Hydrometry - Water level measuring devices (ISO 4373:2022)

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.

Hydrometrie - Geräte zur Wasserstandsmessung (ISO 4373:2022)

Diese Internationale Norm legt die funktionellen Anforderungen an die Messgeräteausrüstung zum Messen des Pegels der Wasseroberfläche (Höhe des Wasserstandes), vor allem zur Bestimmung des Durchflusses, fest.
Die vorliegende Internationale Norm wird durch einen Anhang ergänzt, der einen Leitfaden zu gegen-wärtig verfügbaren Messgerätetypen zur Wasserstandsmessung und die damit verbundene Messunsicherheit enthält (siehe Anhang A).
Diese Norm umfasst sowohl kontaktbasierte als auch kontaktlose Messverfahren. Die kontaktlosen Verfahren stehen nicht in direktem Materialkontakt mit der Wasseroberfläche, sondern messen die Höhe des Wasserstandes mit Ultraschall oder elektromagnetischen Wellen.

Hydrométrie - Appareils de mesure du niveau de l'eau (ISO 4373:2022)

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.

Hidrometrija - Naprave za merjenje višine gladine vode (ISO 4373:2022)

General Information

Status
Published
Public Enquiry End Date
01-Aug-2021
Publication Date
05-May-2022
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
28-Apr-2022
Due Date
03-Jul-2022
Completion Date
06-May-2022

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SLOVENSKI STANDARD
SIST EN ISO 4373:2022
01-junij-2022
Nadomešča:
SIST EN ISO 4373:2009
Hidrometrija - Naprave za merjenje višine gladine vode (ISO 4373:2022)
Hydrometry - Water level measuring devices (ISO 4373:2022)
Hydrometrie - Geräte zur Wasserstandsmessung (ISO 4373:2022)
Hydrométrie - Appareils de mesure du niveau de l'eau (ISO 4373:2022)
Ta slovenski standard je istoveten z: EN ISO 4373:2022
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
SIST EN ISO 4373:2022 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 4373:2022
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SIST EN ISO 4373:2022
EN ISO 4373
EUROPEAN STANDARD
NORME EUROPÉENNE
April 2022
EUROPÄISCHE NORM
ICS 17.120.20 Supersedes EN ISO 4373:2008
English Version
Hydrometry - Water level measuring devices (ISO
4373:2022)

Hydrométrie - Appareils de mesure du niveau de l'eau Hydrometrie - Geräte zur Wasserstandsmessung (ISO

(ISO 4373:2022) 4373:2022)
This European Standard was approved by CEN on 13 March 2022.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this

European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references

concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN

member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by

translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,

Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and

United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 4373:2022 E

worldwide for CEN national Members.
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SIST EN ISO 4373:2022
EN ISO 4373:2022 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 4373:2022
EN ISO 4373:2022 (E)
European foreword

This document (EN ISO 4373:2022) has been prepared by Technical Committee ISO/TC 113

"Hydrometry" in collaboration with Technical Committee CEN/TC 318 “Hydrometry” the secretariat of

which is held by BSI.

This European Standard shall be given the status of a national standard, either by publication of an

identical text or by endorsement, at the latest by October 2022, and conflicting national standards shall

be withdrawn at the latest by October 2022.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN shall not be held responsible for identifying any or all such patent rights.

This document supersedes EN ISO 4373:2008.

Any feedback and questions on this document should be directed to the users’ national standards

body/national committee. A complete listing of these bodies can be found on the CEN website.

According to the CEN-CENELEC Internal Regulations, the national standards organizations of the

following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,

Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,

Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of

North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the

United Kingdom.
Endorsement notice

The text of ISO 4373:2022 has been approved by CEN as EN ISO 4373:2022 without any modification.

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SIST EN ISO 4373:2022
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SIST EN ISO 4373:2022
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
---------------------- Page: 7 ----------------------
SIST EN ISO 4373:2022
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

or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
© ISO 2022 – All rights reserved
---------------------- Page: 8 ----------------------
SIST EN ISO 4373:2022
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

iii
© ISO 2022 – All rights reserved
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SIST EN ISO 4373:2022
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.
© ISO 2022 – All rights reserved
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SIST EN ISO 4373:2022
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.
© ISO 2022 – All rights reserved
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SIST EN ISO 4373:2022
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SIST EN ISO 4373:2022
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.

© ISO 2022 – All rights reserved
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SIST EN ISO 4373:2022
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
© ISO 2022 – All rights reserved
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SIST EN ISO 4373:2022
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.

© ISO 2022 – All rights reserved
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SIST EN ISO 4373:2022
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

© ISO 2022 – All rights reserved
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SIST EN ISO 4373:2022
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 (re
...

SLOVENSKI STANDARD
oSIST prEN ISO 4373:2021
01-julij-2021
Hidrometrija - Naprave za merjenje višine gladine vode (ISO/DIS 4373:2021)
Hydrometry - Water level measuring devices (ISO/DIS 4373:2021)
Hydrometrie - Geräte zur Wasserstandsmessung (ISO/DIS 4373:2021)
Hydrométrie - Appareils de mesure du niveau de l'eau (ISO/DIS 4373:2021)
Ta slovenski standard je istoveten z: prEN ISO 4373
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
oSIST prEN ISO 4373:2021 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
oSIST prEN ISO 4373:2021
---------------------- Page: 2 ----------------------
oSIST prEN ISO 4373:2021
DRAFT INTERNATIONAL STANDARD
ISO/DIS 4373
ISO/TC 113/SC 5 Secretariat: ANSI
Voting begins on: Voting terminates on:
2021-05-13 2021-08-05
Hydrometry — Water level measuring devices
Hydrométrie — Appareils de mesure du niveau de l'eau
ICS: 17.120.20
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
ISO/CEN PARALLEL PROCESSING
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 4373:2021(E)
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 SUPPORTING DOCUMENTATION. ISO 2021
---------------------- Page: 3 ----------------------
oSIST prEN ISO 4373:2021
ISO/DIS 4373:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© 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 or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
oSIST prEN ISO 4373:2021
ISO/DIS 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 ................................................................................................................................................................ 2

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 ........................................................................................................................................................................................ 3

4.5.3 Analogue ................................................................................................................................................................................. 4

5 Recording ..................................................................................................................................................................................................................... 4

5.1 Chart recorders ....................................................................................................................................................................................... 4

5.2 Data loggers ............................................................................................................................................................................................... 4

6 Enclosure ...................................................................................................................................................................................................................... 4

7 Installation ................................................................................................................................................................................................................. 4

8 Maintenance .............................................................................................................................................................................................................. 5

9 Estimation of measurement uncertainty .................................................................................................................................... 5

9.1 General ........................................................................................................................................................................................................... 5

9.2 Type-A estimation ................................................................................................................................................................................ 5

9.3 Type-B estimation ................................................................................................................................................................................ 6

9.4 Uncertainty in case of low stage conditions .................................................................................................................. 6

9.5 Level measurement datum ........................................................................................................................................................... 6

9.6 Combining primary measurement uncertainties...................................................................................................... 7

Annex A (informative) Types of water level measuring devices .............................................................................................. 8

Annex B (informative) Manually operated measuring devices ..............................................................................................21

Annex C (informative) Recording devices ....................................................................................................................................................24

Bibliography .............................................................................................................................................................................................................................26

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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 on 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 the following

URL: 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.

This fourth edition cancels and replaces the third edition (ISO 4373:2008), which has been technically

revised.
The main changes compared to the previous edition are as follows:
— Incorporation of improvements in water level measuring devices;
— The use of mercury has been removed from the standard;
— Splitting up the old Annex A into three new separate annexes A, B and C;

— In the new Annex A the nowadays more commonly used electronic techniques have been brought

to the front in order to emphasize them more.
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Introduction

Measuring the level of water surface is very important in hydrometry among other things for the

purpose of determining flow rates. Information about water levels is also used in operational water

management, for the design of dikes, for storm surge warning services and guidance of shipping.

ISO 4373:2008 is now in revision because of numerous improvements in water level measuring devices

since the standard was published in 2008.
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DRAFT INTERNATIONAL STANDARD ISO/DIS 4373:2021(E)
Hydrometry — Water level measuring devices
1 Scope

This International Standard specifies the functional requirements of instrumentation for measuring

the level of water surface (stage), primarily for the purpose of determining flow rates.

This International Standard is supplemented by an annex providing guidance on the types of water

level measurement devices currently available and the measurement uncertainty associated with them

(see Annex A).

This standard covers 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 60529, Degrees of protection provided by enclosures (IP Code)

IEC 60079-10, Electrical apparatus for explosive gas atmospheres — Part 10: Classification of

hazardous areas
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 may

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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It should be made clear whether these levels of attainment can only be achieved using special works, for

example 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 must state the physical principle of the measuring device to allow the user to judge

the device's suitability for the proposed environment. In Table 2 the various physical principles of

operational water level measuring devices, that are being used in the field, are listed 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 measure- Typical resolu-
measurement ment range tion
Mechanical de- Float Yes 20 m 10 mm
vices
Peak level No 15 m 10 - 20 mm
Staff and ramp gauge Yes 10 m 10 - 20 mm
Electrical devices Bubbler Yes 30 m 2 - 10 mm
Pressure transducer Yes 20 m 10 mm
Capacitance Yes 15 m 10 mm
Resistance – 15 m 10 mm
Non-contact de- Radar/Laser Yes 10 - 50 m 1 mm
vices
Ultrasonic (through Yes 3 - 30 m 1 - 2 mm
air)
Ultrasonic (through Yes 3 - 30 m 2 - 5 mm
water)
4.3 Maximum rate of change

As water levels may 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

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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.

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.

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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 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.

5.2 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.

NOTE The above mentioned statement of nine digits for the timestamp is based on the format DDDYYHHMM

(day, year, hour, minute).

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 in contact with water are suitable for contact

with water. National regulations and by-laws relating to materials in contact with water shall be obeyed.

It shall be stated whether or not the equipment may 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 must 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 ten times smaller than the diameter of the stilling well

itself in order to sufficiently reduce any disturbances originating from the wavy water in the main

channel.

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 could introduce errors in the measurement. There should be a clear path from the sensor face to

the water surface, free from obstacles that might give false reflections. Many non-contact instruments

include signal diagnostics that help the user when commissioning the instrument.
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Careful selection of the measurement technique is required when foam or bubbles 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 might be necessary vary with instrument type but could 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 may 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 e.g. waves on the water surface

or due to noise in electronic systems); or

b) resolution of the measurement process (resolution of the sensor or of the human eye);

c) gradual drift from the original calibration due to sensitivity to, and variability in environmental

conditions e.g. temperature, relative humidity, atmospheric pressure etc.;

d) gradual drift from the original calibration due to sensitivity to, and variability in electrical

conditions e.g. supply voltage, supply frequency etc.;

e) gradual shift in vertical position of the gauge structure and consequent drift from the last datum

check (this is elaborated upon in clause 9.5).

Two methods of estimation, Type A and Type B are described in the Guide to the expression of uncertainty

[1]

in measurement for relating the dispersion of values to the probability of "closeness" to mean value.

9.2 Type-A estimation

A Type-A estimation is determined directly from the standard deviation of a large number of

measurements. Note that the distribution of these results need not to 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 might be 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 neighboring 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 uncertainties are

uncorrelated, the combined variance is twice the variance of each individual station. Thus, the standard

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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 2.

Yet another Type-A estimation is the comparison of instrument water level measures and manual

observations using reference gauges such as staffs, ramps, hooks of wire-weight gauges.

9.3 Type-B 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 formula defining the relationship between the value of this result and

its primary measurements. For instance, the primary measurement for a non-contact sensor could

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. The relationship is described by the formula, mentioned above.

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.
9.4 Uncertainty in case of low stage conditions

It is important to remember that in the measurement of stage, absolute uncertainty expressed

as a percentage of water level range gives rise to worst case relative uncertainty in the determination

of stage at low values of stage. For instance, 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 water level that becomes

large when the water level decreases. Therefore, it becomes increasingly difficult to measure low water

levels with sufficient relative accuracy.

This is highly significant in situations where flow information is derived from local stage measurements.

...

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