Hydrometry — Measurement of free surface flow in closed conduits

ISO/TR 9824:2007 provides a synopsis of the methods of flow gauging that can be deployed in closed conduits flowing part full, i.e. with a free open water surface. This Technical Report provides a brief description of each method with particular reference to other International Standards where appropriate, the attributes and limitations of each technique, possible levels of uncertainty in the flow determinations and specific equipment requirements. The uncertainties quoted herein are expanded uncertainties with a coverage factor of 2 and an approximate confidence level of 95 %.

Hydrométrie — Mesurage du débit des écoulements à surface dénoyée dans les conduites fermées

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Published
Publication Date
25-Mar-2007
Current Stage
6060 - International Standard published
Start Date
26-Mar-2007
Completion Date
26-Mar-2007
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TECHNICAL ISO/TR
REPORT 9824
First edition
2007-04-01
Hydrometry — Measurement of free
surface flow in closed conduits
Hydrométrie — Mesurage du débit des écoulements à surface dénoyée
dans les conduites fermées
Reference number
ISO/TR 9824:2007(E)
ISO 2007
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ISO/TR 9824:2007(E)
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© ISO 2007

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ii © ISO 2007 – All rights reserved
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ISO/TR 9824:2007(E)
Contents Page

Foreword............................................................................................................................................................ iv

1 Scope ..................................................................................................................................................... 1

2 Normative references ........................................................................................................................... 1

3 Terms and definitions........................................................................................................................... 1

4 Characteristics of a closed conduit system ......................................................................................1

4.1 Physical structure................................................................................................................................. 1

4.2 Construction.......................................................................................................................................... 2

4.3 Flow conditions..................................................................................................................................... 2

4.4 Environment .......................................................................................................................................... 2

5 Selection of method.............................................................................................................................. 3

5.1 General................................................................................................................................................... 3

5.2 Factors ................................................................................................................................................... 3

6 Methods of measurement .................................................................................................................... 4

6.1 Volumetric methods ............................................................................................................................. 4

6.2 Tracer and dilution method ................................................................................................................. 5

6.3 Flow measurement structures............................................................................................................. 6

6.4 Ultrasonic Doppler.............................................................................................................................. 17

6.5 ‘Transit time’ ultrasonic flow meters ................................................................................................ 18

6.6 Electromagnetic method.................................................................................................................... 21

6.7 Slope-area method.............................................................................................................................. 23

6.8 Non-contact methods......................................................................................................................... 25

6.9 Spot flow measurements, evaluation and verification.................................................................... 26

7 Final selection of method .................................................................................................................. 26

Annex A (informative) Guide to the selection of methods........................................................................... 27

Bibliography ..................................................................................................................................................... 30

© ISO 2007 – All rights reserved iii
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ISO/TR 9824:2007(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.

In exceptional circumstances, when a technical committee has collected data of a different kind from that

which is normally published as an International Standard (“state of the art”, for example), it may decide by a

simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely

informative in nature and does not have to be reviewed until the data it provides are considered to be no

longer valid or useful.

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/TR 9824 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1, Velocity

area methods.

This first edition of ISO/TR 9824 cancels and replaces ISO/TR 9824-1:1990 and ISO/TR 9824-2:1990, of

which it constitutes a technical revision.
iv © ISO 2007 – All rights reserved
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TECHNICAL REPORT ISO/TR 9824:2007(E)
Hydrometry — Measurement of free surface flow in closed
conduits
1 Scope

This Technical Report provides a synopsis of the methods of flow gauging that can be deployed in closed

conduits flowing part full, i.e. with a free open water surface. It provides a brief description of each method

with particular reference to other International Standards where appropriate, the attributes and limitations of

each technique, possible levels of uncertainty in the flow determinations and specific equipment requirements.

The uncertainties quoted herein are expanded uncertainties with a coverage factor of 2 and an approximate

confidence level of 95 %.
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, Hydrometric determinations — Vocabulary and symbols
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 772 and the following apply.

3.1
free surface flow in closed conduits

flow within closed conduits, under the influence of gravity only, and normally having a free surface

4 Characteristics of a closed conduit system
4.1 Physical structure

Closed conduits can be located below ground (e.g. sewer) or above ground (e.g. culvert). Systems

constructed underground usually incorporate a means of access through a suitable sized shaft (manhole)

sealed at the surface with a secure, but removable, cover. Access shafts may be provided at frequent

intervals along the length of the conduit. It is normal to locate shafts at points of structural change in the

system, such as bends, or junctions, or where for some reason, inspection or entry to the system may be

required. Access will be subject to strict health and safety conditions and operatives may require special

training. Also, access may not be allowed during or following a period of rainfall.

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ISO/TR 9824:2007(E)
4.2 Construction
4.2.1 Material

Conduits can be made from a variety of materials such as dry stone blocks, vitreous clayware, concrete, cast

iron, steel, galvanized iron or steel, asbestos and glass reinforced plastic. In addition, the conduit may have

been formed out of the natural bedrock.

The roughness of the surface of the conduit may range from smooth to extremely rough. The roughness may

be influenced by organic growths, deposits of sediment, rust, cracks, holes and other imperfections.

4.2.2 Cross-sectional shape

Closed conduits are most commonly circular or rectangular in shape. They may also be ovoid, horseshoe,

barrel or triangular. For the purposes of this Technical Report, they are considered to range in diameter from

150 mm upwards.
4.3 Flow conditions

Flow in closed conduits can vary from clear water, free from contaminants (e.g. spring flows), to liquids

containing both floating and suspended material (e.g. foul sewer), and in some cases effluents of a corrosive

nature. The fluid itself may be an admixture of several substances each with its own characteristic properties.

Discharges may vary over a wide range from reverse flow, through zero to many cubic metres per second. For

some applications, measurement equipment should be capable of withstanding inundation and measuring

surcharge flow. The flow, especially that generated from impervious catchments, may exhibit rapid changes in

discharge over short durations and may range from subcritical to supercritical.
4.4 Environment
4.4.1 Within the conduit

The atmosphere within a conduit system may be assumed to be in equilibrium with the liquid in the conduit. If

the atmosphere is of a toxic and/or corrosive nature, precautions should be taken to protect the equipment

from its effects or choose a method for which this is not a problem.

It is possible that under certain circumstances, the atmosphere may be of a potentially highly explosive nature.

Therefore, the equipment to be installed within the confines of the conduit system should be intrinsically safe.

For example, all electrical circuits should be constructed so that they cannot cause ignition of the atmosphere.

The extremes of the atmospheric environment within which the equipment is expected to operate need to be

ascertained in terms of temperature, humidity, pressure and gases.
4.4.2 External environment

Where elements of the equipment are situated outside the conduit system, the external environmental

conditions should be ascertained. Examples of these external conditions are
a) atmospheric temperature and relative humidity ranges,
b) likelihood of electrical interference, and
c) likelihood of mechanical shock.
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ISO/TR 9824:2007(E)
5 Selection of method
5.1 General

In selecting the most appropriate method, the factors in 5.2 should be taken into account.

5.2 Factors
5.2.1 Frequency and duration of measurement

The response of the conduit system to inputs of storm run-off may require measurements to be taken at

frequent intervals to allow the hydrograph to be defined. The recording intervals may need to be one minute or

less. The duration of flow measurement at a site should be consistent with the intended use of the data.

5.2.2 Physical conditions
The physical conditions that may affect the choice of method are
a) ease of access to the site,
b) dimensions of the conduit,
c) upstream and downstream conduit integrity,
d) junctions, bends, connections, bifurcations, inlets and outlets,
e) bed load, silt load and suspended solids,
f) range of depth and discharge,
g) range of velocity,
h) flow directions,
i) atmosphere within the conduit, e.g. temperature, humidity and quality,

j) the nature and concentration of dissolved, floating and suspended solids. The material/pollutant may be

classified into four groups:
1) pollutants and sediments in solution;
2) finely suspended sediments with median diameter = 0,062 mm;
3) coarse sediments where median diameter = 3,5 mm;

4) gross solids where particulate matter is greater than 6 mm in any two dimensions.

5.2.3 Site surveys

It is desirable that a preliminary survey is made to decide on the suitability of the site taking due account of the

various physical conditions as listed in 5.2.2. In addition, it may be necessary to abide by specific national or

local health and safety regulations that could be in force for persons working in closed conduits or confined

spaces.
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ISO/TR 9824:2007(E)
6 Methods of measurement
6.1 Volumetric methods
6.1.1 Description

In the volumetric method, the change in level of fluid in a reservoir is measured over time to deduce flow-rate,

given a known relationship between fluid depth and volume. Account needs to be taken of any simultaneous

inflows and outflows that are occurring. For example, for a wet well system that is emptied by a pump turned

on and off by high and low level switches, the inflow may be calculated from the time to fill, i.e. when the

pumps are off. The discharge may also be calculated from the time to empty when the pumps are on,

assuming that the inflow is constant.

This method may be applied where fluid depth is monitored by fixed point level switches or a continuous level

sensor.

Using this method, flow-rate is averaged over the time period to fill or empty the tank, hence short-term peaks

or troughs in the instantaneous flow-rate may not be captured. The use of continuous level measurement

equipment to take intermediate readings may enable any variations in the flow-rate to be identified.

6.1.2 Attributes and limitations

Fluid level can be measured by non-contact means minimizing maintenance requirements.

This method requires a flow computer but otherwise no additional sensors need be installed, as those already

in place to control the pump switching are used.

The method can only be used where there is an appropriate tank, wet well or reservoir.

The volume/depth relationship can be difficult to determine for irregularly shaped reservoirs or those with

intrusions and internal structures.
This method is usually not practical in an underground system.

Sediment and sludge building up in the reservoir can change the volume/depth relationship.

6.1.3 Equipment
The following will be needed to apply this method:
a) a suitable reservoir;
b) a means of determining fluid level at two or more points in the reservoir;
c) a suitably programmed flow computer.
6.1.4 Application

This method may either be used for short-term flow surveys, calibration and verification of permanently

installed equipment or as a permanent means of measurement.
6.1.5 Uncertainties

The performance of this method is dependent on the certainty with which the volume/depth relationship of the

reservoir is known, the resolution and accuracy of the equipment used to measure the fluid depth in the tank

and the presence and character of the inflows and outflows. The resolution of the level sensor(s) should be

considered against the changes in depth which may be encountered using a reservoir with large surface area,

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ISO/TR 9824:2007(E)

i.e. the volume change in a reservoir with a large cross-sectional area will change greater for a given change

in fluid depth than one with a smaller cross-sectional area.

Uncertainties (with a coverage factor of 2 and an approximate confidence level of 95 %) of less than 2 % are

achievable for a clean reservoir having a precisely defined volume/depth relationship and using a high

resolution depth sensor with an uncertainty within 0,5 %. Generally with a good installation, the uncertainty will

be of the order of 5 %.
6.2 Tracer and dilution method
6.2.1 Basic principles

The basis of the tracer and dilution method is to inject a substance into the flow that can be easily

distinguished from the bulk liquid and thereafter detect that substance at a point downstream. There are two

distinct ways in which the method may be applied:

a) transit time that measures the time taken for a sudden injection of tracer to travel from one point to

another;

b) dilution gauging which compares the concentration of the tracer injected into the bulk fluid with the

concentration detected downstream of the injection point. This can be further subdivided into constant

rate injection or tracer integration methods.

The choice of tracer depends upon the nature of the fluid and the installation. The tracer should not appear in

significant quantities in the fluid nor be absorbed by, or react with the fluid or the walls of the conduit. The

latter may be a consideration if the walls are subject to biological or chemical fouling.

The three principal types of tracer which are used are

1) radioactive, e.g. Tritium, Na(24) (as sodium carbonate solution), Br(82) (as potassium bromide

solution),
2) fluorescent, e.g. Pyranine,
3) chemical, e.g. sodium chloride, lithium chloride, sodium iodide.

The use of radioactive tracers in discharges to sewers or the environment is nowadays generally

unacceptable. For water applications, lithium chloride and sodium chloride are the most widely used tracers.

The application of dilution gauging to free surface flows is described further in ISO 9555 (parts1 through 4).

6.2.2 Attributes and limitations

The method has been successfully used across a wide range of applications and is covered by established

International Standards.
There is minimal disruption to the flow.

There is a need to ensure good mixing between the injection and sampling points whilst keeping this distance

as short as possible. ISO 2975 looks specifically at the requirements to ensure good mixing. To help mixing,

flow should be fully turbulent, i.e. with a Reynolds number greater than 5 000.

With some tracers, e.g. lithium chloride, results are not available on site as samples require laboratory

analysis to determine the tracer concentration. If sodium chloride is used as the tracer, detection may be done

using a conductivity sensor to provide an immediate result.
Two access points are required, one for injection and one for sampling.

Flow needs to be kept stable for the duration of the test that may take many minutes.

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ISO/TR 9824:2007(E)
6.2.3 Equipment

The equipment required for the application of this method comprises: reservoir of tracer solution, timing

mechanism, tracer injection apparatus and means for extracting samples or detecting tracer downstream of

injection point. For some chemical tracers, access to a laboratory for determination of tracer concentration in

the downstream samples will be needed.
6.2.4 Application

These methods are generally used for short-term tests such as flow surveys or for the calibration and

verification of permanently installed equipment.
6.2.5 Uncertainties

An uncertainty (with a coverage factor of 2 and an approximate confidence level of 95 %) of 5 % may be

expected.
6.3 Flow measurement structures
6.3.1 Basic principles

A flow measurement structure is generally designed to act as a control in the channel in order to provide a

unique, stable relationship between head (stage) and discharge. The relationship can usually either be

derived empirically or from physical principles. Most flow measurement structures require critical flow to occur

at the control section, i.e. the upstream head or water level is independent of downstream conditions. This

condition is normally referred to as modular flow. If the upstream water level (head) is affected by downstream

conditions, the flow is said to be non-modular. Some structures can still be utilized when the flow is non-

modular, but an additional downstream head measurement is required in order to determine a reduction factor

that is applied to the modular head-discharge relationship. However, the uncertainty of the flow determination

under non-modular conditions will be larger. Therefore, the use of flow measurement structures under non-

modular flows should be avoided whenever possible.

For most of the commonly used flow measurement structures, there are International Standards that outline

the design requirements and their physical limitations, e.g. minimum head requirements. Reference is made to

the relevant International Standards, where appropriate, in 6.3.4.

The performance of a flow measurement structure is dependent on the following factors:

a) the hydraulic and other site conditions over the target discharge range;
b) the quality and accuracy of construction and installation;

c) the accuracy and reliability of the upstream head, and where appropriate downstream head,

measurements.

Flow measurement structures cause an obstruction in the watercourse to create sufficient head difference in

order to determine flow. When designing or installing flow measurement structures, it is therefore important to

establish the impact of the structure on upstream water levels in the channel, i.e. how much afflux or

backwater is being created under different flow conditions. It is particularly important when installing flow

measurement structures in closed conduits to ensure that the liquid carrying capacity of the conduit is not

significantly reduced resulting in the potential to do damage or harm.
Guidelines on the selection of structures are contained in ISO 8368.
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ISO/TR 9824:2007(E)
6.3.2 The use of flow measurement structures in closed conduits

The use and selection of flow measurement structures in closed conduits will depend on the hydraulic and

other physical conditions that prevail. In some closed conduit situations, conventional open channel flow

measurement structures for which there are appropriate International Standards may be used. Conversely in

some closed conduits where pressure conditions as well as free surface flow conditions occur, or where the

hydraulic conditions are unsuitable, it will not be possible to measure flows using conventional structures. For

this reason, special types of structures have been designed to operate under such conditions. For the

purposes of this Technical Report, flow measurement structures have therefore been categorized under the

following headings:

a) special structures appropriate for use in closed conduits or structures for which there is currently no

International Standard;
b) conventional structures for which there is an International Standard.
6.3.3 Special structures not covered by any specific International Standard
These are summarized in Table 1.

Table 1 — Special structures for use in closed conduits not covered by a specific

International Standard
Type Figure Comments, attributes and limitations Applications
number
Vertical slot weir

1 Allows solid materials to pass the installation. Larger conduits carrying solid

Rating curve may be affected by the conduit materials, where there is free
slope and roughness, particularly for wide slots surface flow
at low flows.

Trapezoidal weir 2 Allows passage of solids and does not obstruct Larger conduits carrying solids and

flow. Rating curve is affected by the conduit where there is free surface flow
slope and roughness, but can measure lower
depths and smaller flows than vertical slot weir.

US Geological 3 Device that acts as a flume under free surface Closed conduits where both free

Survey (USGS) flow conditions and a nozzle when surcharged. surface and surcharge conditions

meter Under the free surface flow conditions, the will occur
greater the constriction, the better the
accuracy.

University of Illinois 4 Similar to USGS meter. The transition from free Closed conduits where both free

meter
surface to pressure flow is not as smooth surface and surcharge conditions
however, owing to the shape of the constriction will occur
in the conduit soffit.
Palmer-Bowlus 5 A critical depth flume that has been tested in Larger conduits
Flume sloping conduits and exhibits a successful
head-discharge calibration for both subcritical
and supercritical free surface flow.
Modifications to the design are recommended
to improve the head-discharge relationship in
the transition zone from free surface to
pressure flow.
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ISO/TR 9824:2007(E)
Dimensions in metres
Figure 1 — Vertical slot weir
Dimensions in metres
Key
D internal diameter of the conduit
h head
Figure 2 — Trapezoidal weir
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ISO/TR 9824:2007(E)
Key
1 throat cross section
2 side view
3 piezometer taps
throat area
4 = 0,709
pipe area
D internal diameter of the conduit
Figure 3 — US Geological Survey (USGS) meter
Key
1 throat cross section
2 critical section
3 flow
D internal diameter of the conduit
Figure 4 — University of Illinois meter
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ISO/TR 9824:2007(E)
Key
1 throat zero datum 5 approach section
2 approach zero datum 6 critical section
3 flow 7 throat section
4 water surface
D internal diameter of the conduit
D/2 half the diameter of the conduit
D/3 one third of the diameter of the conduit
D/6 one sixth of the diameter of the conduit
Figure 5 — Palmer-Bowlus flume
6.3.4 Conventional structures
These are summarized in Table 2.
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ISO/TR 9824:2007(E)
Table 2 — Conventional structures with International Standards
Type/ Figure Comments, attributes and limitations Applications
ISO International number
Standard number
Parshall flume/

6 Critical depth flume originally developed for use Free surface flows in larger conduits

ISO 9826 in irrigation canals.

Thin plate weirs/ 7 and 8 Rectangular thin plate weirs and V–notches are Free surface flows in sediment free

ISO 1438-1
relatively easy to install but manufacture of crest water, e.g. spring flows
requires care. Accurate. V-notches can be
particularly suitable where ratio of high to low
flows is large and low flow accuracy important.
Susceptible to poor approach conditions and
high sediment loads.

Triangular profile 9 Good accuracy, discharge range and modular Free surface flows in larger conduits

weirs/ limit. Relatively robust and have the ability to with large amounts of suspended

ISO 4360 pass heavy silt. Can be used under non-modular solids
conditions if crest tapping and/or downstream
head measurements are made.
Triangular profile

10 Have similar attributes to horizontal crested Free surface flows in larger conduits

flat-v weir/ triangular profile weirs but have greater with large amounts of suspended

ISO 4377
sensitivity. solids
Flumes: 11 and 12 Flumes are often recommended where material Free surface flows
Rectangular and is being transported along the conduit. Suitable
Rectangular flumes in rectangular
U-shaped/ where afflux needs to be kept to minimum.
conduits and U-shaped flumes in
ISO 4359 Rectangular flumes in particular can be relatively
circular sewers and other conduits
cheap to install. They are universally used for
measuring inflows to waste water treatment
works.
Dimensions in metres
Key
1 level floor 4 converging section
2 flow 5 throat section
3 water surface 6 diverging section
Figure 6 — Parshall flume
© ISO 2007 – All rights reserv
...

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