ISO 18481:2017

INTERNATIONAL ISO
STANDARD 18481
First edition
2017-12
Hydrometry — Liquid flow
measurement using end depth method
in channels with a free overfall
Hydrométrie — Mesure du débit liquide dans les chenaux à déversoir
sans pelle
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .v
1 Scope . 1
2  Normative references . 1
3  Terms and definitions . 1
4  Symbols and abbreviated terms . 2
5  Principle . 2
6  Installation . 2
6.1 General . 2
6.2 Selection of site . 2
7  Measurement of end depth . 3
7.1 General . 3
7.2 Head measuring devices . 3
7.3 Gauge datum . 3
8 Maintenance . 3
8.1 General . 3
8.2 Types. 4
8.3 Specifications for the drop structure . 5
8.4 Specifications for installation . 6
8.5 Determination of gauge zero . 6
8.6 Discharge relationship . 6
8.7 Coefficient of discharge . 6
8.7.1 Confined nappe . 6
8.7.2 Unconfined nappe . 6
8.8 Practical limitations . 6
8.9 Uncertainty of measurement . 7
9  Triangular channel drop structure . 7
9.1 Specifications for the drop structure . 7
9.2 Specifications for installation . 7
9.3 Specifications for head measurement . 7
9.3.1 General. 7
9.3.2 Determination of channel angle . 7
9.3.3 Determination of gauge zero . 8
9.4 Discharge formula — Unconfined . 8
9.5 Practical limitations . 8
9.6 Uncertainty of measurement . 8
10  Trapezoidal channel drop structure . 9
10.1 Specifications for the drop structure . 9
10.2 Specifications for head measurement . 9
10.2.1 General. 9
10.2.2 Determination of gauge zero . 9
10.3 Discharge formula — Unconfined . 9
10.4 Practical limitations .10
10.5 Uncertainty of measurement .10
11  Circular channel drop structure .11
11.1 Specifications for the drop structure .11
11.2 Specifications for head measurement .11
11.2.1 General.11
11.2.2 Determination of gauge zero .11
11.3 Discharge formula — Unconfined .11
11.4 Practical limitations .13
11.5 Uncertainty of measurement .13
12  Parabolic channel drop structure .14
12.1 Specifications for the drop structure .14
12.2 Specifications for head measurement .14
12.2.1 General.14
12.2.2 Geometry .14
12.2.3 Determination of gauge zero .14
12.3 Discharge formula — Unconfined .15
12.4 Practical limitations .15
13  Uncertainties of flow measurement .15
13.1 General .15
13.2 Sources of error .15
13.3 Kinds of error .16
13.4 Uncertainties in coefficient values .16
13.5 Uncertainties in measurements made by the user .17
13.6 Combination of uncertainties to give total uncertainty on discharge .17
13.7 Example .17
Bibliography .20
iv © ISO 2017 – All rights reserved

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 st
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