ISO/TS 15769:2000
(Main)Hydrometric determinations — Liquid flow in open channels and partly filled pipes — Guidelines for the application of Doppler-based flow measurements
Hydrometric determinations — Liquid flow in open channels and partly filled pipes — Guidelines for the application of Doppler-based flow measurements
Déterminations hydrométriques — Débit des liquides dans les canaux découverts et dans les conduites partiellement remplies — Lignes directrices pour l'application de mesurages du débit basés sur l'effet Doppler
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Standards Content (Sample)
TECHNICAL ISO/TS
SPECIFICATION 15769
First edition
2000-12-01
Hydrometric determinations — Liquid flow
in open channels and partly filled pipes —
Guidelines for the application of
Doppler-based flow measurements
Déterminations hydrométriques — Débit des liquides dans les canaux
découverts et dans les conduites partiellement remplies — Lignes
directrices pour l'application de mesurages du débit basés sur l'effet
Doppler
Reference number
ISO/TS 15769:2000(E)
©
ISO 2000
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ISO/TS 15769:2000(E)
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ISO/TS 15769:2000(E)
Contents Page
Foreword.iv
Introduction.v
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Principle.1
5 Factors affecting operation and accuracy .3
5.1 General.3
5.2 Characteristics of the instrument .3
5.3 Channel and water characteristics .6
6 General expectations of performance .10
7 Considerations when selecting equipment .10
8 Evaluation and verification.10
Annex A (informative) Other practical considerations.11
Annex B (informative) Sample questionnaire — Doppler-based flowmeter.16
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ISO/TS 15769:2000(E)
Foreword
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member bodies). The work of preparing International Standards is normally carried out through ISO technical
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The main task of technical committees is to prepare International Standards. Draft International Standards adopted
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Standard requires approval by at least 75 % of the member bodies casting a vote.
In other circumstances, particularly when there is an urgent market requirement for such documents, a technical
committee may decide to publish other types of normative documents:
� an ISO Publicly Available Specification (ISO/PAS) represents an agreement between technical experts in an
ISO working group and is accepted for publication if it is approved by more than 50 % of the members of the
parent committee casting a vote;
� an ISO Technical Specification (ISO/TS) represents an agreement between the members of a technical
committee and is accepted for publication if it is approved by 2/3 of the members of the committee casting a
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International Standard.
Attention is drawn to the possibility that some of the elements of this Technical Specification may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TS 15769 was prepared by Technical Committee ISO/TC 113, Hydrometric determinations, Subcommittee
SC 1, Velocity area methods.
Annexes A and B of this Technical Specification are for information only.
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ISO/TS 15769:2000(E)
Introduction
Flow measurement using Doppler-based flowmeters fall into the category of velocity-area methods and, as with all
variants of this approach, flow estimation is a two-stage process. Measurements are made to derive estimates of:
a) mean channel velocity, using the principle of Doppler shift,
b) depth, from which cross-sectional area is computed with a knowledge of the relationship between depth and
area (i.e. the profile of the cross-section).
Most difficulties governing this method relate to the estimation of mean channel velocity and the degree to which
computed velocities can be assumed to be representative of the true mean velocity through the measurement
section. The accuracy with which the calculated velocities represent the mean velocity is influenced by various
factors which are considered in clause 4. This guide focuses on the process of velocity estimation and the
conditions and practices which may help to deliver optimum results. However, it should be recognized that the
accuracy of overall flow determination also depends on the accuracy of depth measurement. The influence of
meter location and sensitivity of cross-sectional area to depth variation will have a bearing on performance.
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TECHNICAL SPECIFICATION ISO/TS 15769:2000(E)
Hydrometric determinations — Liquid flow in open channels and
partly filled pipes — Guidelines for the application of Doppler-
based flow measurements
1 Scope
This Technical Specification gives guidelines for the selection and use of Doppler-based flowmeters for the
measurement of liquid flow in small open channels and partly filled pipes. It is applicable to whole-channel flow
measurements using a single meter at a fixed point in the cross-section.
It is not applicable to flow measurements made using Doppler-based current meters for point velocity measurement
nor using mobile profiling systems.
A limitation of this technique is that measurement is made of the velocity of particles or other reflectors being
transported by the liquid rather than that of the liquid itself.
NOTE Though the title refers to liquid flow in general, the main use of flowmeters which use the Doppler principle is for
water or water-based liquid and particular reference is often made in the text to water.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this Technical Specification. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this Technical Specification are encouraged to
investigate the possibility of applying the most recent editions of the normative documents indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 772, Hydrometric determinations — Vocabulary and symbols.
ISO/TR 8363, Measurement of liquid flow in open channels — General guidelines for selection of method.
3 Terms and definitions
For the purposes of this Technical Specification, the terms and definitions given in ISO 772 apply.
4Principle
This method of velocity measurement is based on a phenomenon first identified by Christian Doppler in 1843. The
principle of “Doppler shift” describes the difference or shift which occurs in the frequency of emitted sound waves
as they are reflected back from a moving body.
The sensors of Doppler systems normally contain a transmitting and a receiving device (see Figure 1). A sound
wave of high frequency (f ) is transmitted into the flow of water and intercepted and reflected back at a different
s
frequency by tiny particles or air bubbles (reflectors). A typical reflector “n” produces a frequency shift f .The
D,n
“shift” between transmitted and reflected frequencies is proportional to the movement of particles relative to the
position of the sound source (i.e. the sensor).
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ISO/TS 15769:2000(E)
� is the angle between direction of motion of particle n and the sensor
n
� is the angle between centre of beam and assumed flow direction (this is referred to as the “beam angle” or “projection
angle”)
� is the beam width or spread
w
Key
1Surface
2Flow
3 Sensor
4 “Envelope” of ultrasonic beam
5Bed
Figure 1 — Principle of Doppler-ultrasonic flow measurement
Doppler shift only occurs if there is relative movement between the transmitted sound source and the reflected
sound source. The velocity of moving reflector “n” can be calculated from the following:
a) the magnitude of the Doppler shift;
b) the angle between the transmitted beam and the direction of movement;
c) the velocity of sound in the water.
Itcanbeshownthat:
fc�
D,n
v �
n
2cf �os�
s n
where
v is the relative velocity between transmitted sound source and reflector n;
n
f is the Doppler frequency shift produced by reflector n;
D,n
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ISO/TS 15769:2000(E)
f is the frequency of sound with no movement;
s
c is the velocity of sound in water;
� is the angle between the reflectors’ line of motion (the assumed flow path) and the direction of the
n
acoustic beam.
A Doppler flowmeter measures the resultant frequency shift produced by a large number of reflectors of which
reflector n is typical and from which the mean velocity can be computed. Consequently, the velocity of moving
particles, and not the velocity of water, is measured. By measuring the velocity of many particles, the mean water
velocity in front of the sensor can be estimated. Although the particles, if small, travel at almost the same speed as
the water, sampling errors may occur. These errors depend on the spatial and velocity distribution of the particles
as well as the extent of penetration of the ultrasonic beam. Instruments are neither point-velocity meters nor whole-
channel meters. The area sampled by the ultrasonic beam can be visualized, by analogy, to shining a torch in front
of the sensor. The purpose of using such an approach is to obtain a representative sample of the full cross-
sectional velocity.
The cross-sectional area is also required for the velocity-area calculation of discharge. Most systems incorporate a
pressure transducer in their sensors and by combining water depth with knowledge of the cross-sectional profile,
the flow can be calculated.
5 Factors affecting operation and accuracy
5.1 General
The factors affecting the performance of Doppler flowmeters may be broadly divided into:
� characteristics of the instrument,
� characteristics of either the channel or the liquid flowing in it.
However, these factors interact and shall be considered together.
5.2 Characteristics of the instrument
5.2.1 General considerations
The characteristics of the instrument and in particular the sensor have a bearing on its performance in any given
situation. There is no optimal set of characteristics. Certain characteristics may make a particular instrument
perform better under certain conditions but worse under others. A list of characteristics and their influence on the
performance of the instrument are given in 5.2.2 to 5.2.6.
5.2.2 Ultrasonic beam angle
The ultrasonic “beam” is usually transmitted in the approximate shape of a cone. The term “beam angle”,or
“projection angle” in this context, refers to the angle between the cone axis and the flow direction. The effects of
beam “angle” should be considered together with those due to beam “width” (5.2.3).
The sensor has to be installed so as to remain beneath the surface of the liquid under all conditions and is
positioned so that the beam cone reaches the lateral extremities of the channel as far as possible. The installation
position is often a compromise and the installer is frequently obliged to install the sensor on the channel bed,
somewhere near the centre of the cross-section. An off-centre position is also sometimes used.
Assuming the sensor is installed on the bed of the channel, a high angle between the flow direction and ultrasonic
beam (e.g. between 30° and 50°) enable signals to be obtained throughout the depth up to the limit of the
penetration of the beam. However, no signals are obtained close to the bed on either side of the sensor. Serious
sampling errors can occur particularly when the ratio between the depth of water and width of the channel is low.
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ISO/TS 15769:2000(E)
Conversely, a shallow beam angle allows flow to be measured close to the bed and is best for shallow depths.
However, if the channel is too wide or not sufficiently long, a beam at a shallow angle may not reach the lateral
extremities of the channel. In a long channel where, theoretically, the beam angle should be able to reach the
extremities of the channel, the penetration (range) of the beam may not be sufficient to do so.
Beam width also has a bearing on the velocity sampling. See 5.2.3.
Figure 2 indicates the significance of the beam angle in relation to sampling.
It may be possible to adjust the beam angle, to improve suitability to the given site conditions, provided that an
appropriate correction is made during the velocity calculation.
5.2.3 Beam “width”
Beam width is a loose term indicating the spread of the beam as shown in Figure 1. It is a function of sound
frequency and diameter of the transmitter. The designer of the instrument may be constrained by other factors in
his scope to vary the beam width.
A wide beam, i.e. one with a cone having a large spread, gives the best coverage because signals are obtained
over a greater area of the channel. However, there is uncertainty in the velocity measurement since the wide beam
means that the actual angle made by a particular reflector (� in Figure 1) may be different from the mean beam
n
angle assumed by the instrument (� in Figure 1). Furthermore, a bias can possibly occur and is dependent on the
distribution of the velocity and on the reflector concentration.
A narrow beam width would have less angular uncertainty but a poorer coverage (sampling).
If the distribution of reflectors and velocity are both fairly uniform, sampling is unimportant and a narrow beam width
would give the best results because the measurement uncertainty relating to beam angle is minimized.
If the velocity distribution is non-uniform, a wide beam width gives a better sample of velocity than a narrow one.
Furthermore, if the reflector distribution is uniform, the error relating to the beam angle may be acceptable and a
wide beam width would be preferable.
If neither the velocity profile nor the reflector density is uniform, a significant uncertainty of measurement can be
expected whatever the beam width.
5.2.4 Ultrasonic frequency
A lower frequency generally penetrates further (greater range) but requires a larger transducer for a given beam
width. Therefore, a lower frequency transducer is preferable for a channel of larger width or greater depth if the
larger sensor size does not cause serious obstruction.
In practice, frequencies between 500 kHz and 1 MHz are generally used.
5.2.5 Method of determining velocity of sound
The velocity of sound in water varies with density, which is a function of temperature and dissolved material (e.g.
salinity). Since velocity of sound appears in the flow formula, errors can be made if no adjustment is made. Some
instruments have no dynamic adjustment though it is possible to put in a fixed calibration factor. This is acceptable
provided the conditions of measurement do not change. Other instruments have a temperature sensor and provide
a dynamic correction for temperature effects. This is acceptable for conditions where the content of dissolved
material in water remains constant but the temperature changes.
When the temperature and dissolved particle content both vary, the only satisfactory solution is for the instrument
to actually measure the velocity of sound.
Alternatively, if the temperature and salinity are measured or estimated separately, a retrospective correction can
be made to the recorded data.
The effect of not making full or partial allowance for these variations is described in 5.3.5.
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ISO/TS 15769:2000(E)
Medium depth
low �
Medium depth
high �
a) At low to medium depth a small � is preferable
Medium depth
high �
High depth
low �
b) At high depth a high � is preferable
Key
1Surface
2 Sensor
3Bed
4 Unsampled area (hatched)
Figure 2 — Beam angle effects
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ISO/TS 15769:2000(E)
5.2.6 Signal processing
The calculation of flow velocity is based on the assumption that a frequency shift results from a single moving
reflector. However, in reality, many reflectors are involved, moving at different speeds in different parts of the beam.
The processor has to employ averaging methods for measuring frequency shifts.
Processing methods vary. Simple, analogue methods are likely to give a higher weighting to stronger signals from
nearby reflectors. This method may give erroneous results if the velocity profile is not uniform and an additional
non-uniform effect may need to be considered with respect to beam angle and width.
Instruments employing more sophisticated processing methods attempt to remove the signal strength effect, for
example by using Fourier transform techniques. Though this is an improvement, such instruments remain sensitive
to non-uniform effects in the water itself.
Some instruments employing “time-gating” or “range-gating” methods attempt to separate the signals from different
parts of the space in the beam so as to produce information about the distribution of velocity. This is possible by
transmitting signals in timed bursts and examining the received reflections at different delays so as to estimate the
velocity variation with respect to the distance from the sensor. However, it is not possible to determine the angle
from which the signals have come within the beam width. Although this information is useful for profiling-type
instruments where velocity profiles are determined in deep water in which the beam is generally aimed across the
flow (usually downwards), it is of little value in flowmeters where the beam angle is generally aimed lengthwise in
the channel. This is the consequence of information being received from different distances along the channel and
not across it. Nonetheless, such methods prevent the processor from becoming saturated from an overload of very
close, strong signals since they can be identified by the short-time delay.
An exception to these observations would be the case of an instrument incorporating multiple narrow beams or a
single narrow beam whose direction is capable of automatic variation. In such cases, velocities from small defined
volumes within the channel can be measured. Such facilities are not normally available in instruments that fall
within the scope of this Technical Specification.
It is important to remember that although instruments employing techniques like time gating or Fourier transform
analysis are likely to perform better in terms of short range bias, their range is still limited by beam penetration. As
the channel size increases, these techniques are prone to another type of range-related sampling error.
5.2.7 Depth measurement
In instruments of this type, depth is commonly measured by means of an integral pressure-activated sensor. The
accuracy of these devices is dependent on the range for which they are designed (� 0,5 % of range is a typical
figure). The manufacturer’s specification should be consulted. The effect of surface irregularities may degrade the
accuracy further. The significance of the depth uncertainty depends on the channel shape, see section 5.3.7.
Care should be taken in the case of a closed conduit which sometimes runs full and at other times only partly full.
When full, the pressure can significantly exceed that produced by a head of water equal to the height of the
conduit. This can cause problems for a pressure depth-transducer. The depth transducer should be capable of
withstanding the maximum pressure. If it is not capable, then alternatively a low-sensitivity transducer (i.e. with a
range much greater than the conduit height) should be used, thus limiting the accuracy. Again the manufacturer’s
specification should be consulted. A special transducer or an alternative method of measuring depth may be
required.
5.3 Channel and water characteristics
5.3.1 Channel geometry
As the channel becomes wider, the limited range of the instrument causes the sampled velocity to be obtained from
a limited part of the whole channel. The consequences of this limited sampling range depend on the homogeneity
of flow velocity and particle distribution. Moreover, the density of particles also affect this range as does the depth
of water. As a general guide, care should be taken if the width is greater than 2 m.
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ISO/TS 15769:2000(E)
For a small channel, particularly when the depth is also low, the size of the sensor can cause serious obstruction
resulting in channel back-up and possibly silt-up (discussed below). The size of the sensor can cause turbulence
thus disrupting the velocity distribution in close proximity.
It is preferable to place the sensor in a long, straight reach of uniform cross-section so as to maintain a regular flow
pattern over the length of flow from which the instrument samples particle velocities. The straight reach should
usually be a longer distance than that required by other methods (e.g. ultrasonic time-of-flight or electromagnetic),
particularly if the beam is at a small acute angle to the flow, as is often the case. As a general rule, the sampling
length should be at least five times the width. It may be possible to employ flow straighteners though they tend to
block in dirty water conditions.
The length may not be the major problem if the water is particularly shallow since lack of depth is likely to be the
greater limitation.
Most instruments allow the cross-sectional shape to be entered into their software programme. This is used to
convert the depth measurement into area which is then multiplied by the velocity measured by the instrument to
calculate the flowrate. Some systems can only process symmetrical cross-sections. Others require complex
geometrical calculations to specify the channel shape. In most cases, the measured velocity is assumed to be the
mean velocity. Some systems take the highest velocity and calculate the mean velocity with a calibrated conversion
factor. However, there can be parts of the cross-section not reached by the beam, so the velocity cannot be
sampled from the whole area. This effect can be worse for channels with complex shapes with step changes or low
flow channels in the bottom. Even a rectangular channel is not necessarily a good shape, particularly if the water is
shallow. Again, the importance of this effect depends on the homogeneity of the velocity and reflector distribution.
Circular, U- or V-shaped channels tend to be the best geometric shape, although they can suffer from problems
relating to siltation.
A sensor facing downstream may be preferable in some circumstances provided the instrument can cope with
negative velocity measurements. The required straight reach may be reduced by facing the sensor downstream. If
a regular flow pattern is established at the point where the sensor is mounted, this solution may be acceptable
because the beam would be localized near its source. Another advantage for such a configuration would be to
reduce the tendency for silt or ragging to build-up on the sensor face. However, in shallow conditions or small
channels, in cases where the sensor is the cause of flow disturbance, this configuration would not be acceptable
because the sensor would make erroneous readings in an area of turbulence created by itself.
Further information about installation is given in A.3.
5.3.2 Depth
A large depth can be a problem for the range of the beam, particularly if the sensor is designed with an acute beam
angle. In order to operate at shallow depth, an acute angle is preferred, in which case, the range of the beam is
unlikely to allow penetration to the surface as the depth increases.
A shallow depth in a relatively wide channel can cause sampling problems in cases where a small surface area is
measured. Generally, ratios of channel depth to width of less than 1 : 5 should be treated with caution.
A large depth range restricts the use of the alternative side-mounting arrangement which may otherwise be
employed to avoid the siltation problems as described in 5.3.6.
5.3.3 Reflector density and efficiency
These instruments require sound reflectors to operate. In general most water courses, with the exception of
extremely clean water, contain sufficient reflectors to allow operation.
Suspended solids or air bubbles act as reflectors. The content of particles (i.e. density of water) required depends
to some extent on the instrument. However, little guidance is offered by most manufacturers.
Air bubbles act as good reflectors but it is almost impossible to estimate their concentration, size or spatial
distribution. Air bubbles tend to be located towards the surface, although it is unlikely they would be moving at the
average velocity. Too much air can restrict the range of the beam.
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Though samples can be taken to assess suspended solids content, the size and nature of the particles can affect
their efficiency as reflectors. A high concentration of suspended solids gives good local signals but limits the range.
The particle distribution throughout the water affects the velocity sampling (see 5.2.6 and 5.3.1). This distribution is
affected by the settling rate, itself a function of the density and size of the particles.
Best results are obtained when there is good mixing and an adequate concentration of particles is present, for
example a few hundred milligrams per litre.
5.3.4 Homogeneity
It has already been pointed out that inhomogeneous distributions of either velocity or reflector concentration affect
the accuracy of flow determination to some extent. If both occur at the same time, errors are inevitable because the
velocity sampled is not the average velocity of the water. It is possible to obtain velocity distribution data in
relatively large channels using a current meter and to take samples at different locations of the cross-section so as
to measure the content of suspended solids. Such measuremen
...
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