

• Plans for the electrical and lighteningprotection set up

• Specifications to meet state regulationsfor hydrometric projects

• Cost estimate

Required documentation• Local site plan (measurement scale at

least 1:5.000)• Cross-section pictures of the

designated measurement site• Map of the site´s infrastructure (electric

and telephone lines; gas and waterpipes)

• Abstract from the register´s plan

Permits for the following proceduresinvolves the responsible institution in thefollowing areas:

• Environmental protection agency

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• Water or ground water publicauthorities

• Water and land organizations• Office for water and ship transportation• City and community administrations• Energy public utilities• Gas public utilities• Phone companies

According to the regulations, an applicationhas to include a description of the plans,including the project drawings, installationlayouts, cable layouts, etc.

10.2 Building specificationsThe building specifications can be outlinedby Quantum or by a contractor hired by thecustomer. Suitable to services potentiallyperformed by contracted labor include:

The digging of a trench for the undergroundcables• The laying of the pipe to protect the

cable and the insertion of the signalcable

• Energy feeding• Telephone feeding• The mounting of a water level measure• The purchase of fixture devices for the

mounting of the transducers• Construction of an outdoor housing or

cabinet

The manufacturer accepts responsibility forthe following services:

• The installation and alignment of thetransducers

• The installation and alignment of themeasurement system, including theexternal sensors (for example, thereadable water level, the output W andQ on the external data collector)

• The starting of the measurementsystem

10.3 System inspection and warrantyThe system inspection follows standardhydrometric regulations. The componentsare tested in the following combinations:

• All of the input and output interfaces• The measured value outputs for the site

and the remote data transfer

• The accuracy of the measurementvalues for the water level (with the helpof a known reference value)

• The accuracy of the dischargemeasurement values (with help fromcomparative measurement procedure)

• The ordering of the buildingspecifications

• The ordering of the transduceralignment procedure and the labeling ofthe components.

• Documentation (Handbook,documentation of the measurementsite, etc.)

The warranty for all of the performedbuilding services lasts 24 months; for theflowmeter system, the warranty lasts 12months.

11. The management andmaintenance of the ultrasonicmeasurement system

The technical management andmaintenance of the ultrasonicmeasurement system can be carried outbetter as a complete working system ratherthan as a group of individual components.

11.1 Transducer• It is possible for the transducers to

meet different measurement conditionsdue to different frequency possibilities

• The simplest and most securetransducer installation

• The simplest alignment and lockingdevice

• Visible marking of the transducerposition in the river bank or channel

11.2 Signal cable• Protection against underground and

above ground shifts• Explicit and systematic labeling of the

lines• Documentation in a cable log

11.3 Measurement electronic• Simple modular construction

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• Easy access and exchangeability ofparts in case of repair; for example theability to change circuit boards

• Ability to receive information from easyto understand visual elements; forexample through LED, etc

• Availability of an interface for datamigration

• Simple on-site and remotecommunication with ultrasonicflowmeter system

• Warranty of the data collection and datastorage over a predetermined timeperiod in the case of power failure (forexample with the installation of anback-up battery unit).

• Automatic start in case of power failure• Possibility of using a system simulator

to test the system

11.4 Housing• Documentation of labeling system• Documentation of all of the components• Easy exchangeability of housed

components, for example modem,heating system, air ventilator, securityequipment, etc.

11.5 Software• Easy graphic user interface for the

regulation and servicing of theflowmeter

• Password security• Data control of the measurement

values with a concise time-variationcurve

• On-line graphic quality test for theacoustic signal

• On-line graphics of the flow velocity,discharge, water level, and ultrasonicsignal velocity

• Protocol of the flowmeter system• Data output in ASCII-format• Documentation of the software and the

employed algorithm

11.6 Water and site maintenanceThe effort needed to care for the measuredcross-section depends on the conditions ofthe banks and the watercourse floor. Theimportant points are:

• to keep the measurement paths freefrom vegetation and other blockages

• to record changes in the cross-sectiongeometry after high waters

• to test the condition of the watercoursefloor when a cross-section exists in aslow moving river, the danger beingthat the geometry of the cross-sectioncould change due to sedimentation.

The exact overall requirements are difficultto specify; the amount and frequency ofthe maintenance must be individuallydetermined. Quantum can help thecustomer develop a specific and effectivemaintenance plan.

12. Work safety regulations

In the installation of the ultrasonicflowmeter systems, it is important to takeinto account work safety and transportsafety.

It is especially important to pay attention tothe regulations pertaining to work withelectrical installations involving highvoltages in close proximity to water.

Underwater work is only permitted bytrained, professional divers.

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13. Literature in English

Cole. J:A: (1979): The deflection of anacoustic beam by temperature and salinitygradients. WDU/WRC Ultrasonic rivergauging seminar reading.

Del Crosso (1974): Journal of theAcoustical Society of America, No. 56,1084

Laenen, A.; Smith, W. (1983): AcousticSystems for the Measurement ofStreamflow. U.S. Geological Survey Water-Supply Paper 2213

A complete list of literature can be found atthe end of the Germany copy of thishandbook.