ISO 1088:2007

INTERNATIONAL ISO
STANDARD 1088
Third edition
2007-07-01
Hydrometry — Velocity-area methods
using current-meters — Collection and
processing of data for determination of
uncertainties in flow measurement
Hydrométrie — Méthodes d'exploration du champ des vitesses à l'aide
de moulinets — Recueil et traitement des données pour la
détermination des incertitudes de mesurage du débit

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

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Symbols and abbreviated terms . 2
4 Types of errors and procedure for estimating the uncertainties in flow measurement. 3
4.1 Principle. 3
4.2 Occurrence of error . 4
4.3 Sources of error . 5
4.4 Determination of the individual components of the uncertainty . 6
4.5 Total uncertainty in discharge. 7
5 Collection and processing of data for the investigation of component uncertainties – type
A evaluation of uncertainties. 8
5.1 Data on the local point velocity. 8
5.2 Data on the average velocity . 9
5.3 Data on the velocity-area method . 10
5.4 Integration method . 11
5.5 Calibration curves. 11
5.6 Distance measurements . 11
5.7 Depth measurements . 12
6 Data processing. 12
6.1 General. 12
6.2 Error-type i. 13
6.3 Error-type ii — Approximation of mean velocity in the vertical. 15
6.4 Error-type iii — Limited number of verticals.17
Annex A (informative) Characteristics of rivers from which data were collected . 21
Annex B (normative) Effect of increasing measuring time on uncertainty. 26
Annex C (normative) Local point velocity measurements - Report form. 27
Annex D (normative) Average velocity measurements — Report form . 31
Annex E (normative) Velocity-area method — Report form . 34
Annex F (informative) Examination of Error Types i, ii, and iii. 38
Annex G (informative) Uncertainties in velocity-area measurement components. 41
Annex H (informative) Calculation of the uncertainty in a current-meter gauging. 45
Bibliography . 48

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 1088 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 5, Instruments,
equipment and data management.
This third edition cancels and replaces the second edition (ISO 1088:1985), which has been revised to
incorporate ISO/TR 7178 (based on ISO/DATA No. 2) and edited in accordance with ISO/IEC Guide 98:1995,
Guide to the expression of uncertainty in measurement (GUM). This third edition of ISO 1088 also cancels
and replaces ISO/TR 7178, all provisions of which have been incorporated into this edition.

iv © ISO 2007 – All rights reserved

Introduction
All measurements of physical quantities are subject to uncertainties, which can be due to biases (systematic
errors) introduced in the manufacture, calibration, and maintenance of measurement instruments, or to
random scatter caused by a lack of sensitivity of the instruments, and to other sources of error.
During the preparation of the first edition of ISO 748, much discussion was given to the question of the
magnitude of errors in measurements, and it was concluded that recommendations could only be formulated
on the basis of an analysis of sufficient data. Moreover, it was recognized that to be able to analyze such data
statistically, it was essential that the data be collected and recorded on a standardized basis and in a
systematic manner, and this recognition led to the preparation of ISO 1088 and ISO/TR 7178.
On the basis of the procedures given in the first editions of ISO 748 (1968) and ISO 1088 (1973), data were
subsequently collected and processed from the following rivers (see Annex A for the characteristics of these
rivers) and ISO/TR 7178 was accordingly published:
a) Rivers Ganga, Jalangi, Yamuna, and Visvesvaraya Canal, in India;
b) River IJssel, in the Netherlands;
c) Rivers Derwent, Eden, Lambourne, Ouse, Tyne, and Usk in the United Kingdom;
d) Rivers Columbia and Mississippi, in the United States.
Further data obtained on the Rivers Ganga and Krishna, in India, and the Spey,Tay, Tweed, Tyne, Gala Water,
Yarrow Water, Ettrick Water, and the Clyde, in the United Kingdom, were received later, but could not be
included in the processing.
The procedures for estimating the component uncertainties and the uncertainty in discharge in this
International Standard conform to the ISO/IEC Guide 98, Guide to the expression of uncertainty in
measurement (GUM).
INTERNATIONAL STANDARD ISO 1088:2007(E)

Hydrometry — Velocity-area methods using current-meters —
Collection and processing of data for determination of
uncertainties in flow measurement
1 Scope
This International Standard provides a standard basis for the collection and processing of data for the
determination of the uncertainties in measurements of discharge in open channels by velocity-area methods
using current-meters.
To determine the discharge in open channels by the velocity-area method, components of the flow (velocity,
depth and breadth) need to be measured. The component measurements are combined to compute the total
discharge. The total uncertainty in the computed discharge is a combination of the uncertainties in the
measured components.
Clause 4 of this International Standard deals with the types of errors and uncertainties involved. Clauses 5
and 6 present a standard procedure to estimate the component uncertainties by the collection and processing
of the necessary data.
This International Standard is intended to be applied to velocity-area methods that involve measurement of
point velocities at a relatively small number of discrete depths and transverse positions in the flow cross-
section, as described in ISO 748. This International Standard is not intended to be applied to measurements
made by Acoustic Doppler Velocity Profilers (ADVP) or other instruments that produce essentially continuous
velocity profiles of the flow field.
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 748, Measurement of liquid flow in open channels — Velocity-area methods
ISO 4363, Measurement of liquid flow in open channels — Methods for measurement of characteristics of
suspended sediment
ISO 4364, Measurement of liquid flow in open channels — Bed material sampling

3 Symbols and abbreviated terms
a coefficient of linear regression, slope of trend line
b breadth (width) of segment i
i
d depth at vertical in segment i
i
L number of sets of measurements (error type ii)
J number of measurements per set (error types ii and iii)
k′ time displacement in autocorrelation function (of time interval, etc.)
k coverage factor for expanded uncertainty (taken as 2, corresponding to a level of confidence of
approximately 95 %)
m number of verticals or sections per measurement cross-section
n multiple of basic exposure-time for velocity measurement (error type i)
n number of depths in vertical i at which velocity measurements are made
i
Q discharge
Q discharge of measurement j in a set of measurements (error type iii)
j
S standard deviation of the relative mean velocities (error type ii)
rel
S mean standard deviation of all measurement sets together due to velocity fluctuations (error
F
type ii)
S standard deviation of sampling error in measurement set i (error type ii)
F,i
S standard deviation of the sampling error due to the computation rule (error type ii)
s
S stochastic sampling error of mean velocity in vertical i (error type ii)
i
S unobservable random sampling error of mean velocity in vertical i (error type ii)
vi,
S (m) standard deviation of relative error when m verticals are applied (error type iii)
s,hd
t instant of time of observation i (error type i)
i
t initial measuring time (basic time interval)
t mean of observation times t in a linear trend segment (error type i)
i
u standard relative (percentage) uncertainty in uncertainty component i
i
u standard relative (percentage) combined uncertainty of measurement
U expanded relative (percentage) uncertainty with coverage factor k
u standard relative (percentage) uncertainty due to responsiveness of current-meter
c
u standard relative (percentage) uncertainty in width measurement
b
u standard relative (perce
...