EN ISO 4373:2022
(Main)Hydrometry - Water level measuring devices (ISO 4373:2022)
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)
Dieses Dokument legt die funktionellen Anforderungen an die Messgeräteausrüstung zum Messen des Pegels der Wasseroberfläche (Höhe des Wasserstandes), vor allem zur Bestimmung des Durchflusses, fest.
Dieses Dokument wird durch Anhang A ergänzt, der einen Leitfaden zu gegenwärtig verfügbaren automatischen Mess¬gerätetypen zur Wasserstandsmessung und die damit verbundene Messunsicherheit enthält. Manuell betätigte Messgeräte sind in Anhang B beschrieben.
Dieses Dokument ist sowohl für kontaktbasierte als auch kontaktlose Messverfahren anwendbar. 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)
Ta dokument določa funkcionalne zahteve instrumentov za merjenje višine vodne gladine (nivo), predvsem za namene določanja pretokov. Ta dokument dopolnjuje dodatek A, ki vsebuje smernice za vrste instrumentov za samodejno merjenje višine vodne gladine, ki so trenutno na voljo, in z njimi povezano merilno negotovost. Ročno upravljani merilni instrumenti so opisani v dodatku B. Ta dokument se uporablja za kontaktne in brezkontaktne metode merjenja. Brezkontaktne metode niso v neposrednem stiku z vodno gladino, temveč merijo višino vodne gladine z ultrazvočnimi ali elektromagnetnimi valovi.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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.
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COMITÉ EUROPÉEN DE NORMALISATION
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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.
Contents Page
European foreword . 3
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.
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 4373:2022(E)
© 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
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Published in Switzerland
ii
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 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.
iv
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.
v
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 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 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 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 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;
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.
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 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 water level
measurements. Low flows are related to low water levels and low flows are thus difficult to measure
with a sufficient relative accuracy. This should be considered in the design of equipment for this
purpose.
9.5 Level measurement datum
Level measurement is not an absolute measurement; it is always relative to a datum, e.g. a local
benchmark or the elevation of a weir crest. It is, therefore, essential that the water level measuring
instrument contains a clear and precise height mark (e.g. the topside of the flange of a radar sensor),
with which it can accurately be referenced to a local datum level. The uncertainty associated with
the datum should be combined with the uncertainty of the water level measurement, which is further
described in 9.6.
When the water level measurement is desired to be meaningful in a wide area, so that it can be
compared with neighbouring water level measuring stations, care should be taken to relate all the local
datum heights to each other or, in other words, to create one consistent regional datum plane. When the
water levels are measured for inferring flow characteristics over a wide area, it is preferred to couple
all the local datum heights in a so-called “geodic or gravitationally equipotential frame of reference”.
Of course, when doing so, all local datum levels should be checked regularly for any sinking or rising
with respect to the overall regional geodetic plane. The required inspection frequency at any station
depends on the vertical stability of the local datum level as a function of time.
9.6 Combining primary measurement uncertainties
To determine the standard uncertainty u of the water level, it is necessary to combine the standard
uncertainties u of all primary measurements. Thus, assuming the measurements are uncorrelated, this
results in Formula (1):
uh = ud +um (1)
() () ()
where:
u(h) is the total uncertainty in the resulting water level;
u(d) is the uncertainty in the datum level;
u(m) is the uncertainty in the water level measurement.
This illustrates the method taking into account the uncertainty of the reference level datum value.
Other components of measurement uncertainty are added in the same way by inclusion of their squared
value within the square root expression.
Uncertainties are evaluated and combined as standard uncertainties related to the standard deviation
of the dispersion distribution. However, a coverage factor k can be applied to report an expanded
uncertainty with a higher level of confidence. Usually, the coverage factor k is 2, resulting in a level
of confidence of approximately 95 %.
ISO 4373:2022(E)
Annex A
(informative)
Types of water level measuring devices
A.1 Echo-location, radar instruments
A.1.1 Description
A downward-looking radar unit (see Figure A.1) can determine the relative position of a water surface
under the radar antenna by measuring the vertical distance between them. It consists of a microwave
transmitter and receiver (transceiver in short), a form of modulation by which the time elapsed
between transmitting a signal and receiving the echo from the air/water interface can be measured
as well as the conversion of the elapsed time to distance using the speed of light. The modulation that
is used to detect the targeted water surface can be a short pulse or a continuous signal that is being
modulated in frequency. The height of the water level is inferred by subtracting the measured distance
(i.e. the distance between transducer face and water surface) from gauge datum, where gauge datum
is to be understood as the height of the transducer above chart datum.
The water level measured relates to the area covered by the beam. The diameter of this area is in meters
and can be estimated by using the two-way radiation beam width (in radians) of the antenna multiplied
by the vertical distance in meters. For example, this can be in the order of 1 m when the radar is 10 m
above the water surface.
Key
1 transceiver
Figure A.1 — Remotely sensed water level height
The frequency of the used electromagnetic waves is usually in the order of 10 GHz, but sometimes other
frequencies are used. The electromagnetic echo location principle is sometimes used at much higher
frequencies extending into the visible light region. However, in such cases an optical or infrared laser is
used instead of an antenna fed by a microwave source.
The user shall accurately know the null level when the absolute height of the water level is to be
measured. Often this null level is indicated by a marker on the outside of the radar. This marker shall be
used to transfer a level datum to the local water level measurement.
In very exposed conditions, such as on a large lake or near the coast, high breaking waves can be
present that can disturb the water level measurement. In such conditions, it is advised to place the
radar measurement in an enclosed cylindrical housing, shielding it from the rough conditions outside.
Deposition of sediment in such a housing can be prevented by leaving the bottom of the protective
housing completely open. When the danger of freezing exists, the housing should be heated.
When contained in a protective housing, the electromagnetic free space condition no longer applies
and consequently the group velocity becomes lower than the speed of light in a vacuum. Since this
ISO 4373:2022(E)
group velocity is the velocity by which information is propagated and the distance is measured, care
should be taken to account for this lower velocity in the calculation of distance between the radar and
the water surface. The reduction factor of the propagation velocity is dependent on the diameter of
the cylindrical housing and can be calculated by standard cylindrical waveguide theory. The diameter
must be larger than half the radar wavelength because otherwise the propagation of electromagnetic
waves is impossible.
A.1.2 Strengths
A radar echo-location instrument is mounted above water and is readily accessible for maintenance. It
has no moving parts and is not subject to fouling by vegetation and debris. The temperature of the air
column through which the signal passes does not affect the measurement. Precipitation and/or wind
do not influence the measurement.
Furthermore, radar will penetrate most surface foams and will give a true reading for the water level. A
radar echo-location instrument can operate without a stilling well when the water is calm. Depending
on the instrument, the accuracy is typically 0,1 % or better.
Tight spaces or reflection problems can sometimes be avoided by using guided wave radar
configurations (i.e. a solid rod acts as the transmission line).
A.1.3 Weaknesses
An echo-location instrument usually needs to be mounted on an arm extending over the flow to ensure
that the conical beam does not strike channel walls. Any blocking objects in the radar beam can hamper
the water level measurement.
Most radar instruments have a region, often referred to as the “dead zone” or “blanking distance” in
which they cannot detect the water surface. It typically extends between 100 mm to 300 mm beneath
the sensor face. This is because the microwave receiver is electrically isolated from the antenna for
a short while after the transmitter has emitted an electromagnetic pulse. This serves the purpose of
protecting the sensitive receiver from the powerful emitted pulse. In this isolated state, the receiver
cannot detect the reflected signal. The user shall take this blanking distance, as specified by the
manufacturer, into account, when calculating the highest water level to be measured.
Radar equipment tends to have a high-energy requirement (several watts) and consequently it can
be necessary to connect the radar to a main power supply. However, radar equipment is sometimes
powered by batteries charged by solar panels.
When short waves are present on the water surface, the radar tends to pick up the echo from the
smooth, concavely shaped and thus strongly reflective wave troughs rather than the rough, spiky wave
crests, which scatter the incident radar wave in all directions yielding a low reflection. The result is
that the water level is measured slightly lower than the actual water level.
Radar installations are potentially vulnerable to vandalism.
A.1.4 Uncertainty
Radar echo-location instruments will show a dispersion with most measurements around the mean
value. The distribution of the measurement resembles a triangular shape, so Formula (A.1) applies:
()xx−
maxmin
ux()= (A.1)
mean
where
x is the discernible upper limit;
max
x is the discernible lower limit.
min
ISO 4373:2022(E)
EXAMPLE If, from inspection, the discernible upper limit is 0,150 and the discernible lower limit is 0,140,
then the best estimate is 0,145 with an uncertainty of 0,002.
A.2 Echo-location, acoustic instruments
A.2.1 Instruments with sound path in air
A.2.1.1 Description
An instrument with its sound path in air (see Figure A.1) has an acoustic transducer/receiver, mounted
above the maximum water level, that transmits an ultrasonic pulse and receives the echo of that pulse
from the water/air interface.
The elapsed time between transmission and reception is converted to distance by using the
speed of sound in air. However, the speed of sound in
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