Hydrometry - Open channel flow measurement using thin-plate weirs

ISO 1438:2008 defines the requirements for the use of rectangular and triangular (V-notch) thin-plate weirs for the measurement of flow of clear water in open channels under free flow conditions. It includes the requirements for the use of full-width rectangular thin-plate weirs in submerged (drowned) flow conditions.

Hydrométrie - Mesure de débit dans les canaux découverts au moyen de déversoirs à paroi mince

Hidrometrija - Meritev pretoka odprtega kanala z uporabo jezov iz tanke plošče

Ta mednarodni standard določa zahteve glede opreme, pritrdišča, podpore in dodatne opreme za žičniške sisteme za uporabo pri merjenju toka. Obravnavani so sistemi, ki se v celoti upravljajo
z rečnega brega ali iz viseče kabine za osebje (z drugim imenom »žičnica«).
Ta mednarodni standard obravnava le žičniške sisteme, ki bodo uporabljeni za hidrometrične meritve. Če naj bi bila žičnica certificirana kot oprema za dviganje, lahko
veljajo drugi standardi ali predpisi. Ta mednarodni standard ne zajema metod za
merjenje izpusta, ki so opisane v standardu ISO 748.

General Information

Status
Withdrawn
Publication Date
06-Jan-2015
Withdrawal Date
15-Apr-2018
Technical Committee
Current Stage
9900 - Withdrawal (Adopted Project)
Start Date
16-Apr-2018
Due Date
09-May-2018
Completion Date
16-Apr-2018

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INTERNATIONAL ISO
STANDARD 1438
Second edition
2008-04-15

Hydrometry — Open channel flow
measurement using thin-plate weirs
Hydrométrie — Mesure de débit dans les canaux découverts au moyen
de déversoirs à paroi mince




Reference number
ISO 1438:2008(E)
©
ISO 2008

---------------------- Page: 1 ----------------------
ISO 1438:2008(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


COPYRIGHT PROTECTED DOCUMENT


©  ISO 2008
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2008 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 1438:2008(E)
Contents Page
Foreword. v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols and abbreviated terms . 1
5 Principle. 2
6 Installation . 2
6.1 General. 2
6.2 Selection of site . 2
6.3 Installation conditions. 2
7 Measurement of head. 4
7.1 Head measuring devices. 4
7.2 Stilling or float well. 5
7.3 Head-measurement section. 5
7.4 Head-gauge datum (gauge zero) . 5
8 Maintenance . 6
9 Rectangular thin-plate weir. 6
9.1 Types. 6
9.2 Specifications for the standard weir. 8
9.3 Specifications for installation. 8
9.4 Determination of gauge zero . 8
9.5 Discharge formulae — General . 11
9.6 Formulae for the basic weir form (all values of b/B). 11
9.7 Formulae for full-width weirs (b/B = 1,0) . 14
10 Triangular-notch thin-plate weir. 16
10.1 Specifications for the standard weir. 16
10.2 Specifications for the installation . 18
10.3 Specifications for head measurement. 18
10.4 Discharge formulae — General . 19
10.5 Formula for all notch angles between π/9 and 5 π/9 radians (20° and 100°). 19
10.6 Formula for specific notch angles (fully-contracted weir) . 21
10.7 Accuracy of discharge coefficients — Triangular-notch weirs . 22
11 Uncertainties of flow measurement. 22
11.1 General. 22
11.2 Combining measurement uncertainties . 23
*
11.3 Uncertainty of discharge coefficient uC for thin-plate weirs . 25
( )
d
11.4 The uncertainty budget . 25
12 Example . 25
12.1 General. 25
12.2 Characteristics — Gauging structure. 26
12.3 Characteristics — Gauged head instrumentation . 26
12.4 Discharge coefficient. 26
12.5 Discharge estimate. 26
12.6 Uncertainty statement . 27
Annex A (informative) Flow measurement with small weir tanks . 29
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ISO 1438:2008(E)
Annex B (informative) Guide to the design and installation of a flow straightener. 31
Annex C (informative) Introduction to measurement uncertainty. 33
Annex D (informative) Sample measurement performance for use in
hydrometric worked examples . 41
Annex E (informative) Specimen tables . 43
Bibliography . 59

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ISO 1438:2008(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 1438 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 2, Flow
measurement structures.
This second edition cancels and replaces the first edition (ISO 1438-1:1980), of which it constitutes a technical
revision. It also incorporates the Amendment ISO 1438-1:1980/Amd 1:1988.

© ISO 2008 – All rights reserved v

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INTERNATIONAL STANDARD ISO 1438:2008(E)

Hydrometry — Open channel flow measurement using
thin-plate weirs
1 Scope
This International Standard defines the requirements for the use of rectangular and triangular (V-notch) thin-
plate weirs for the measurement of flow of clear water in open channels under free flow conditions. It includes
the requirements for the use of full-width rectangular thin-plate weirs in submerged (drowned) flow conditions.
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, Hydrometry — Vocabulary and symbols
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Symbols and abbreviated terms
2
A m area of approach channel
B m width of approach channel
b m measured width of the notch
b m width of notch at maximum head (V-notch)

max
C discharge coefficient (gauged head)
C coefficient of discharge
d
f drowned flow reduction factor
fC combined coefficient of discharge
C coefficient of velocity

v
e m random uncertainty in the width measurement

b

2
g m/s acceleration due to gravity
H m total head above crest level

h m upstream gauged head above crest level (upstream head is inferred if no subscript is used)

J numerical constant
l m distance of the head measurement section upstream of the weir
n number of measurements in a set
p m height of the crest relative to the floor
3
Q m /s volumetric rate of flow
S submergence ratio, h /h

2 1
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ISO 1438:2008(E)
S modular limit

1
m/s mean velocity
V
U % expanded percentage uncertainty

u*(b) % percentage uncertainty in b

u*(C) % percentage uncertainty in C

u*(E) % percentage uncertainty in datum measurement

u*(h ) % percentage uncertainty in h

1 1
u*(Q) % percentage uncertainty in Q

α ° notch angle
Subscripts:
1 upstream
2 downstream
e effective
r rectangular
t triangular
5 Principle
The discharge over thin-plate weirs is a function of the upstream head on the weir (for free-flow), upstream
and downstream head (for drowned flow), the size and shape of the discharge area and an experimentally
determined coefficient which takes into account the head, the geometrical properties of the weir and approach
channel and the dynamic properties of the water.
6 Installation
6.1 General
General requirements of weir installations are described in the following clauses. Special requirements of
different types of weirs are described in clauses which deal with specific weirs (see Clauses 9 and 10).
6.2 Selection of site
The type of weir to be used for discharge measurement is determined in part by the nature of the proposed
measuring site. Under some conditions of design and use, weirs shall be located in rectangular flumes or in
weir boxes which simulate flow conditions in rectangular flumes. Under other conditions, weirs may be located
in natural channels as well as flumes or weir boxes, with no significant difference in measurement accuracy.
Specific site-related requirements of the installation are described in 6.3.
6.3 Installation conditions
6.3.1 General
Weir discharge is critically influenced by the physical characteristics of the weir and the weir channel.
Thin-plate weirs are especially dependent on installation features which control the velocity distribution in the
approach channel and on the construction and maintenance of the weir crest in meticulous conformance with
standard specifications.
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ISO 1438:2008(E)
6.3.2 Weir
Thin-plate weirs shall be vertical and perpendicular to the walls of the channel. The intersection of the weir
plate with the walls and floor of the channel shall be watertight and firm, and the weir shall be capable of
withstanding the maximum flow without distortion or damage.
Stated practical limits associated with different discharge formulae such as minimum width, minimum weir
height, minimum head, and maximum values of h/p and b/B (where h is the measured head, p is the height of
crest relative to floor, b is the measured width of the notch and B is the width of the approach channel), are
factors which influence both the selection of weir type and the installation.
6.3.3 Approach channel
For the purposes of this International Standard, the approach channel is the portion of the weir channel which
extends upstream from the weir a distance not less than 5 times the width of the nappe at maximum head. If
the weir is located in a weir tank, ideally the length of the tank should equal to 10 times the width of the nappe
at maximum head. Information on the use of small weir tanks is given in Annex A.
The flow in the approach channel shall be uniform and steady, with the velocity distribution approximating that
in a channel of sufficient length to develop satisfactory flow in smooth, straight channels. Figure 1 shows
measured velocity distributions perpendicular to the direction of flow in rectangular channels, upstream from
the influence of a weir. Baffles and flow straighteners can be used to simulate satisfactory velocity distribution,
but their location with respect to the weir shall be not less than the minimum length prescribed for the
approach channel.
The influence of approach-channel velocity distribution on weir flow increases as h/p and b/B increase in
magnitude. If a weir installation unavoidably results in a velocity distribution that is appreciably non-uniform,
the possibility of error in calculated discharge should be checked by means of an alternative discharge-
measuring method for a representative range of discharges.
If the approach conditions are judged to be unsatisfactory, then flow straighteners shall be introduced in
accordance with Annex B.
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ISO 1438:2008(E)

a)

b)

c)
NOTE The contours refer to values of local flow velocity relative to the mean cross-sectional velocity.
Figure 1 — Examples of normal velocity distribution in rectangular channels
6.3.4 Downstream channel
For most applications, the level of the water in the downstream channel shall be a sufficient vertical distance
below the crest to ensure free, fully ventilated discharges. Free (non-submerged) discharge occurs when the
discharge is independent of the downstream water level. Fully ventilated discharge is ensured when the air
pressure on the lower surface of the nappe is fully ventilated. Drowned flow operation is permitted for full width
weirs under certain conditions (see 9.7.2). Under these circumstances, downstream water levels may rise
above crest level.
7 Measurement of head
7.1 Head measuring devices
In order to obtain the discharge measurement accuracies specified for the standard weirs, the head on the
weir shall be measured with a laboratory-grade hook gauge, point gauge, manometer, or other gauge of
equivalent accuracy. For a continuous record of head variants, precise float gauges and servo-operated point
gauges can be used. Staff and tape gauges can be used when less accurate measurements are acceptable.
[1]
Additional specifications for head-measuring devices are given in ISO 4373 .
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ISO 1438:2008(E)
7.2 Stilling or float well
For the exceptional case where surface velocities and disturbances in the approach channel are negligible,
the headwater level can be measured directly (for example, by means of a point gauge mounted over the
water surface). Generally, however, to avoid water-level variations caused by waves, turbulence or vibration,
the headwater level should be measured in a separate stilling well.
Separate stilling wells are connected to the approach channel by means of a suitable conduit, equipped if
necessary with a throttle valve to damp oscillations. At the channel end of the conduit, the connection is made
to floor or wall piezometers or a static tube at the head-measurement section.
[2]
Additional specifications for stilling wells are given in ISO 1100-1 .
7.3 Head-measurement section
7.3.1 Upstream head-measurement
The head-measurement section shall be located a sufficient distance upstream from the weir to avoid the
region of surface drawdown caused by the formation of the nappe. On the other hand, it shall be sufficiently
close to the weir that the energy loss between the head-measurement section and the weir is negligible. For
the weirs included in this International Standard, the location of the head-measurement section will be
satisfactory if it is at a distance equal to 2 to 4 times the maximum head (2h to 4h ) upstream from the
max max
weir.
If high velocities occur in the approach channel or if water-surface disturbances or irregularities occur at the
head-measurement section because of high values of h/p or b/B, it may be necessary to install several
pressure intakes to ensure that the head measured in the gauge well is representative of the average head
across the measurement section.
In the case of a full-width thin-plate weir, the effect of frictional effects upon the upstream channel requires an
adjustment to the standard coefficient of discharge. The correction is in terms of both l/h and h/p and given in
Table 1.
Table 1 — Factors to be applied to the standard discharge coefficient values
h/p l/h
2 4 6 8
3,5 to 4,0
1,00 1,00 0,96 0,92
3,0 to 3,5 1,00 1,00 0,97 0,94
2,5 to 3,0 1,00 1,00 0,98 0,96
2,0 to 2,5 1,00 1,00 0,99 0,98
Less than 2,0 1,00 1,00 1,00 1,00
7.3.2 Downstream head measurement
If the weir is to be operated in the submerged (drowned) flow range, a measurement of downstream head is
required in addition to that upstream. The downstream head measurement position shall be 10 h
max
downstream from the upstream face of the weir. If a stilling well is included in the design, it is recommended
that the downstream head measurement be located no closer to the weir than 4 h .
max
7.4 Head-gauge datum (gauge zero)
Accuracy of head measurements is critically dependent upon the determination of the head-gauge datum or
gauge zero, which is defined as the gauge reading corresponding to the level of the weir crest (rectangular
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ISO 1438:2008(E)
weirs) or the level of the vertex of the notch (triangular-notch weirs). When necessary, the gauge zero shall be
checked. Numerous acceptable methods of determining the gauge zero are in use. Typical methods are
described in subsequent clauses dealing specifically with rectangular and triangular weirs. (See Clauses 9
and 10.)
Because of surface tension, the gauge zero cannot be determined with sufficient accuracy by reading the
head gauge with the water in the approach channel drawn down to the apparent crest (or notch) level.
8 Maintenance
Maintenance of the weir and the weir channel is necessary to ensure accurate measurements.
The approach channel shall be kept free of silt, vegetation and obstructions which might have deleterious
effects on the flow conditions specified for the standard installation. The downstream channel shall be kept
free of obstructions which might cause submergence or inhibit full ventilation of the nappe under all conditions
of flow.
The weir plate shall be kept clean and firmly secured. In the process of cleaning, care shall be taken to avoid
damage to the crest or notch, particularly the upstream edges and surfaces. Construction specifications for
these most sensitive features should be reviewed before maintenance is undertaken.
Head-measurement piezometers, connecting conduits and the stilling well shall be cleaned and checked for
leakage. The hook or point gauge, manometer, float or other instrument used to measure the head shall be
checked periodically to ensure accuracy.
If a flow straightener is used in the approach channel, perforated plates shall be kept clean so that the
percentage open area remains greater than 40 %.
9 Rectangular thin-plate weir
9.1 Types
The rectangular thin-plate weir is a general classification in which the rectangular-notch weir is the basic form
and the full-width weir is a limiting case. A diagrammatic illustration of the basic weir form is shown in Figure 2
with intermediate values of b/B and h/p. When b/B = 1,0, that is when the width of the weir (b) is equal to the
width of the channel at the weir section (B), the weir is of full-width type (also referred to as a “suppressed”
weir, because its nappe lacks side contractions).
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ISO 1438:2008(E)
Dimensions in millimetres

a)

b)
Key
1 upstream face of weir plate
2 head measurement section, h
1
Figure 2 — Rectangular-notch, thin-plate weir
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ISO 1438:2008(E)
9.2 Specifications for the standard weir
The basic weir form consists of a rectangular notch in a vertical, thin plate. The plate shall be plane and rigid
and perpendicular to the walls and the floor of the approach channel. The upstream face of the plate shall be
smooth (in the vicinity of the notch it shall be equivalent in surface finish to that of rolled sheet-metal).
The vertical bisector of the notch shall be equidistant from the two walls of the channel. The crest surface of
the notch shall be a horizontal, plane surface, which shall form a sharp edge at its intersection with the
upstream face of the weir plate. The width of the crest surface, measured perpendicular to the face of the
plate, shall be between 1 mm and 2 mm. The side surfaces of the notch shall be vertical, plane surfaces which
shall make sharp edges at their intersection with the upstream face of the weir plate. For the limiting case of
the full-width weir, the crest of the weir shall extend to the walls of the channel, which in the vicinity of the
crest shall be plane and smooth (see also 9.3).
To ensure that the upstream edges of the crest and the sides of the notch are sharp, they shall be machined
or filed, perpendicular to the upstream face of the weir plate, free of burrs or scratches and untouched by
abrasive cloth or paper. The downstream edges of the notch shall be chamfered if the weir plate is thicker
than the maximum allowable width of the notch surface. The surface of the chamfer shall make an angle of
not less than π/4 radians (45°) with the crest and side surfaces of the notch (see detail, Figure 2). The weir
plate in the vicinity of the notch preferably shall be made of corrosion-resistant metal; but if it is not, all
specified smooth surfaces and sharp edges shall be kept coated with a thin, protective film (for example, oil,
wax, silicone) applied with a soft cloth.
9.3 Specifications for installation
The specifications stated in 6.3 shall apply. In general, the weir shall be located in a straight, horizontal,
rectangular approach channel if possible. However, if the effective opening of the notch is so small in
comparison with the area of the upstream channel that the approach velocity is negligible, the shape of the
channel is not significant. In any case, the flow in the approach channel shall be uniform and steady, as
specified in 6.3.3.
If the width of the weir is equal to the width of the channel at the weir section (i.e. a full-width weir), the sides
of the channel upstream from the plane of the weir shall be vertical, plane, parallel and smooth (equivalent in
surface finish to that of rolled sheet-metal). The sides of the channel above the level of the crest of a full-width
weir shall extend at least 0,3 h downstream from the plane of the weir. Fully ventilated discharge shall be
max
ensured as specified in 6.3.4.
The approach channel floor shall be smooth, flat and horizontal when the height of the crest relative to the
floor (p) is small and/or h/p is large. For rectangular weirs, the floor should be smooth, flat and horizontal,
particularly when p is less than 0,1 m and/or h /p is greater than 1. Additional conditions are specified in
max
connection with the recommended discharge formulae.
9.4 Determination of gauge zero
The head-gauge datum or gauge zero shall be determined with great care, and it shall be checked when
necessary. A typical, acceptable method of determining the gauge zero for rectangular weirs is described as
follows.
a) Still water in the approach channel is drawn to a level below the weir crest.
b) A temporary hook gauge is mounted over the approach channel, a short distance upstream from the weir
crest.
c) A precise machinists' level is placed with its axis horizontal, with one end lying on the weir crest and the
other end on the point of the temporary hook gauge (the gauge having been adjusted to hold the level in
this position). The reading of the temporary gauge is recorded.
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ISO 1438:2008(E)
d) The temporary hook gauge is lowered to the water surface in the approach channel and its reading is
recorded. The permanent gauge is adjusted to read the level in the gauge well, and this reading is
recorded.
e) The computed difference between the two readings of the temporary gauge is added to the reading of the
permanent gauge. The sum is the gauge zero for the permanent gauge.
Figure 3 illustrates the use of this procedure with a form of temporary hook gauge which is conveniently
mounted on the weir plate.
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ISO 1438:2008(E)


a) b)

c)
Key
1 permanent gauge
2 temporary hook gauge
3 precision level
4 vernier micrometer
5 set screw
6 weir crest
Figure 3 — Determination of gauge zero for rectangular weir
10 © ISO 2008 – All rights reserved

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ISO 1438:2008(E)
9.5 Discharge formulae — General
Recommended discharge formulae for rectangular thin-plate weirs are presented in three categories:
a) modular discharge equation for the basic weir form (all values of b/B);
b) modular discharge equation for full-width weirs (b/B = 1,0);
c) non-modular discharge equat
...

SLOVENSKI STANDARD
SIST ISO 1438:2015
01-februar-2015
+LGURPHWULMD0HULWHYSUHWRNDRGSUWHJDNDQDOD]XSRUDERMH]RYL]WDQNHSORãþH
Hydrometry - Open channel flow measurement using thin-plate weirs
Hydrométrie - Mesure de débit dans les canaux découverts au moyen de déversoirs à
paroi mince
Ta slovenski standard je istoveten z: ISO 1438:2008
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
SIST ISO 1438:2015 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------

SIST ISO 1438:2015

---------------------- Page: 2 ----------------------

SIST ISO 1438:2015

INTERNATIONAL ISO
STANDARD 1438
Second edition
2008-04-15

Hydrometry — Open channel flow
measurement using thin-plate weirs
Hydrométrie — Mesure de débit dans les canaux découverts au moyen
de déversoirs à paroi mince




Reference number
ISO 1438:2008(E)
©
ISO 2008

---------------------- Page: 3 ----------------------

SIST ISO 1438:2015
ISO 1438:2008(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


COPYRIGHT PROTECTED DOCUMENT


©  ISO 2008
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2008 – All rights reserved

---------------------- Page: 4 ----------------------

SIST ISO 1438:2015
ISO 1438:2008(E)
Contents Page
Foreword. v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols and abbreviated terms . 1
5 Principle. 2
6 Installation . 2
6.1 General. 2
6.2 Selection of site . 2
6.3 Installation conditions. 2
7 Measurement of head. 4
7.1 Head measuring devices. 4
7.2 Stilling or float well. 5
7.3 Head-measurement section. 5
7.4 Head-gauge datum (gauge zero) . 5
8 Maintenance . 6
9 Rectangular thin-plate weir. 6
9.1 Types. 6
9.2 Specifications for the standard weir. 8
9.3 Specifications for installation. 8
9.4 Determination of gauge zero . 8
9.5 Discharge formulae — General . 11
9.6 Formulae for the basic weir form (all values of b/B). 11
9.7 Formulae for full-width weirs (b/B = 1,0) . 14
10 Triangular-notch thin-plate weir. 16
10.1 Specifications for the standard weir. 16
10.2 Specifications for the installation . 18
10.3 Specifications for head measurement. 18
10.4 Discharge formulae — General . 19
10.5 Formula for all notch angles between π/9 and 5 π/9 radians (20° and 100°). 19
10.6 Formula for specific notch angles (fully-contracted weir) . 21
10.7 Accuracy of discharge coefficients — Triangular-notch weirs . 22
11 Uncertainties of flow measurement. 22
11.1 General. 22
11.2 Combining measurement uncertainties . 23
*
11.3 Uncertainty of discharge coefficient uC for thin-plate weirs . 25
( )
d
11.4 The uncertainty budget . 25
12 Example . 25
12.1 General. 25
12.2 Characteristics — Gauging structure. 26
12.3 Characteristics — Gauged head instrumentation . 26
12.4 Discharge coefficient. 26
12.5 Discharge estimate. 26
12.6 Uncertainty statement . 27
Annex A (informative) Flow measurement with small weir tanks . 29
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SIST ISO 1438:2015
ISO 1438:2008(E)
Annex B (informative) Guide to the design and installation of a flow straightener. 31
Annex C (informative) Introduction to measurement uncertainty. 33
Annex D (informative) Sample measurement performance for use in
hydrometric worked examples . 41
Annex E (informative) Specimen tables . 43
Bibliography . 59

iv © ISO 2008 – All rights reserved

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SIST ISO 1438:2015
ISO 1438:2008(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 1438 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 2, Flow
measurement structures.
This second edition cancels and replaces the first edition (ISO 1438-1:1980), of which it constitutes a technical
revision. It also incorporates the Amendment ISO 1438-1:1980/Amd 1:1988.

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SIST ISO 1438:2015
INTERNATIONAL STANDARD ISO 1438:2008(E)

Hydrometry — Open channel flow measurement using
thin-plate weirs
1 Scope
This International Standard defines the requirements for the use of rectangular and triangular (V-notch) thin-
plate weirs for the measurement of flow of clear water in open channels under free flow conditions. It includes
the requirements for the use of full-width rectangular thin-plate weirs in submerged (drowned) flow conditions.
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, Hydrometry — Vocabulary and symbols
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Symbols and abbreviated terms
2
A m area of approach channel
B m width of approach channel
b m measured width of the notch
b m width of notch at maximum head (V-notch)

max
C discharge coefficient (gauged head)
C coefficient of discharge
d
f drowned flow reduction factor
fC combined coefficient of discharge
C coefficient of velocity

v
e m random uncertainty in the width measurement

b

2
g m/s acceleration due to gravity
H m total head above crest level

h m upstream gauged head above crest level (upstream head is inferred if no subscript is used)

J numerical constant
l m distance of the head measurement section upstream of the weir
n number of measurements in a set
p m height of the crest relative to the floor
3
Q m /s volumetric rate of flow
S submergence ratio, h /h

2 1
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S modular limit

1
m/s mean velocity
V
U % expanded percentage uncertainty

u*(b) % percentage uncertainty in b

u*(C) % percentage uncertainty in C

u*(E) % percentage uncertainty in datum measurement

u*(h ) % percentage uncertainty in h

1 1
u*(Q) % percentage uncertainty in Q

α ° notch angle
Subscripts:
1 upstream
2 downstream
e effective
r rectangular
t triangular
5 Principle
The discharge over thin-plate weirs is a function of the upstream head on the weir (for free-flow), upstream
and downstream head (for drowned flow), the size and shape of the discharge area and an experimentally
determined coefficient which takes into account the head, the geometrical properties of the weir and approach
channel and the dynamic properties of the water.
6 Installation
6.1 General
General requirements of weir installations are described in the following clauses. Special requirements of
different types of weirs are described in clauses which deal with specific weirs (see Clauses 9 and 10).
6.2 Selection of site
The type of weir to be used for discharge measurement is determined in part by the nature of the proposed
measuring site. Under some conditions of design and use, weirs shall be located in rectangular flumes or in
weir boxes which simulate flow conditions in rectangular flumes. Under other conditions, weirs may be located
in natural channels as well as flumes or weir boxes, with no significant difference in measurement accuracy.
Specific site-related requirements of the installation are described in 6.3.
6.3 Installation conditions
6.3.1 General
Weir discharge is critically influenced by the physical characteristics of the weir and the weir channel.
Thin-plate weirs are especially dependent on installation features which control the velocity distribution in the
approach channel and on the construction and maintenance of the weir crest in meticulous conformance with
standard specifications.
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6.3.2 Weir
Thin-plate weirs shall be vertical and perpendicular to the walls of the channel. The intersection of the weir
plate with the walls and floor of the channel shall be watertight and firm, and the weir shall be capable of
withstanding the maximum flow without distortion or damage.
Stated practical limits associated with different discharge formulae such as minimum width, minimum weir
height, minimum head, and maximum values of h/p and b/B (where h is the measured head, p is the height of
crest relative to floor, b is the measured width of the notch and B is the width of the approach channel), are
factors which influence both the selection of weir type and the installation.
6.3.3 Approach channel
For the purposes of this International Standard, the approach channel is the portion of the weir channel which
extends upstream from the weir a distance not less than 5 times the width of the nappe at maximum head. If
the weir is located in a weir tank, ideally the length of the tank should equal to 10 times the width of the nappe
at maximum head. Information on the use of small weir tanks is given in Annex A.
The flow in the approach channel shall be uniform and steady, with the velocity distribution approximating that
in a channel of sufficient length to develop satisfactory flow in smooth, straight channels. Figure 1 shows
measured velocity distributions perpendicular to the direction of flow in rectangular channels, upstream from
the influence of a weir. Baffles and flow straighteners can be used to simulate satisfactory velocity distribution,
but their location with respect to the weir shall be not less than the minimum length prescribed for the
approach channel.
The influence of approach-channel velocity distribution on weir flow increases as h/p and b/B increase in
magnitude. If a weir installation unavoidably results in a velocity distribution that is appreciably non-uniform,
the possibility of error in calculated discharge should be checked by means of an alternative discharge-
measuring method for a representative range of discharges.
If the approach conditions are judged to be unsatisfactory, then flow straighteners shall be introduced in
accordance with Annex B.
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a)

b)

c)
NOTE The contours refer to values of local flow velocity relative to the mean cross-sectional velocity.
Figure 1 — Examples of normal velocity distribution in rectangular channels
6.3.4 Downstream channel
For most applications, the level of the water in the downstream channel shall be a sufficient vertical distance
below the crest to ensure free, fully ventilated discharges. Free (non-submerged) discharge occurs when the
discharge is independent of the downstream water level. Fully ventilated discharge is ensured when the air
pressure on the lower surface of the nappe is fully ventilated. Drowned flow operation is permitted for full width
weirs under certain conditions (see 9.7.2). Under these circumstances, downstream water levels may rise
above crest level.
7 Measurement of head
7.1 Head measuring devices
In order to obtain the discharge measurement accuracies specified for the standard weirs, the head on the
weir shall be measured with a laboratory-grade hook gauge, point gauge, manometer, or other gauge of
equivalent accuracy. For a continuous record of head variants, precise float gauges and servo-operated point
gauges can be used. Staff and tape gauges can be used when less accurate measurements are acceptable.
[1]
Additional specifications for head-measuring devices are given in ISO 4373 .
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7.2 Stilling or float well
For the exceptional case where surface velocities and disturbances in the approach channel are negligible,
the headwater level can be measured directly (for example, by means of a point gauge mounted over the
water surface). Generally, however, to avoid water-level variations caused by waves, turbulence or vibration,
the headwater level should be measured in a separate stilling well.
Separate stilling wells are connected to the approach channel by means of a suitable conduit, equipped if
necessary with a throttle valve to damp oscillations. At the channel end of the conduit, the connection is made
to floor or wall piezometers or a static tube at the head-measurement section.
[2]
Additional specifications for stilling wells are given in ISO 1100-1 .
7.3 Head-measurement section
7.3.1 Upstream head-measurement
The head-measurement section shall be located a sufficient distance upstream from the weir to avoid the
region of surface drawdown caused by the formation of the nappe. On the other hand, it shall be sufficiently
close to the weir that the energy loss between the head-measurement section and the weir is negligible. For
the weirs included in this International Standard, the location of the head-measurement section will be
satisfactory if it is at a distance equal to 2 to 4 times the maximum head (2h to 4h ) upstream from the
max max
weir.
If high velocities occur in the approach channel or if water-surface disturbances or irregularities occur at the
head-measurement section because of high values of h/p or b/B, it may be necessary to install several
pressure intakes to ensure that the head measured in the gauge well is representative of the average head
across the measurement section.
In the case of a full-width thin-plate weir, the effect of frictional effects upon the upstream channel requires an
adjustment to the standard coefficient of discharge. The correction is in terms of both l/h and h/p and given in
Table 1.
Table 1 — Factors to be applied to the standard discharge coefficient values
h/p l/h
2 4 6 8
3,5 to 4,0
1,00 1,00 0,96 0,92
3,0 to 3,5 1,00 1,00 0,97 0,94
2,5 to 3,0 1,00 1,00 0,98 0,96
2,0 to 2,5 1,00 1,00 0,99 0,98
Less than 2,0 1,00 1,00 1,00 1,00
7.3.2 Downstream head measurement
If the weir is to be operated in the submerged (drowned) flow range, a measurement of downstream head is
required in addition to that upstream. The downstream head measurement position shall be 10 h
max
downstream from the upstream face of the weir. If a stilling well is included in the design, it is recommended
that the downstream head measurement be located no closer to the weir than 4 h .
max
7.4 Head-gauge datum (gauge zero)
Accuracy of head measurements is critically dependent upon the determination of the head-gauge datum or
gauge zero, which is defined as the gauge reading corresponding to the level of the weir crest (rectangular
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weirs) or the level of the vertex of the notch (triangular-notch weirs). When necessary, the gauge zero shall be
checked. Numerous acceptable methods of determining the gauge zero are in use. Typical methods are
described in subsequent clauses dealing specifically with rectangular and triangular weirs. (See Clauses 9
and 10.)
Because of surface tension, the gauge zero cannot be determined with sufficient accuracy by reading the
head gauge with the water in the approach channel drawn down to the apparent crest (or notch) level.
8 Maintenance
Maintenance of the weir and the weir channel is necessary to ensure accurate measurements.
The approach channel shall be kept free of silt, vegetation and obstructions which might have deleterious
effects on the flow conditions specified for the standard installation. The downstream channel shall be kept
free of obstructions which might cause submergence or inhibit full ventilation of the nappe under all conditions
of flow.
The weir plate shall be kept clean and firmly secured. In the process of cleaning, care shall be taken to avoid
damage to the crest or notch, particularly the upstream edges and surfaces. Construction specifications for
these most sensitive features should be reviewed before maintenance is undertaken.
Head-measurement piezometers, connecting conduits and the stilling well shall be cleaned and checked for
leakage. The hook or point gauge, manometer, float or other instrument used to measure the head shall be
checked periodically to ensure accuracy.
If a flow straightener is used in the approach channel, perforated plates shall be kept clean so that the
percentage open area remains greater than 40 %.
9 Rectangular thin-plate weir
9.1 Types
The rectangular thin-plate weir is a general classification in which the rectangular-notch weir is the basic form
and the full-width weir is a limiting case. A diagrammatic illustration of the basic weir form is shown in Figure 2
with intermediate values of b/B and h/p. When b/B = 1,0, that is when the width of the weir (b) is equal to the
width of the channel at the weir section (B), the weir is of full-width type (also referred to as a “suppressed”
weir, because its nappe lacks side contractions).
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Dimensions in millimetres

a)

b)
Key
1 upstream face of weir plate
2 head measurement section, h
1
Figure 2 — Rectangular-notch, thin-plate weir
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9.2 Specifications for the standard weir
The basic weir form consists of a rectangular notch in a vertical, thin plate. The plate shall be plane and rigid
and perpendicular to the walls and the floor of the approach channel. The upstream face of the plate shall be
smooth (in the vicinity of the notch it shall be equivalent in surface finish to that of rolled sheet-metal).
The vertical bisector of the notch shall be equidistant from the two walls of the channel. The crest surface of
the notch shall be a horizontal, plane surface, which shall form a sharp edge at its intersection with the
upstream face of the weir plate. The width of the crest surface, measured perpendicular to the face of the
plate, shall be between 1 mm and 2 mm. The side surfaces of the notch shall be vertical, plane surfaces which
shall make sharp edges at their intersection with the upstream face of the weir plate. For the limiting case of
the full-width weir, the crest of the weir shall extend to the walls of the channel, which in the vicinity of the
crest shall be plane and smooth (see also 9.3).
To ensure that the upstream edges of the crest and the sides of the notch are sharp, they shall be machined
or filed, perpendicular to the upstream face of the weir plate, free of burrs or scratches and untouched by
abrasive cloth or paper. The downstream edges of the notch shall be chamfered if the weir plate is thicker
than the maximum allowable width of the notch surface. The surface of the chamfer shall make an angle of
not less than π/4 radians (45°) with the crest and side surfaces of the notch (see detail, Figure 2). The weir
plate in the vicinity of the notch preferably shall be made of corrosion-resistant metal; but if it is not, all
specified smooth surfaces and sharp edges shall be kept coated with a thin, protective film (for example, oil,
wax, silicone) applied with a soft cloth.
9.3 Specifications for installation
The specifications stated in 6.3 shall apply. In general, the weir shall be located in a straight, horizontal,
rectangular approach channel if possible. However, if the effective opening of the notch is so small in
comparison with the area of the upstream channel that the approach velocity is negligible, the shape of the
channel is not significant. In any case, the flow in the approach channel shall be uniform and steady, as
specified in 6.3.3.
If the width of the weir is equal to the width of the channel at the weir section (i.e. a full-width weir), the sides
of the channel upstream from the plane of the weir shall be vertical, plane, parallel and smooth (equivalent in
surface finish to that of rolled sheet-metal). The sides of the channel above the level of the crest of a full-width
weir shall extend at least 0,3 h downstream from the plane of the weir. Fully ventilated discharge shall be
max
ensured as specified in 6.3.4.
The approach channel floor shall be smooth, flat and horizontal when the height of the crest relative to the
floor (p) is small and/or h/p is large. For rectangular weirs, the floor should be smooth, flat and horizontal,
particularly when p is less than 0,1 m and/or h /p is greater than 1. Additional conditions are specified in
max
connection with the recommended discharge formulae.
9.4 Determination of gauge zero
The head-gauge datum or gauge zero shall be determined with great care, and it shall be checked when
necessary. A typical, acceptable method of determining the gauge zero for rectangular weirs is described as
follows.
a) Still water in the approach channel is drawn to a level below the weir crest.
b) A temporary hook gauge is mounted over the approach channel, a short distance upstream from the weir
crest.
c) A precise machinists' level is placed with its axis horizontal, with one end lying on the weir crest and the
other end on the point of the temporary hook gauge (the gauge having been adjusted to hold the level in
this position). The reading of the temporary gauge is recorded.
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d) The temporary hook gauge is lowered to the water surface in the approach channel and its reading is
recorded. The permanent gauge is adjusted to read the level in the gauge well, and this read
...

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