ISO 4373:2022
(Main)Hydrometry — Water level measuring devices
Hydrometry — Water level measuring devices
This document specifies the functional requirements of instrumentation for measuring the level of water surface (stage), primarily for the purpose of determining flow rates. This document is supplemented by Annex A, which provides guidance on the types of automatic water level measurement devices currently available and the measurement uncertainty associated with them. The manually operated measuring devices are described in Annex B. This document is applicable to both contact and non-contact methods of measurement. The non-contact methods are not in direct material contact with the water surface but measure the height of the water level with ultrasonic or electromagnetic waves.
Hydrométrie — Appareils de mesure du niveau de l'eau
Le présent document spécifie les caractéristiques de fonctionnement des instruments employés pour mesurer le niveau de la surface de l'eau, essentiellement afin de déterminer des débits. Le présent document est complété par l'Annexe A qui fournit des indications sur les types d'appareils de mesure du niveau de l'eau actuellement disponibles et sur l'incertitude de mesure qui leur est associée. Les appareils de mesure à fonctionnement manuel sont décrits dans l'Annexe B. Le présent document s'applique à la fois aux méthodes de mesure avec et sans contact. Les méthodes sans contact concernent des appareils qui ne sont pas en contact direct avec la surface de l'eau, mais mesurent la hauteur du niveau d'eau avec des ondes ultrasoniques ou électromagnétiques.
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INTERNATIONAL ISO
STANDARD 4373
Fourth edition
2022-03
Hydrometry — Water level measuring
devices
Hydrométrie — Appareils de mesure du niveau de l'eau
Reference number
ISO 4373:2022(E)
© ISO 2022
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ISO 4373:2022(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
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Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
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ISO 4373:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Instrument specification . 1
4.1 Performance parameters . 1
4.2 Performance classification . 1
4.3 Maximum rate of change . 3
4.4 Environment . 3
4.4.1 General . 3
4.4.2 Temperature . 3
4.4.3 Relative humidity . 3
4.5 Timing . 3
4.5.1 General . 3
4.5.2 Digital . 4
4.5.3 Analogue . 4
5 Recording . 4
5.1 General . 4
5.2 Chart recorders . 4
5.3 Data loggers . . 4
6 Enclosure . 4
7 Installation .5
8 Maintenance . 5
9 Estimation of measurement uncertainty . 5
9.1 General . 5
9.2 Type A uncertainty estimation . 6
9.3 Type B uncertainty estimation . 6
9.4 Uncertainty in case of low water level conditions . 7
9.5 Level measurement datum . 7
9.6 Combining primary measurement uncertainties . 7
Annex A (informative) Types of water level measuring devices . 8
Annex B (informative) Manually operated measuring devices .22
Annex C (informative) Recording devices .25
Bibliography .27
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ISO 4373:2022(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 113 Hydrometry, Subcommittee
SC 5, Instruments, equipment and data management, in collaboration with the European Committee
for Standardization (CEN) Technical Committee CEN/TC 318, Hydrometry, in accordance with the
Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This fourth edition cancels and replaces the third edition (ISO 4373:2008), which has been technically
revised. The main changes are as follows:
— improvements in water level measuring devices have been incorporated;
— the use of mercury has been removed;
— the old Annex A has been divided into three new separate Annexes A, B and C;
— in the new Annex A, the electronic techniques that are currently more commonly used have been
brought to the front in order to give them a greater emphasis.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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ISO 4373:2022(E)
Introduction
Measuring the level of water surface is very important in hydrometry for the purpose of, among
other things, determining flow rates. Information about water levels is also used in operational water
management, including the design of dikes and storm surge warning services. Water level information
also provides decision-making guidance to shipping activities.
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INTERNATIONAL STANDARD ISO 4373:2022(E)
Hydrometry — Water level measuring devices
1 Scope
This document specifies the functional requirements of instrumentation for measuring the level of
water surface (stage), primarily for the purpose of determining flow rates.
This document is supplemented by Annex A, which provides guidance on the types of automatic water
level measurement devices currently available and the measurement uncertainty associated with them.
The manually operated measuring devices are described in Annex B.
This document is applicable to both contact and non-contact methods of measurement. The non-contact
methods are not in direct material contact with the water surface but measure the height of the water
level with ultrasonic or electromagnetic waves.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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
IEC 60079-10, Electrical apparatus for explosive gas atmospheres — Part 10: Classification of hazardous
areas
IEC 60529, Degrees of protection provided by enclosures (IP Code)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Instrument specification
4.1 Performance parameters
The performance parameters of a water level measuring device are uncertainty, measurement range,
temperature range and relative humidity range. Thus, the overall performance of the equipment can be
summarized by a few characterizing parameters.
4.2 Performance classification
Water level measuring devices shall be classified in accordance with the performance classes given
in Table 1 that account for the resolution to be achieved and the limits of uncertainty required
over specified measurement ranges. Measurement range is to be understood as the difference between
the highest and the lowest water level that can be measured. When measuring short ranges with class 1
and 2 devices, the uncertainty is a few millimetres, and this is difficult to achieve.
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ISO 4373:2022(E)
It should be made clear whether these levels of attainment can only be achieved using special works,
e.g. installation within a stilling well, also referred to as a “gauge well”.
Table 1 — Performance classes of water level measuring devices
Class Resolution Range Nominal uncertainty
Performance class 1 ≤ 1 mm ≤ 1,0 m < ±0,1 % of range
≤ 2 mm ≤ 5,0 m
≤ 10 mm ≤ 20 m
Performance class 2 ≤ 2 mm ≤ 1,0 m < ±0,3 % of range
≤ 5 mm ≤ 5,0 m
≤ 20 mm ≤ 20 m
Performance class 3 ≤ 10 mm ≤ 1,0 m < ±1 % of range
≤ 50 mm ≤ 5,0 m
≤ 200 mm ≤ 20 m
The manufacturer shall state the physical principle of the measuring device to allow the user to judge
the device’s suitability for the proposed environment. Table 2 lists the various physical principles of
operational water level measuring devices being used in the field against their characteristics. These
different techniques are described in more detail in Annex A.
Table 2 — Characteristics of operational water level measuring devices
Device Type Suitable for continuous Typical Typical
measurement measurement uncertainty
range
Mechanical Float and counterweight in a Yes 20 m 5 mm to 10 mm
devices stilling well
Wire weight gauge No 20 m 5 mm to 10 mm
Peak level No 15 m 10 mm to
20 mm
Staff and ramp gauge Yes 10 m 10 mm to
20 mm
Electrical Bubbler Yes 30 m 10 mm to
devices 20 mm
Pressure transducer Yes 20 m 10 mm to
20 mm
Capacitance Yes 15 m 10 mm to
20 mm
Resistance Yes 15 m 10 mm to
20 mm
Non-contact Radar/laser Yes 10 m to 50 m 5 mm to 10 mm
devices
Ultrasonic Yes 3 m to 30 m 10 mm to
(through air) 20 mm
Ultrasonic Yes 3 m to 30 m 10 mm to
(through water) 20 mm
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ISO 4373:2022(E)
4.3 Maximum rate of change
As water levels can rise and fall rapidly in some applications, to provide guidance on suitability,
for mechanical devices the manufacturer shall state on the equipment specification sheet and
in the instruction manual:
a) the maximum rate of change which the instrument can follow without damage;
b) the maximum rate of change which the instrument can tolerate without suffering a change
in calibration;
c) the response time of the instrument.
The response time is the time interval between the instant when the level sensor is subjected to an
abrupt change in liquid level and the instant when the readings cross the limits of (and remain inside)
a band defined by the 90 % and the 110 % of the difference between the initial and final value of the
abrupt change. The response time should be short enough for the instrument to follow even the fastest
relevant changes in water level, e.g. tides and flood waves. The response time should not be too short.
Therefore, in many electronic devices, it is possible to enlarge the response time through the setting
of certain parameters within the instrument. This can be useful, for example, to damp out the rapid
excursions caused by short waves. Such rapid disturbances are due to local hydraulic phenomena and
are thus not representative for the water level over a large section of the water course. The locally
excited disturbances are thus to be discarded as much as possible.
4.4 Environment
4.4.1 General
Water level measuring devices shall operate within the ranges of temperature in 4.4.2 and the ranges
of relative humidity in 4.4.3.
4.4.2 Temperature
Water level measuring devices shall operate within the following ambient air temperature classes:
Temperature class 1: –30 °C to +55 °C
Temperature class 2: –10 °C to +50 °C
Temperature class 3: 0 °C to + 50 °C
4.4.3 Relative humidity
Water level measuring devices shall operate within the following relative humidity classes:
Relative humidity class 1: 5 % to 95 % including condensation
Relative humidity class 2: 10 % to 90 % including condensation
Relative humidity class 3: 20 % to 80 % including condensation
4.5 Timing
4.5.1 General
Where timing, either analogue or digital, is part of the instrument specification, the timing method
used shall be clearly stated on the instrument and in the instruction manual.
NOTE It is recognized that digital timing is potentially more accurate than analogue timing.
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ISO 4373:2022(E)
Moreover, when several raw data samples are assembled in order to calculate a time averaged
measurement value, it should be clearly stated to which moment in time the final result applies. It is
preferred to have this time label be at exactly the middle of the averaging time window, because this
moment is the most representative. However, many commercially available loggers add time and data
stamps at the beginning or at the end of the averaging time window.
4.5.2 Digital
The uncertainty of digital timing devices used in water level measuring devices shall be within ±60 s
at the end of a period of 30 days, within the range of environmental conditions defined in 4.4.
4.5.3 Analogue
The uncertainty of analogue timing devices used in water level measuring devices shall be within ±5 min
at the end of a period of 30 days, within the range of environmental conditions defined in 4.4.
5 Recording
5.1 General
Recording devices serve the purpose of storing the water level data for immediate or later use. Such
devices can be divided into analogue chart recorders and digital data loggers. For more information
about the strengths and weaknesses of these recording devices, see Annex C.
5.2 Chart recorders
Where a chart recorder is to be used as the primary source of data, the resolution and uncertainty
parameters shall take account of changes in the dimensions of the recording medium due to atmospheric
variables.
NOTE Chart recorders have been superseded to a large extent by data logging services. However, they are
still used as back-up units or to provide rapid visual assessment of flow changes on site.
5.3 Data loggers
A data logger shall be able to store at least the measured value and a timestamp. The data logger shall
be able to store at least the equivalent of four digits per measurement and at least the equivalent of nine
digits for the timestamp. In practice, however, the minimum requirement of four digits per measurement
does not always suffice. Therefore, the data logger can store readings which are sufficiently resolute
to record the full range of measured water level values including all increments possible at the level
sensor’s resolution. This means that there shall be sufficient decimal places, or equivalent, to record
all possible step changes in measured values across the sensor’s range. Consequently, for some high-
resolution water level measurements, there is a need for more than four digits per measurement.
The nine digits for the timestamp are based on the format YYDDDHHMM (year, day, hour, minute).
However, a more time resolute and practical date time stamp such as a DDMMYYYYHHMMSS (day,
month, year, hour, minute, second), or similar, format is preferred. Furthermore, it is advised to properly
mention the local time zone and its reference to coordinated universal time (UTC) as well as any applied
daylight-saving time shifts.
Where a data logger includes the interface electronics, the resolution and uncertainty shall relate to the
stored value.
6 Enclosure
The performance of the enclosure shall be stated in terms of the IP classification system in accordance
with IEC 60529. It shall be stated whether or not any parts potentially in contact with water are suitable
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ISO 4373:2022(E)
for contact with water. It shall be stated whether or not the equipment can be used in a potentially
explosive environment in accordance with IEC 60079-10.
7 Installation
The manufacturer shall provide clear instructions for the installation of water level measuring devices.
The water level measuring device shall have a clearly visible reference mark, which can be used for
tying the device to the local gauge datum.
If a float measuring system is equipped with a stilling well, the diameter of the horizontal inlet pipe
or orifice to the stilling well should be about 10 times smaller than the diameter of the stilling well itself
to sufficiently reduce any disturbances originating from waves on the water surface.
Furthermore, the vertical cylindrical pipes, in which the float can move up and down, should be at least
10 cm wider than the float diameter and shall be erected exactly along the local vertical to ensure free
movement of the float over the entire range.
Ensure that a non-contact sensor is set up with its beam perpendicular to the water surface. Non-
contact sensors shall be installed on rigid and well secured brackets to prevent movement of the sensor
that can introduce errors in the measurement. There should be a clear path from the sensor face to
the water surface, free from obstacles that can give false reflections. Many non-contact instruments
include signal diagnostics that help the user when commissioning the instrument.
Careful selection of the measurement technique is required when foam, bubbles or other disturbances
are likely to be present on the water surface (see Annex A).
8 Maintenance
Clear instructions shall be given regarding the proper maintenance of the measuring device. This also
includes regular inspections and possibly regular calibrations. It is important that measurements from
installed devices are checked periodically and, when necessary, the instrument should be recalibrated.
Reasons why recalibration is sometimes necessary vary with instrument type but can include: change
in the datum, cable stretch, electronics drift, etc.
Maintenance needs to include the periodic check of the gauge reference mark(s) to the gauge datum.
The frequency of the reference mark/datum checks depends on the stability of the gauge structure.
The level of maintenance required will vary depending on instrument type and site conditions. Annex A
gives basic maintenance considerations against each instrument type.
NOTE The above-mentioned maintenance instructions do not only apply to the measuring device, but also
to any ancillary equipment (e.g. inlet pipes and stilling wells) that can affect the proper operation of a water level
measuring station.
9 Estimation of measurement uncertainty
9.1 General
The uncertainty of a value derived from primary measurements may be due to:
a) unsteadiness of the measured value (noisy fluctuations due to, for example, waves on the water
surface or due to noise in electronic systems);
b) resolution of the measurement process (resolution of the sensor or of the human eye);
c) measurement errors due to changes in temperature, sediment content, salinity of the water or
Bernoulli effects caused by the water velocity;
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ISO 4373:2022(E)
d) gradual drift from the original calibration due to sensitivity to the varying environmental
conditions, e.g. temperature, relative humidity or atmospheric pressure;
e) gradual drift from the original calibration due to sensitivity to the varying electrical conditions,
e.g. supply voltage or supply frequency;
f) gradual shift in vertical position of the gauge structure and consequent drift from the last datum
check (this is elaborated upon in 9.5).
Under the GUM uncertainty framework (GUM stands for Guide to the expression of uncertainty in
[1]
measurement ), measurement uncertainty is expressed in terms of “standard uncertainty” and
“expanded uncertainty”. Standard uncertainty is denoted by u. Expanded uncertainty is denoted by
U and U = ku, where k is the coverage factor depending on the desired level of confidence. The GUM
describes two methods for estimating uncertainties that are classified as Type A and Type B. These two
estimation methods are used for relating the dispersion of values to the probability of “closeness” to the
mean value.
9.2 Type A uncertainty estimation
A Type A uncertainty is estimated as the standard deviation of a large number of measurements
under a steady-state condition. Note that the distribution of these results need not be Gaussian.
Type A estimations can be readily computed from continuous measurements when the dispersion
is not masked by hysteresis of the measurement process. Of course, the dispersion must exceed by a
significant margin the resolution of the measurement process.
Another approach for a Type A estimation is to compare the readings from two water level measuring
stations in the same water course within a very short distance of each other. When carefully examining
the difference between the two neighbouring stations, a randomly fluctuating signal can be discerned
that represents the combined effect of the two individual uncertainties at both water level measuring
stations. When the two stations are of identical construction and their measurements are uncorrelated,
the combined variance is twice the variance of each individual station. Thus, the standard deviation of
each station can be calculated by dividing the standard deviation in the random part of the water level
difference between both stations by the square root of two.
Yet another Type A estimation is the comparison of instrument water level measures and manual
observations using reference gauges such as staffs, ramps and wire-weight gauges.
9.3 Type B uncertainty estimation
A Type B estimation is assigned to a measurement process for which sufficiently large numbers
of measurements are not available or to a measurement with defined limits of resolution. To define
a Type B uncertainty, the upper and lower limits of the dispersion or the upper and lower limits of
resolution are used to define the limits of a probability diagram whose shape is selected to represent
the dispersion, i.e. uniform dispersions would have a rectangular distribution, dispersions with most
measurements congregated about the mean value would have a triangular distribution. Allocation of
probability distributions is described in Annex A.
The relationship between the uncertainty of primary measurements and the value of the uncertainty
of the result is derived from the relationship between the value of this result and its primary
measurements. For instance, the primary measurement for a non-contact sensor can be the measured
travelling time elapsed between transmission and reception of an echo from the water surface. Any
uncertainty in measuring this travelling time will lead to a correlated uncertainty in the resulting
water level.
In the case of level, this relationship to primary measurements is generally linear. Sensitivities that
describe the dependencies of the uncertainty in the result to the uncertainty in the individual primary
measurements are the partial derivatives of the value of the result with respect to each primary
measurement.
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ISO 4373:2022(E)
9.4 Uncertainty in case of low water level conditions
It is important to remember that in the measurement of water level, uncertainty expressed
as a percentage of water level range gives rise to worst case relative uncertainty in the determination
of low values of water level. For instance, say the uncertainty is ±1 % of range and the local range
in water level is two metres. Then there is an absolute uncertainty in all water level measurements
of ±2 cm. This leads to a relative uncertainty expressed as a percentage of the wat
...
NORME ISO
INTERNATIONALE 4373
Quatrième édition
2022-03
Hydrométrie — Appareils de mesure
du niveau de l'eau
Hydrometry — Water level measuring devices
Numéro de référence
ISO 4373:2022(F)
© ISO 2022
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ISO 4373:2022(F)
DOCUMENT PROTÉGÉ PAR COPYRIGHT
© ISO 2022
Tous droits réservés. Sauf prescription différente ou nécessité dans le contexte de sa mise en œuvre, aucune partie de cette
publication ne peut être reproduite ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique,
y compris la photocopie, ou la diffusion sur l’internet ou sur un intranet, sans autorisation écrite préalable. Une autorisation peut
être demandée à l’ISO à l’adresse ci-après ou au comité membre de l’ISO dans le pays du demandeur.
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Publié en Suisse
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ISO 4373:2022(F)
Sommaire Page
Avant-propos .iv
Introduction .v
1 Domaine d'application .1
2 Références normatives .1
3 Termes et définitions . 1
4 Spécification des instruments . 1
4.1 Paramètres de performances . 1
4.2 Classification des performances . 1
4.3 Vitesse maximale de variation . 2
4.4 Environnement . 3
4.4.1 Généralités . 3
4.4.2 Température . 3
4.4.3 Humidité relative . 3
4.5 Horodatage. 3
4.5.1 Généralités . 3
4.5.2 Horodatage numérique . 4
4.5.3 Horodatage analogique . 4
5 Enregistrement .4
5.1 Généralités . 4
5.2 Enregistreurs graphiques . . 4
5.3 Enregistreurs de données numériques . 4
6 Enveloppe . 5
7 Installation .5
8 Entretien . 5
9 Estimation de l'incertitude de mesure . 6
9.1 Généralités . 6
9.2 Estimation de l'incertitude de Type A . 6
9.3 Estimation de l'incertitude de Type B . 6
9.4 Incertitude dans le cas de conditions de niveau d'eau bas . 7
9.5 Système de référence des mesurages de niveau . 7
9.6 Combinaison des incertitudes de mesures primaires . 8
Annexe A (informative) Types d'appareils de mesure du niveau de l'eau .9
Annexe B (informative) Appareils de mesure à fonctionnement manuel .24
Annexe C (informative) Appareils enregistreurs .27
Bibliographie .29
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ISO 4373:2022(F)
Avant-propos
L'ISO (Organisation internationale de normalisation) est une fédération mondiale d'organismes
nationaux de normalisation (comités membres de l'ISO). L'élaboration des Normes internationales est
en général confiée aux comités techniques de l'ISO. Chaque comité membre intéressé par une étude
a le droit de faire partie du comité technique créé à cet effet. Les organisations internationales,
gouvernementales et non gouvernementales, en liaison avec l'ISO participent également aux travaux.
L'ISO collabore étroitement avec la Commission électrotechnique internationale (IEC) en ce qui
concerne la normalisation électrotechnique.
Les procédures utilisées pour élaborer le présent document et celles destinées à sa mise à jour sont
décrites dans les Directives ISO/IEC, Partie 1. Il convient, en particulier, de prendre note des différents
critères d'approbation requis pour les différents types de documents ISO. Le présent document a
été rédigé conformément aux règles de rédaction données dans les Directives ISO/IEC, Partie 2 (voir
www.iso.org/directives).
L'attention est attirée sur le fait que certains des éléments du présent document peuvent faire l'objet de
droits de propriété intellectuelle ou de droits analogues. L'ISO ne saurait être tenue pour responsable
de ne pas avoir identifié de tels droits de propriété et averti de leur existence. Les détails concernant
les références aux droits de propriété intellectuelle ou autres droits analogues identifiés lors de
l'élaboration du document sont indiqués dans l'Introduction et/ou dans la liste des déclarations de
brevets reçues par l'ISO (voir www.iso.org/brevets).
Les appellations commerciales éventuellement mentionnées dans le présent document sont données
pour information, par souci de commodité, à l’intention des utilisateurs et ne sauraient constituer un
engagement.
Pour une explication de la nature volontaire des normes, la signification des termes et expressions
spécifiques de l'ISO liés à l'évaluation de la conformité, ou pour toute information au sujet de l'adhésion
de l'ISO aux principes de l’Organisation mondiale du commerce (OMC) concernant les obstacles
techniques au commerce (OTC), voir www.iso.org/avant-propos.
Le présent document a été élaboré par le comité technique ISO/TC 113, Hydrométrie, sous-comité SC 5,
Instruments, équipement et gestion des données, en collaboration avec le comité technique CEN/TC 318,
Hydrométrie, du Comité européen de normalisation (CEN), conformément à l'Accord de coopération
technique entre l'ISO et le CEN (Accord de Vienne).
Cette quatrième édition annule et remplace la troisième édition (ISO 4373:2008), qui a fait l'objet d'une
révision technique. Les principales modifications sont les suivantes:
— des améliorations liées aux appareils de mesure du niveau de l'eau ont été introduites;
— l'usage du mercure a été éliminé;
— l'ancienne Annexe A a été scindée en trois Annexes A, B et C distinctes;
— dans la nouvelle Annexe A, les techniques électroniques les plus couramment utilisées de nos jours
ont été ramenées au premier plan pour être davantage mises en exergue.
Il convient que l’utilisateur adresse tout retour d’information ou toute question concernant le présent
document à l’organisme national de normalisation de son pays. Une liste exhaustive desdits organismes
se trouve à l’adresse www.iso.org/fr/members.html.
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ISO 4373:2022(F)
Introduction
Dans le domaine de l'hydrométrie, il est très important de mesurer le niveau de la surface de l'eau afin,
notamment, de déterminer les débits. Les informations relatives aux niveaux de l'eau sont également
utilisées pour la gestion opérationnelle de l'eau, notamment la conception de digues et les services
d'alerte de crue. Les informations relatives aux niveaux de l'eau fournissent également des conseils de
prise de décision pour l'aide à la navigation.
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NORME INTERNATIONALE ISO 4373:2022(F)
Hydrométrie — Appareils de mesure du niveau de l'eau
1 Domaine d'application
Le présent document spécifie les caractéristiques de fonctionnement des instruments employés pour
mesurer le niveau de la surface de l'eau, essentiellement afin de déterminer des débits.
Le présent document est complété par l'Annexe A qui fournit des indications sur les types d'appareils de
mesure du niveau de l'eau actuellement disponibles et sur l'incertitude de mesure qui leur est associée.
Les appareils de mesure à fonctionnement manuel sont décrits dans l'Annexe B.
Le présent document s'applique à la fois aux méthodes de mesure avec et sans contact. Les méthodes
sans contact concernent des appareils qui ne sont pas en contact direct avec la surface de l'eau, mais
mesurent la hauteur du niveau d'eau avec des ondes ultrasoniques ou électromagnétiques.
2 Références normatives
Les documents suivants sont cités dans le texte de sorte qu’ils constituent, pour tout ou partie de leur
contenu, des exigences du présent document. Pour les références datées, seule l’édition citée s’applique.
Pour les références non datées, la dernière édition du document de référence s'applique (y compris les
éventuels amendements).
ISO 772, Hydrométrie — Vocabulaire et symboles
IEC 60079-10, Matériel électrique pour atmosphères explosives gazeuses — Partie 10: Classement des
emplacements dangereux
IEC 60529, Degrés de protection procurés par les enveloppes (Code IP)
3 Termes et définitions
Pour les besoins du présent document, les termes et les définitions de l’ISO 772 s’appliquent.
L’ISO et l’IEC tiennent à jour des bases de données terminologiques destinées à être utilisées en
normalisation, consultables aux adresses suivantes:
— ISO Online browsing platform: disponible à l’adresse https:// www .iso .org/ obp
— IEC Electropedia: disponible à l’adresse https:// www .electropedia .org/
4 Spécification des instruments
4.1 Paramètres de performances
Les paramètres de performance d'un appareil de mesure du niveau de l'eau sont l'incertitude, la plage
de mesure, la plage de température et la plage d'humidité relative. Les performances globales de
l'équipement peuvent donc être résumées par quelques paramètres caractéristiques.
4.2 Classification des performances
Les appareils de mesure du niveau de l'eau doivent être classés selon les classes de performance
indiquées dans le Tableau 1, qui tiennent compte de la résolution devant être atteinte et des limites
d'incertitude exigées sur des plages de mesure spécifiées. La plage de mesure doit s'entendre au sens
de la différence entre le niveau d'eau le plus haut et le niveau d'eau le plus bas pouvant être mesuré.
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ISO 4373:2022(F)
Pour la mesure de courtes distances à l'aide d'appareils de classes 1 et 2, l'incertitude est de quelques
millimètres, et cela est difficile à atteindre.
Il convient de déterminer si ces niveaux de performance ne peuvent être atteints qu'à l'aide d'installations
spéciales, par exemple dans des puits de mesurage (également appelés «puits de limnigraphe»).
Tableau 1 — Classes de performance des appareils de mesure du niveau de l'eau
Classe Résolution Plage Incertitude nominale
Classe de performance 1 ≤ 1 mm ≤ 1,0 m < ±0,1 % de la plage
≤ 2 mm ≤ 5,0 m
≤ 10 mm ≤ 20 m
Classe de performance 2 ≤ 2 mm ≤ 1,0 m < ±0,3 % de la plage
≤ 5 mm ≤ 5,0 m
≤ 20 mm ≤ 20 m
Classe de performance 3 ≤ 10 mm ≤ 1,0 m < ±1 % de la plage
≤ 50 mm ≤ 5,0 m
≤ 200 mm ≤ 20 m
Le fabricant doit indiquer le principe physique de l'appareil de mesure pour permettre à l'utilisateur
d'évaluer l'adéquation de l'appareil à l'environnement proposé. Le Tableau 2 répertorie les différents
principes des appareils de mesure du niveau de l'eau en exploitation utilisés sur le terrain, ainsi que
leurs caractéristiques. Ces différentes techniques sont décrites plus en détail à l'Annexe A.
Tableau 2 — Caractéristiques des appareils de mesure du niveau de l'eau en exploitation
Appareil Type Adapté à un mesurage Plage de Incertitude
en continu mesure type typique
Appareils Flotteur et contrepoids dans un Oui 20 m 5 mm à 10 mm
mécaniques puits de mesurage
Sonde à câble lesté Non 20 m 5 mm à 10 mm
À maximum Non 15 m 10 mm à 20 mm
Échelle limnimétrique verticale Oui 10 m 10 mm à 20 mm
et inclinée
Appareils Bulleur Oui 30 m 10 mm à 20 mm
électriques
Capteur de pression Oui 20 m 10 mm à 20 mm
Mesure de la capacité Oui 15 m 10 mm à 20 mm
Mesure de la résistance Oui 15 m 10 mm à 20 mm
Appareils Radar/Laser Oui 10 m à 50 m 5 mm à 10 mm
sans contact
Ultrasons Oui 3 m à 30 m 10 mm à 20 mm
(dans l'air)
Ultrasons Oui 3 m à 30 m 10 mm à 20 mm
(immergés)
4.3 Vitesse maximale de variation
Étant donné que les niveaux d'eau peuvent monter et baisser rapidement dans certaines applications,
le fabricant doit donner les informations suivantes dans la notice technique et le mode d'emploi de
l'équipement, dans le cas d'appareils mécaniques, afin de fournir des indications sur leur adéquation:
a) vitesse maximale de variation que l'instrument peut suivre sans dommage;
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b) vitesse maximale de variation que l'instrument peut tolérer sans subir de modification de
l'étalonnage;
c) temps de réponse de l'instrument.
Le temps de réponse est l'intervalle de temps entre le moment où le capteur de niveau est exposé à
un changement brusque du niveau de liquide et le moment où les valeurs lues dépassent les limites
d'une bande comprise entre 90 % et 110 % de la différence entre la valeur initiale et la valeur finale
du changement brusque et restent à l'intérieur de cette bande. Il convient que le temps de réponse soit
suffisamment court pour permettre à l'instrument de suivre les variations du niveau d'eau même les
plus rapides, par exemple les marées et les ondes de crue. Il convient, aussi, que le temps de réponse ne
soit pas trop court. Dans de nombreux appareils électroniques, il est donc possible d'élargir le temps de
réponse en définissant certains paramètres dans l'instrument. Cela peut être utile, par exemple, pour
atténuer les variations rapides causées par des ondes courtes. Ces perturbations rapides sont dues à
des phénomènes hydrauliques locaux et ne sont, par conséquent, pas représentatives du niveau de l'eau
sur une large section du cours d'eau. Les perturbations causées par une variation localisée doivent donc
être ignorées autant que possible.
4.4 Environnement
4.4.1 Généralités
Les appareils de mesure du niveau de l'eau doivent fonctionner dans les plages de température indiquées
en 4.4.2 et les plages d'humidité relative indiquées en 4.4.3.
4.4.2 Température
Les appareils de mesure du niveau de l'eau doivent fonctionner dans les classes de température d'air
ambiant suivantes:
Classe de température 1: –30 °C à +55 °C
Classe de température 2: –10 °C à +50 °C
Classe de température 3: 0 °C à +50 °C
4.4.3 Humidité relative
Les appareils de mesure du niveau de l'eau doivent fonctionner dans les classes d'humidité relative
suivantes:
Classe d'humidité relative 1: 5 % à 95 %, condensation comprise
Classe d'humidité relative 2: 10 % à 90 %, condensation comprise
Classe d'humidité relative 3: 20 % à 80 %, condensation comprise
4.5 Horodatage
4.5.1 Généralités
Lorsqu'un horodatage, analogique ou numérique, fait partie de la spécification de l'instrument, la
méthode d'horodatage utilisée doit être clairement indiquée sur l'instrument et dans le mode d'emploi.
NOTE Il est reconnu que l'horodatage numérique est potentiellement plus précis que l'horodatage
analogique.
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De plus, lorsque plusieurs échantillons de données brutes sont combinés afin de calculer une valeur
de mesure moyennée dans le temps, il convient d'indiquer clairement à quel moment s'applique le
résultat final. Il est préférable que cette étiquette temporelle se situe exactement au milieu de la fenêtre
temporelle de moyennage, qui représente le moment le plus représentatif. Cependant, de nombreux
enregistreurs disponibles dans le commerce ajoutent les étiquettes temporelles au début ou à la fin de
la fenêtre temporelle de moyennage.
4.5.2 Horodatage numérique
L'incertitude des dispositifs d'horodatage numérique utilisés dans les appareils de mesure du niveau de
l'eau doit être de ±60 s à la fin d'une période de 30 jours dans la gamme des conditions environnementales
définies en 4.4.
4.5.3 Horodatage analogique
L'incertitude des dispositifs d'horodatage analogique utilisés dans les appareils de mesure du
niveau de l'eau doit être de ±5 min à la fin d'une période de 30 jours dans la gamme des conditions
environnementales définies en 4.4.
5 Enregistrement
5.1 Généralités
Les appareils enregistreurs permettent d'enregistrer les données relatives au niveau de l'eau en vue
d'une utilisation immédiate ou ultérieure. Lesdits appareils peuvent être répartis entre enregistreurs
graphiques analogiques et enregistreurs numériques de données. Pour plus d'informations sur les
points forts et les points faibles de ces appareils enregistreurs, consultez l'Annexe C.
5.2 Enregistreurs graphiques
Lorsqu'un enregistreur graphique est utilisé comme principale source de données, les paramètres
de résolution et d'incertitude doivent tenir compte des variations dimensionnelles du support
d'enregistrement liées aux variables atmosphériques.
NOTE Les enregistreurs graphiques ont, dans une large mesure, été remplacés par des enregistreurs
numériques de données. Néanmoins, ils sont encore utilisés comme solutions de secours ou pour obtenir une
évaluation visuelle rapide des variations de débit sur le terrain.
5.3 Enregistreurs de données numériques
Un enregistreur numérique de données doit être capable de stocker au moins la valeur mesurée et un
horodatage. L'enregistreur numérique de données doit être capable de stocker au moins l'équivalent
de quatre chiffres par mesure et au moins l'équivalent de neuf chiffres pour l'horodatage. Cependant,
en pratique, l'exigence minimale de quatre chiffres par mesure n'est pas toujours suffisante. Par
conséquent, l'enregistreur numérique de données peut enregistrer des relevés avec une résolution
suffisante pour enregistrer la plage complète des valeurs de niveau de l'eau mesurées, y compris
l'ensemble des incréments possibles dans la résolution du capteur de niveau. Cela signifie qu'il doit y
avoir suffisamment de décimales, ou leur équivalent, pour permettre d'enregistrer toutes les variations
progressives possibles dans les valeurs mesurées sur toute la plage du capteur. Par conséquent,
certaines mesures du niveau de l'eau haute résolution nécessitent plus de quatre chiffres par mesure.
Les neuf chiffres utilisés pour l'horodatage reposent sur le format AAJJJHHMM (année, jour, heure,
minute). Cependant, on privilégie un horodatage plus pratique, avec une résolution temporelle
supérieure, tel que JJMMAAAAHHMMSS (jour, mois, année, heure, minute, seconde), ou similaire. En
outre, il est conseillé de mentionner correctement le fuseau horaire local et sa référence au temps
universel coordonné (UTC), ainsi que les éventuels passages à l'heure d'été appliqués.
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ISO 4373:2022(F)
Lorsqu'un enregistreur numérique de données contient l'électronique d'interface, la résolution et
l'incertitude doivent se rapporter à la valeur enregistrée.
6 Enveloppe
Les performances de l'enveloppe doivent être déclarées en termes de système de classification
IP conformément à l'IEC 60529. Il doit être indiqué si les pièces en contact potentiel avec l'eau sont
adaptées ou non au contact avec l'eau. Il doit être indiqué si l'équipement peut ou non être utilisé dans
un environnement potentiellement explosif conformément à l'IEC 60079-10.
7 Installation
Le fabricant doit fournir des instructions claires pour l'installation des appareils de mesure du niveau
de l'eau.
L'appareil de mesure du niveau de l'eau doit porter un repère clairement visible, lequel peut être utilisé
pour lier l'appareil au zéro de l'échelle locale.
Si un système de mesure à flotteur est équipé d'un puits de mesurage, il convient que le diamètre
du tuyau ou de l'orifice d'entrée horizontal sur le puits de mesurage soit environ 10 fois inférieur au
diamètre du puits de mesurage lui-même, afin de réduire suffisamment les perturbations dues à l'eau
agitée à la surface.
En outre, il convient que les tuyaux cylindriques verticaux, dans lesquels le flotteur peut monter et
descendre, soient plus larges d'au moins 10 cm que le diamètre du flotteur, et ces tuyaux doivent être
montés exactement le long de la verticale locale afin de garantir au flotteur toute liberté de mouvement
sur l'ensemble de la plage.
Il faut s'assurer qu'un capteur sans contact est monté de telle sorte que son faisceau soit perpendiculaire
à la surface de l'eau. Les capteurs sans contact doivent être montés sur des supports rigides et solidement
fixés afin d'éviter tout mouvement du capteur susceptible d'induire des erreurs de mesurage. Il convient
de prévoir un passage dégagé entre la face du capteur et la surface de l'eau, sans obstacles pouvant
produire de faux échos. De nombreux instruments sans contact fournissent une aide à l'utilisateur pour
la mise en service de l'instrument.
Il est nécessaire de choisir soigneusement la technique de mesure lorsque de la mousse, des bulles ou
d'autres perturbations sont susceptibles d'être présentes à la surface de l'eau (voir Annexe A).
8 Entretien
Des instructions claires doivent être fournies concernant l'entretien approprié de l'appareil de
mesure. Ces instructions doivent également couvrir les inspections régulières et, éventuellement, les
étalonnages périodiques. Il est important de vérifier régulièrement les mesures obtenues à partir des
appareils installés et il convient de réétalonner l'instrument chaque fois que nécessaire. Les raisons
pour lesquelles un réétalonnage s'avère quelquefois nécessaire sont variables selon le type d'instrument,
mais elles peuvent notamment comprendre un changement du zéro de l'échelle, un étirement du câble,
une dérive dans le circuit électronique, etc.
Il est nécessaire d'inclure dans l'entretien la vérification périodique du ou des repères du limnimètre
par rapport au zéro de l'échelle. La fréquence de vérification du repère/du zéro de l'échelle dépend de la
stabilité de la structure du limnimètre.
Le niveau d'entretien exigé varie en fonction du type d'instrument et des conditions du site. L'Annexe A
décrit les aspects à prendre en compte pour l'entretien de base selon chaque type d'instrument.
NOTE Les instructions d'entretien spécifiées ci-dessus s'appliquent non seulement à l'appareil de mesure,
mais également à tout équipement auxiliaire (tuyaux d'entrée et puits de mesurage, par exemple) qui peut nuire
au fonctionnement du poste de mesurage du niveau de l'eau.
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ISO 4373:2022(F)
9 Estimation de l'incertitude de mesure
9.1 Généralités
L'incertitude d'une valeur dérivée de mesures primaires peut être due:
a) à l'instabilité de la valeur mesurée (fluctuations dues, par exemple, aux ondes à la surface de l'eau
ou au bruit des systèmes électroniques);
b) à la résolution du processus de mesurage (résolution du capteur ou de l'œil humain);
c) aux erreurs de mesure dues aux changements de température, au contenu des sédiments, à la
salinité de l'eau ou aux effets de Bernoulli engendrés par la vitesse de l’eau;
d) au décalage progressif par rapport à l'étalonnage initial en raison de la sensibilité aux conditions
environnementales (par exemple, température, humidité relative ou pression atmosphérique) et de
la variabilité de ces conditions;
e) au décalage progressif par rapport à l'étalonnage initial en raison de la sensibilité aux conditions
électriques (par exemple, tension d'alimentation ou fréquence d'alimentation) et de la variabilité de
ces conditions;
f) à la dérive progressive en position verticale de la structure du limnimètre et au décalage qui
s'ensuit par rapport à la dernière vérification du zéro de l'échelle (ce point est détaillé en 9.5).
En vertu du principe d'incertitude du GUM (GUM signifie Guide to the expression of uncertainty in
[1]
measurement, ou Guide pour l'expression de l'incertitude de mesure ), l'incertitude liée aux mesurages
est exprimée en termes «d'incertitude-type» et «d'incertitude élargie». L'incertitude-type est indiquée
par u. L'incertitude élargie est indiquée par U et U = ku, où k est le facteur d'élargissement en fonction du
niveau de confiance souhaité. Le GUM présente deux méthodes pour estimer les incertitudes, qui sont
classées en Type A et Type B. Ces deux méthodes d'estimation permettent établir une relation entre la
dispersion des valeurs et la probabilité de «proximité» par rapport à la valeur moyenne.
9.2 Estimation de l'incertitude de Type A
L'incertitude de Type A est estimée comme correspondant à l'écart-type d'un grand nombre de
mesurages dans un régime établi. Il est à noter que la distribution de ces résultats peut ne pas être
gaussienne. Les estimations de Type A peuvent être facilement calculées à partir de mesurages
en continu lorsque la dispersion n'est pas masquée par l'hystérésis du processus de mesurage. La
dispersion doit bien sûr dépasser d'une marge significative la résolution du processus de mesurage.
Une autre approche d'une estimation de Type A consiste à comparer les relevés obtenus à partir de
deux postes de mesurage du niveau de l'eau dans le même cours d'eau à une distance très courte l'un
de l'autre. En examinant attentiveme
...
FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 4373
ISO/TC 113/SC 5
Hydrometry — Water level measuring
Secretariat: ANSI
devices
Voting begins on:
2021-12-02
Hydrométrie — Appareils de mesure du niveau de l'eau
Voting terminates on:
2022-01-27
ISO/CEN PARALLEL PROCESSING
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/FDIS 4373:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN-
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. © ISO 2021
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ISO/FDIS 4373:2021(E)
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© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
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Published in Switzerland
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ISO/FDIS 4373:2021(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Instrument specification . 1
4.1 Performance parameters . 1
4.2 Performance classification . 1
4.3 Maximum rate of change . 3
4.4 Environment . 3
4.4.1 General . 3
4.4.2 Temperature . 3
4.4.3 Relative humidity . 3
4.5 Timing . 3
4.5.1 General . 3
4.5.2 Digital . 4
4.5.3 Analogue . 4
5 Recording . 4
5.1 General . 4
5.2 Chart recorders . 4
5.3 Data loggers . . 4
6 Enclosure . 5
7 Installation .5
8 Maintenance . 5
9 Estimation of measurement uncertainty . 5
9.1 General . 5
9.2 Type A uncertainty estimation . 6
9.3 Type B uncertainty estimation . 6
9.4 Uncertainty in case of low water level conditions . 7
9.5 Level measurement datum . 7
9.6 Combining primary measurement uncertainties . 7
Annex A (informative) Types of water level measuring devices . 9
Annex B (informative) Manually operated measuring devices .23
Annex C (informative) Recording devices .26
Bibliography .28
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ISO/FDIS 4373:2021(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 113 Hydrometry, Subcommittee
SC 5, Instruments, equipment and data management, in collaboration with the European Committee
for Standardization (CEN) Technical Committee CEN/TC 318, Hydrometry, in accordance with the
Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This fourth edition cancels and replaces the third edition (ISO 4373:2008), which has been technically
revised. The main changes are as follows:
— improvements in water level measuring devices have been incorporated;
— the use of mercury has been removed;
— the old Annex A has been divided into three new separate Annexes A, B and C;
— in the new Annex A, the electronic techniques that are currently more commonly used have been
brought to the front in order to give them a greater emphasis.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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ISO/FDIS 4373:2021(E)
Introduction
Measuring the level of water surface is very important in hydrometry for the purpose of, among
other things, determining flow rates. Information about water levels is also used in operational water
management, including for the design of dikes, storm surge warning services and guidance of shipping.
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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 4373:2021(E)
Hydrometry — Water level measuring devices
1 Scope
This document specifies the functional requirements of instrumentation for measuring the level of
water surface (stage), primarily for the purpose of determining flow rates.
This document is supplemented by Annex A, which provides guidance on the types of automatic water
level measurement devices currently available and the measurement uncertainty associated with them.
The manually operated measuring devices are described in Annex B.
This document is applicable to both contact and non-contact methods of measurement. The non-contact
methods are not in direct material contact with the water surface but measure the height of the water
level with ultrasonic or electromagnetic waves.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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
IEC 60079-10, Electrical apparatus for explosive gas atmospheres — Part 10: Classification of hazardous
areas
IEC 60529, Degrees of protection provided by enclosures (IP Code)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Instrument specification
4.1 Performance parameters
The performance parameters of a water level measuring device are uncertainty, measurement range,
temperature range and relative humidity range. Thus, the overall performance of the equipment can be
summarized by a few characterizing parameters.
4.2 Performance classification
Water level measuring devices shall be classified in accordance with the performance classes given
in Table 1 that account for the resolution to be achieved and the limits of uncertainty required
over specified measurement ranges. Measurement range is to be understood as the difference between
the highest and the lowest water level that can be measured. When measuring short ranges with class 1
and 2 devices, the uncertainty is a few millimetres, and this is difficult to achieve.
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ISO/FDIS 4373:2021(E)
It should be made clear whether these levels of attainment can only be achieved using special works,
e.g. installation within a stilling well, also referred to as a “gauge well”.
Table 1 — Performance classes of water level measuring devices
Class Resolution Range Nominal uncertainty
Performance class 1 ≤ 1 mm ≤ 1,0 m < ± 0,1 % of range
≤ 2 mm ≤ 5,0 m
≤ 10 mm ≤ 20 m
Performance class 2 ≤ 2 mm ≤ 1,0 m < ± 0,3 % of range
≤ 5 mm ≤ 5,0 m
≤ 20 mm ≤ 20 m
Performance class 3 ≤ 10 mm ≤ 1,0 m < ± 1 % of range
≤ 50 mm ≤ 5,0 m
≤ 200 mm ≤ 20 m
The manufacturer shall state the physical principle of the measuring device to allow the user to judge
the device’s suitability for the proposed environment. Table 2 lists the various physical principles of
operational water level measuring devices being used in the field against their characteristics. These
different techniques are described in more detail in Annex A.
Table 2 — Characteristics of operational water level measuring devices
Device Type Suitable for continuous Typical Typical
measurement measurement uncertainty
range
Mechanical Float and counterweight in a Yes 20 m 5 mm to 10 mm
devices stilling well
Wire weight gauge No 20 m 5 mm to 10 mm
Peak level No 15 m 10 mm to
20 mm
Staff and ramp gauge Yes 10 m 10 mm to
20 mm
Electrical Bubbler Yes 30 m 10 mm to
devices 20 mm
Pressure transducer Yes 20 m 10 mm to
20 mm
Capacitance Yes 15 m 10 mm to
20 mm
Resistance Yes 15 m 10 mm to
20 mm
Non-contact Radar/laser Yes 10 m to 50 m 5 mm to 10 mm
devices
Ultrasonic Yes 3 m to 30 m 10 mm to
(through air) 20 mm
Ultrasonic Yes 3 m to 30 m 10 mm to
(through water) 20 mm
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ISO/FDIS 4373:2021(E)
4.3 Maximum rate of change
As water levels can rise and fall rapidly in some applications, to provide guidance on suitability,
for mechanical devices the manufacturer shall state on the equipment specification sheet and
in the instruction manual:
a) the maximum rate of change which the instrument can follow without damage;
b) the maximum rate of change which the instrument can tolerate without suffering in change
in calibration;
c) the response time of the instrument.
The response time is the time interval between the instant when the level sensor is subjected to an
abrupt change in liquid level and the instant when the readings cross the limits of (and remain inside)
a band defined by the 90 % and the 110 % of the difference between the initial and final value of the
abrupt change. The response time should be short enough for the instrument to follow even the fastest
relevant changes in water level, e.g. tides and flood waves. The response time should not be too short.
Therefore, in many electronic devices, it is possible to enlarge the response time through the setting
of certain parameters within the instrument. This can be useful, for example, to damp out the rapid
excursions caused by short waves. Such rapid disturbances are due to local hydraulic phenomena and
are thus not representative for the water level over a large section of the water course. The locally
excited disturbances are thus to be discarded as much as possible.
4.4 Environment
4.4.1 General
Water level measuring devices shall operate within the ranges of temperature in 4.4.2 and the ranges
of relative humidity in 4.4.3.
4.4.2 Temperature
Water level measuring devices shall operate within the following ambient air temperature classes:
Temperature class 1: –30 °C to +55 °C;
Temperature class 2: –10 °C to +50 °C;
Temperature class 3: 0 °C to + 50 °C.
4.4.3 Relative humidity
Water level measuring devices shall operate within the following relative humidity classes:
Relative humidity class 1: 5 % to 95 % including condensation;
Relative humidity class 2: 10 % to 90 % including condensation;
Relative humidity class 3: 20 % to 80 % including condensation.
4.5 Timing
4.5.1 General
Where timing, either analogue or digital, is part of the instrument specification, the timing method
used shall be clearly stated on the instrument and in the instruction manual.
NOTE It is recognized that digital timing is potentially more accurate than analogue timing.
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ISO/FDIS 4373:2021(E)
Moreover, when several raw data samples are assembled together in order to arrive at a time averaged
measurement value, it should clearly be stated to which moment in time the final result applies. It is
preferred to have this time label at exactly the middle of the averaging time window, because this
moment is the most representative. However, many commercially available loggers add time and data
stamps at the beginning or at the end of the averaging time window.
4.5.2 Digital
The uncertainty of digital timing devices used in water level measuring devices shall be within ± 60 s
at the end of a period of 30 days, within the range of environmental conditions defined in 4.4.
4.5.3 Analogue
The uncertainty of analogue timing devices used in water level measuring devices shall be within ± 5 min
at the end of a period of 30 days, within the range of environmental conditions defined in 4.4.
5 Recording
5.1 General
Recording devices serve the purpose of storing the water level data for immediate or later use. Such
devices can be divided into analogue chart recorders and digital data loggers. For more information
about the strengths and weaknesses of these recording devices, see Annex C.
5.2 Chart recorders
Where a chart recorder is to be used as the primary source of data, the resolution and uncertainty
parameters shall take account of changes in the dimensions of the recording medium due to atmospheric
variables.
NOTE Chart recorders have been superseded to a large extent by data logging services. However, they are
still used as back-up units or to provide rapid visual assessment of flow changes on site.
5.3 Data loggers
A data logger shall be able to store at least the measured value and a timestamp. The data logger shall
be able to store at least the equivalent of four digits per measurement and at least the equivalent of nine
digits for the timestamp. In practice, however, the minimum requirement of four digits per measurement
does not always suffice. It is advised, therefore, that the data logger is capable of storing readings
which are sufficiently resolute to record the full range of measured water level values including all
increments possible at the level sensor’s resolution. This means that there shall be sufficient decimal
places, or equivalent, to record all possible step changes in measured values across the sensor’s range.
Consequently, for some high-resolution water level measurements, there is a need for more than four
digits per measurement.
The nine digits for the timestamp are based on the format YYDDDHHMM (year, day, hour, minute).
However, a more time resolute and practical date time stamp such as a DDMMYYYYHHMMSS (day,
month, year, hour, minute, second), or similar, format is preferred. Furthermore, it is advised to properly
mention the local time zone and its reference to coordinated universal time (UTC) as well as any applied
daylight-saving time shifts.
Where a data logger includes the interface electronics, the resolution and uncertainty shall relate to the
stored value.
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ISO/FDIS 4373:2021(E)
6 Enclosure
The performance of the enclosure shall be stated in terms of the IP classification system in accordance
with IEC 60529. It shall be stated whether or not any parts potentially in contact with water are suitable
for contact with water. It shall be stated whether or not the equipment can be used in a potentially
explosive environment in accordance with IEC 60079-10.
7 Installation
The manufacturer shall provide clear instructions for the installation of water level measuring devices.
The water level measuring device shall have a clearly visible reference mark, which can be used for
tying the device to the local gauge datum.
If a float measuring system is equipped with a stilling well, the diameter of the horizontal inlet pipe
or orifice to the stilling well should be about 10 times smaller than the diameter of the stilling well itself
in order to sufficiently reduce any disturbances originating from waves on the water surface.
Furthermore, the vertical cylindrical pipes, in which the float can move up and down should be at least
10 cm wider than the float diameter and shall be erected exactly along the local vertical in order to
ensure free movement of the float over the entire range.
Ensure that a non-contact sensor is set up with its beam perpendicular to the water surface. Non-
contact sensors shall be installed on rigid and well secured brackets to prevent movement of the sensor
that can introduce errors in the measurement. There should be a clear path from the sensor face to
the water surface, free from obstacles that can give false reflections. Many non-contact instruments
include signal diagnostics that help the user when commissioning the instrument.
Careful selection of the measurement technique is required when foam, bubbles or other disturbances
are likely to be present on the water surface (see Annex A).
8 Maintenance
Clear instructions shall be given regarding the proper maintenance of the measuring device. This also
includes regular inspections and possibly regular calibrations. It is important that measurements from
installed devices are checked periodically and, when necessary, the instrument should be recalibrated.
Reasons why recalibration is sometimes necessary vary with instrument type but can include: change
in the datum, cable stretch, electronics drift, etc.
Maintenance needs to include the periodic check of the gauge reference mark(s) to the gauge datum.
The frequency of the reference mark/datum checks depends on the stability of the gauge structure.
The level of maintenance required will vary depending on instrument type and site conditions. Annex A
gives basic maintenance considerations against each instrument type.
NOTE The above-mentioned maintenance instructions do not only apply to the measuring device, but also
to any ancillary equipment (e.g. inlet pipes and stilling wells) that can affect the proper operation of a water level
measuring station.
9 Estimation of measurement uncertainty
9.1 General
The uncertainty of a value derived from primary measurements may be due to:
a) unsteadiness of the measured value (noisy fluctuations due to, for example, waves on the water
surface or due to noise in electronic systems);
b) resolution of the measurement process (resolution of the sensor or of the human eye);
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ISO/FDIS 4373:2021(E)
c) measurement errors due to changes in temperature, sediment content or salinity of the water or
due to Bernoulli effects caused by the water velocity;
d) gradual drift from the original calibration due to sensitivity to, and variability in, environmental
conditions, e.g. temperature, relative humidity, atmospheric pressure;
e) gradual drift from the original calibration due to sensitivity to, and variability in, electrical
conditions, e.g. supply voltage, supply frequency;
f) gradual shift in vertical position of the gauge structure and consequent drift from the last datum
check (this is elaborated upon in 9.5).
Under the GUM uncertainty framework (GUM stands for Guide to the expression of uncertainty in
[1]
measurement ), measurement uncertainty is expressed in terms of “standard uncertainty” and
“expanded uncertainty”. Standard uncertainty is denoted by u. Expanded uncertainty is denoted by
U and U = ku, where k is the coverage factor depending on the desired level of confidence. The GUM
describes two methods for estimating uncertainties that are classified as Type A and Type B. These two
estimation methods are used for relating the dispersion of values to the probability of “closeness” to the
mean value.
9.2 Type A uncertainty estimation
A Type A uncertainty is estimated as the standard deviation of a large number of measurements
under a steady-state condition. Note that the distribution of these results need not be Gaussian.
Type A estimations can be readily computed from continuous measurements when the dispersion
is not masked by hysteresis of the measurement process. Of course, the dispersion must exceed by a
significant margin the resolution of the measurement process.
Another approach for a Type A estimation is to compare the readings from two water level measuring
stations in the same water course within a very short distance of each other. When carefully examining
the difference between the two neighbouring stations, a randomly fluctuating signal can be discerned
that represents the combined effect of the two individual uncertainties at both water level measuring
stations. When the two stations are of identical construction and their measurements are uncorrelated,
the combined variance is twice the variance of each individual station. Thus, the standard deviation of
each station can be calculated by dividing the standard deviation in the random part of the water level
difference between both stations by the square root of two.
Yet another Type A estimation is the comparison of instrument water level measures and manual
observations using reference gauges such as staffs, ramps and wire-weight gauges.
9.3 Type B uncertainty estimation
A Type B estimation is assigned to a measurement process for which sufficiently large numbers
of measurements are not available or to a measurement with defined limits of resolution. To define
a Type B uncertainty, the upper and lower limits of the dispersion or the upper and lower limits of
resolution are used to define the limits of a probability diagram whose shape is selected to represent
the dispersion, i.e. uniform dispersions would have a rectangular distribution, dispersions with most
measurements congregated about the mean value would have a triangular distribution. Allocation of
probability distributions is described in Annex A.
The relationship between the uncertainty of primary measurements and the value of the uncertainty
of the result is derived from the relationship between the value of this result and its primary
measurements. For instance, the primary measurement for a non-contact sensor can be the measured
travelling time elapsed between transmission and reception of an echo from the water surface. Any
uncertainty in measuring this travelling time will lead to a correlated uncertainty in the resulting
water level.
In the case of level, this relationship to primary measurements is generally linear. Sensitivities that
describe the dependencies of the uncertainty in the result to the uncertainty in the individual primary
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