Hydrometry — Measurement of discharge by the ultrasonic (acoustic) method

ISO 6416:2004 describes the establishment and operation of an ultrasonic (transit-time) gauging station for the continuous measurement of discharge in a river, an open channel or a closed conduit. It also describes the basic principles on which the method is based, the operation and performance of associated instrumentation and procedures for commissioning. It is limited to the transit time of ultrasonic pulses technique, and is not applicable to systems that make use of the Doppler shift or correlation orlevel-to-flow techniques. ISO 6416:2004 is not applicable to measurement in rivers with ice.

Hydrométrie — Mesure du débit à l'aide de la méthode ultrasonique (acoustique)

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Status
Withdrawn
Publication Date
27-Jun-2004
Withdrawal Date
27-Jun-2004
Current Stage
9599 - Withdrawal of International Standard
Completion Date
11-Oct-2017
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ISO 6416:2004 - Hydrometry -- Measurement of discharge by the ultrasonic (acoustic) method
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INTERNATIONAL ISO
STANDARD 6416
Third edition
2004-07-01

Hydrometry — Measurement of discharge
by the ultrasonic (acoustic) method
Hydrométrie — Mesure du débit à l'aide de la méthode ultrasonique
(acoustique)




Reference number
ISO 6416:2004(E)
©
ISO 2004

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ISO 6416:2004(E)
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©  ISO 2004
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ISO 6416:2004(E)
Contents Page
Foreword. v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Applications. 1
4.1 Open channels . 1
4.2 Multiple channels . 2
4.3 Closed conduits . 2
5 Method of measurement. 3
5.1 Discharge. 3
5.2 Calculation of discharge from the transit-time measurement. 3
6 Flow velocity determination by the ultrasonic (transit time) method. 3
6.1 Principle . 3
6.2 Sound propagation in water. 6
7 Gauge configuration . 10
7.1 General. 10
7.2 Single-path systems . 11
7.3 Multi-path systems. 12
7.4 Crossed path systems. 12
7.5 Reflected-path systems. 14
7.6 Systems using transponders. 15
7.7 Systems using divided cross-sections. 16
7.8 Sloping paths . 17
8 Calculation of discharge . 17
8.1 Single-path systems . 17
8.2 Multi-path systems. 17
8.3 Systems with transducers in the channel . 21
9 System calibration . 21
9.1 General. 21
9.2 Single-path systems . 22
10 Site selection . 24
10.1 Practical constraints. 24
10.2 Physical constraints of the measurement site. 25
10.3 Physical constraints which are distant from the measurement site . 25
11 Site survey — Before design and construction. 26
11.1 General. 26
11.2 Visual survey . 26
11.3 Survey of the cross-section. 27
11.4 Survey of velocity distribution . 27
11.5 Survey of signal propagation. 28
11.6 Other survey activities. 28
12 Operational measurement requirements. 28
12.1 General. 28
12.2 Basic components of flow determination. 29
12.3 Water velocity determination. 29
12.4 Determination of water stage or depth . 29
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ISO 6416:2004(E)
12.5 Channel width.30
13 Gauging station equipment.30
13.1 General .30
13.2 Design and construction of equipment.31
13.3 Reflectors .32
13.4 Civil engineering works .35
13.5 Signal timing and processing .35
13.6 System self-checking.37
13.7 Site-specific data (or site parameters) .38
13.8 Clock and calendar.38
13.9 System performance criteria.38
13.10 System output.40
13.11 Installation.40
13.12 Commissioning.41
13.13 Operating manual.41
13.14 Maintenance.41
14 Measurement uncertainties.43
14.1 General .43
14.2 Definition of uncertainty .43
14.3 Uncertainty in discharge .44
Bibliography.50

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ISO 6416:2004(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 6416 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1, Velocity area
methods.
This third edition cancels and replaces the second edition (ISO 6416:1992), which has been technically
revised.

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INTERNATIONAL STANDARD ISO 6416:2004(E)

Hydrometry — Measurement of discharge by the ultrasonic
(acoustic) method
1 Scope
This International Standard describes the establishment and operation of an ultrasonic (transit-time) gauging
station for the continuous measurement of discharge in a river, an open channel or a closed conduit. It also
describes the basic principles on which the method is based, the operation and performance of associated
instrumentation and procedures for commissioning.
It is limited to the “transit time of ultrasonic pulses” technique, and is not applicable to systems that make use
of the “Doppler shift” or “correlation” or “level-to-flow” techniques.
This International Standard is not applicable to measurement in rivers with ice.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 772:1996, Hydrometric determinations — Vocabulary and symbols
ISO 4373:1995, Measurement of liquid flow in open channels — Water-level measuring devices
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Applications
4.1 Open channels
4.1.1 The method is suitable for use in river flow measurement, a significant advantage being additional
freedom from siting constraints in comparison with other available techniques. In particular, the method does
not demand the presence of a natural control or the creation of a man-made control at the proposed gauge
location, as it does not rely upon the establishment of a unique relation between water level and discharge.
4.1.2 Gauges using the method are capable of providing highly accurate flow determinations over a range
of flows contained within a defined gauge cross-section. They are tolerant of the backwater effects created by
tides, downstream tributary discharges, downstream weed growth, reservoir or head-pond water level
manipulation, and periodic channel obstruction.
NOTE For locations subjected to significant bed level or profile instability, it may not be possible to use gauges.
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ISO 6416:2004(E)
4.1.3 Use of the method usually creates no obstruction to navigation. It creates no significant hazard or loss
of amenity for other channel users or riparian interests. However, some species of fish may be sensitive to
some types of ultrasonic signal. The gauge can be designed to be physically unobtrusive.
4.1.4 For use in remote locations, the electronic equipment can be designed to operate from battery power.
To economise on power consumption, the system is usually set to sample the flow for short periods and to
return to a quiescent condition between samples. (see 10.1.3 and 13.9.5).
4.1.5 The method is not really suitable for use when the channel is covered with ice, because of the
difficulty of determining the cross-sectional area of the water. Although this is a limitation of use, the method
may still have value in determining water velocity under the ice, if transducers can be positioned in unfrozen
water.
4.2 Multiple channels
4.2.1 At locations where the total flow is divided between two or more physically separate channels, such
as under a multiple-arched bridge, the instrumentation can be configured to determine individual channel
flows separately and then to combine these to create a single unified determination of flow.
4.2.2 If flow may not readily be contained within a single well-defined cross-section, and in particular if there
is significant flow that bypasses the main gauge cross-section by way of an extensive flood plain, it may be
possible to subdivide the flood plain into a series of “channels” in which the flow can be measured.
4.2.3 A station designer may decide to provide a comprehensive flood-plain measurement capability by this
means or may, alternatively, simply provide a flow or velocity sampling facility. In the latter situation, gauged
cross-sections may be constructed in the flood plain. These do not normally provide total coverage, but merely
provide locations at which flood-plain flow can be sampled for subsequent examination and analysis.
4.2.4 It should be noted that systems designed to determine flood-plain flow may suffer from the practical
difficulties of
a) inability to commission the system due to there being no water in the measurement section,
b) maintenance of the section, including weed cutting, debris clearance and repair of vandalism.
4.3 Closed conduits
The ultrasonic method can also be applied to the measurement of flow in closed conduits, including both
storm-water and foul sewers, under both free-flowing and surcharged conditions.
For systems used in foul sewers, special attention should be paid to the following:
a) the source of the water, especially whether it is from an aeration tank or from a section of channel
containing aerators or from a hydro-electric plant. The air dissolved in the water from such sources may
cause bubbles to form, and these may inhibit the operation of the flow gauge (see 10.3.1);
b) possible aeration of the water caused by a hydraulic jump or weir upstream of the measurement section,
especially under storm conditions (see 10.3.1);
c) the design of transducer mountings, to eliminate the risk of fouling by grease, rags and paper;
d) the need for the system to meet local codes of practice for electrical equipment installed in potentially
explosive atmospheres. This usually requires a certified intrinsically safe design for both the transducers
(which can be piezo-electric sources of ignition) and for the electronic unit (see for example EN 50014);
e) the change in the flow computation algorithm when the conduit is surcharged.
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ISO 6416:2004(E)
For foul sewers which are less than about 4 m in width, a high loading of suspended solids is unlikely to
present a serious problem of signal attenuation (see 6.2.3).
5 Method of measurement
5.1 Discharge
5.1.1 Discharge, as defined in ISO 772, is the volume of liquid flowing through a cross-section in a unit time.
3
It is usually denoted by the symbol q and expressed in cubic metres per second (m /s). The definition of
discharge is the product of the wetted cross-sectional area and the mean velocity vector perpendicular to it.
5.1.2 The measurement methods may either determine the bulk quantity discharge q directly, by measuring
the time taken to fill a tank of known volume, or the methods may be indirect and require calculation of the
discharge from measured flow velocities in all points of the wet cross-section. The latter are generally referred
to as “velocity-area methods”. In practice it is not possible to measure velocities at all points, and so the
velocity-area methods deal with only a limited number of measuring points.
The transit-time method is a velocity-area method using flow velocities which have been determined by the
equipment, and which are averaged along one or more lines which are usually, but not necessarily, horizontal.
5.2 Calculation of discharge from the transit-time measurement
5.2.1 Flow measurement by the ultrasonic transit-time technique is analogous to flow measurement by
current meters. However, while the most commonly used current-metering method is based on the estimation
of mean velocity at a series of verticals dispersed across the gauged cross-section, in the transit-time method
the velocity samples are horizontally orientated (and vertically distributed). In principle, flow can be computed
by exactly the same methods applied to a current meter gauging (see ISO 748). However, in practice, the
different graphical methods available do not lend themselves easily to automatic computation, and only the
arithmetic methods are useable.
5.2.2 Discharge can be computed, provided that a relation can be established between the estimated
(horizontally averaged) flow velocity and the mean cross-sectional velocity. If the measured velocity at a single
elevation is not sufficient to establish this relation, measurements at more elevations can be carried out. The
resulting samples of flow velocity can be vertically integrated to provide an estimate of mean cross-sectional
velocity.
5.2.3 Discharge calculation also requires the cross-sectional area of the water to be known. An ultrasonic
transit-time system will, therefore, normally be capable not only of making sample measurements of velocity,
but also of determining (or accepting a signal from some other device determining) water depth, and of storing
details of the relation between water depth and cross-sectional area. It will also normally be capable of
executing the mathematical functions necessary to compute flow from the relevant stored and directly
determined data.
6 Flow velocity determination by the ultrasonic (transit time) method
6.1 Principle
6.1.1 An ultrasonic pulse travels in a downstream direction faster than a similar pulse travels upstream. The
speed of a pulse of sound travelling diagonally across the flow in a downstream direction will be increased by
the velocity component of the water. Conversely, the speed of a sound pulse moving in the opposite direction
will be decreased. The difference in the transit time in the two directions can be used to resolve both the
velocity of sound in water as well as the component of the velocity along the path taken by the ultrasonic
pulses.
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ISO 6416:2004(E)

Key
1 v component of water velocity along the path
path
2 v component of water velocity in the direction of the flow
line
3 direction of flow
4 channel width
5 ultrasonic path
A, B transducers
θ angle between the path and the direction of flow
y downstream distance between transducers
Figure 1 — Schematic illustrating the general principle
6.1.2 For the path between transducers A and B in Figure 1, the transit time for the ultrasonic pulses are:
t = L/(c − v cosθ ) and t = L/(c + v cosθ ) (1)
AB BA
where
t is the transit time from transducer A to B, in seconds;
AB
t is the transit time from transducer B to A, in seconds;
BA
L is the path length (distance between transducer A and transducer B), in metres;
c is the speed of sound in water, in metres per second;
v is the line velocity or the average velocity of the water across the channel in the direction of flow, in
line
metres per second;
θ is the angle between the path and direction of flow.
Resolving for line velocity:
v = L × (t − t ) / (t × t × 2 cosθ ) (2)
line AB BA AB BA
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ISO 6416:2004(E)
6.1.3 The transit times in Equation (2) are for the water path only, and do not include the fixed delays due to
the travel times through the faces of the transducers and cables, delays in the transmitter and receiver circuits,
and delays in signal detection (which may be affected by signal distortion). These fixed delays do not affect
the transit-time difference (t − t ), but will affect the term (t × t ). This factor is of particular importance
AB BA AB BA
for small channels or where long cable runs to the transducers are required.
Typical delay times for the transducers and electronic circuits are between 4 µs and 20 µs.
The delay time for the cables is typically 1 µs per 200 m of cable, i.e. for 100 m each way, transmit and
receive.
Taking the signal delays into account, Equation (2) for the computed water velocity becomes:
v = L × (t − t ) / [(t − δ ) × (t − δ ) × 2 cosθ ] (3)
R F R F
where
t is the transit time from the electronic unit via transducer A to B and back to the unit, in seconds;
R
t is the transit time from the electronic unit via transducer B to A and back to the unit, in seconds;
F
δ is the signal delay.
For a channel of width 1 m, with path angle of 45° and total signal delay of 10 µs, an error of 2 % in the
computed water velocity would be introduced if the delay effect were to be ignored.
For wider channels, the effect of the signal delay is reduced in proportion to the path length, and may be
insignificant.
6.1.4 It should be noted that the calculation of water velocity is
 independent of the speed of sound in water,
 proportional to the difference in transit times,
 inversely proportional to the product of the transit times,
 critically dependent on the angle between the path and the direction of flow (see Table 1).
Table 1 — Systematic errors incurred if the assumed direction of flow is not parallel to the channel
axis
Path angle Velocity error for 1° difference between
θ actual and assumed flow direction
%
degrees
30 1,0
45 1,7
60 3,0

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ISO 6416:2004(E)
6.1.5 In open-channel flow measurement, practical considerations will normally dictate that
a) the transducers at either end of an “ultrasonic path” are located on opposite banks of the watercourse;
b) the line joining them is at an angle to the mean direction of flow, which should be between 30° and 65°.
6.1.6 The following limitations are encountered in open-channel flow measurement.
a) At intersection angles greater than 65°, the time difference between sound pulses in opposite directions
may become small and therefore subject to a relatively large uncertainty, especially at low velocities.
b) At an angle of 90°, there will be no time difference between forward and reverse pulses, and thus velocity
cannot be determined.
c) With large angles, there is also an increase in the error in velocity computation that results from
assumptions made in the assessment of the angle. This is due to the presence of the cosine function in
the equation relating time difference to velocity (see 6.1.3). Table 1 demonstrates this effect.
d) At intersection angles less than 30°, the following problems can arise.
1) The length of the channel occupied by the gauge can become excessive, and cease to be quasi-
uniform.
2) The direction of flow relative to the path may not be constant.
3) There can be practical problems with site selection, due to the length of the channel which is required
to be set aside for the flow gauge, and maintained free of debris and weeds.
4) The excessive length of the paths can cause problems of signal strength and/or signal reflection from
the channel bed or water surface, especially if vertical temperature gradients are present.
6.1.7 To calculate discharge, the flow gauge should contain a means of storing details of the relation
between water depth and cross-sectional area, determine water depth or stage, determine water velocity for
each path, and be capable of executing the mathematical functions necessary to calculate flow from the
relevant stored and directly determined data (see Clause 7).
6.2 Sound propagation in water
6.2.1 General
Sound is a mechanical disturbance of the medium in which it propagates. It encompasses a wide range of
frequencies. The audible range is from approximately 50 Hz to 15 000 Hz, and is generally referred to as
“sonic”. Frequencies less than 50 Hz are usually termed “subsonic”, and those above 15 000 Hz “ultrasonic”.
Transit-time systems operate in the ultrasonic range at frequencies typically between 100 kHz and 1 MHz.
The performance of transit-time systems depends heavily on the characteristics of sound propagation in water.
These characteristics are briefly described here.
6.2.2 Speed of sound in water
The speed of sound in water is independent of frequency, but depends on the temperature, salinity and
pressure of the water. In open channels, the effect of pressure is negligible. Over the normal ambient
temperature range, the speed of sound in fresh water varies from abo
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