ISO/TR 9824:2007

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 2007
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©  ISO 2007
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ii © ISO 2007 – All rights reserved

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

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

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.
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.
2 © ISO 2007 – All rights reserved

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.
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
...