ISO 23350:2021

Hydrometry — Catching-type liquid precipitation measuring gauges

This document specifies the typical requirements of instrumentation for measuring liquid precipitation, primarily for the purpose of hydrological and meteorological observation. This document is applicable to both non-recording and recording catching-type precipitation gauges for the measurement of liquid precipitation. This document covers design criteria for the gauges and elements to be considered in their construction. This document does not include specification, design and installation conditions. NOTE Since the measurement of solid precipitation is outside the scope, the disadvantages of solid precipitation apply to all gauges listed in Annex A. The measurement of solid precipitation and associated disadvantages are well documented in Reference [1].

Hydrométrie — Appareils de mesure de précipitations liquides de type collecteur

General Information

• Status: Published
• Publication Date: 23-Sep-2021
• ICS: 17.120.20 - Flow in open channels
• Technical Committee: ISO/TC 113/SC 5 - Instruments, equipment and data management
• Drafting Committee: ISO/TC 113/SC 5 - Instruments, equipment and data management
• Current Stage: 6060 - International Standard published
• Start Date: 24-Sep-2021
• Due Date: 11-Apr-2021
• Completion Date: 24-Sep-2021

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23350
First edition
2021-09
Hydrometry — Catching-type liquid
precipitation measuring gauges
Hydrométrie — Appareils de mesure de précipitations liquides de type
collecteur
Reference number
ISO 23350:2021(E)
© ISO 2021

---------------------- Page: 1 ----------------------
ISO 23350:2021(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: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
  © ISO 2021 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 23350:2021(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Instrument specifications . 1
4.1 General . 1
4.2 General gauge specifications . 2
4.3 Environment . 2
4.3.1 General . 2
4.3.2 Temperature . 2
4.3.3 Relative humidity . 3
5 Recording . 3
5.1 Recording format . 3
5.2 Recording interval . 3
6 Environmental protection and housing . 3
7 Installation .4
8 Estimation of measurement uncertainty . 4
8.1 General . 4
8.2 Factors of measurement uncertainty . 4
8.3 Type-A estimation . 7
8.4 Type-B estimation . 7
8.5 Combined uncertainty . 7
Annex A (Informative) Catching-type liquid precipitation measuring gauges .8
Annex B (informative) Uncertainties of catching-type liquid precipitation measuring
gauges .15
Bibliography .18
iii
© ISO 2021 – All rights reserved

---------------------- Page: 3 ----------------------
ISO 23350:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 5,
Instruments, equipment and data management.
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 2021 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 23350:2021(E)
Introduction
Rainfall totals and intensity rates are measured at frequencies ranging from seconds to months,
and the data are required by the user on timescales ranging from immediate to several weeks. This
document covers instrumentation that measures rainfall accumulation and rainfall intensity (RI) by
collecting the volume of water that falls on a fixed area. The gauge user should decide on the gauge
design, measurement unit, period of rainfall and network purpose.
Unlike many other meteorological instruments, there is no absolute physical standard against which
a raingauge (hereafter called catching-type liquid precipitation measuring gauges) can be compared.
Many different types, shapes and sizes of catching-type liquid precipitation measuring gauges are
acceptable for the measurement of rainfall and rainfall depth, each reflecting a specific requirement.
Most consist of a circular collecting device, delineating the fixed area of the sample, and a funnel leading
into a storage reservoir or measuring system, or both. Some types of automatic gauges do not require
a funnel. Since various sizes and shapes of orifice and gauge heights are used in different countries, the
measurements are not strictly comparable.
This document provides general information on the functions of catching-type liquid precipitation
gauges. The annexes in this document provide guidance on the types of catching-type liquid precipitation
measuring gauges currently available (see Annex A) and the measurement uncertainty associated with
them (see Annex B). Although advances in measurement electronic technologies and smart instruments
have led to the development of optical and hybrid type liquid precipitation measuring gauges, they are
not included in this document.
NOTE 1 This document specifies only the general functions of catching-type liquid precipitation measuring
gauges. For in-depth and extensive information on precipitation measuring gauges, see References [1] and [2].
NOTE 2 For detailed information on measurement unit and period of rainfall, see Reference [1].
NOTE 3 For detailed information on the design of a reference raingauge pit as well as installation, see
Reference [3].
v
© ISO 2021 – All rights reserved

---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 23350:2021(E)
Hydrometry — Catching-type liquid precipitation
measuring gauges
1 Scope
This document specifies the typical requirements of instrumentation for measuring liquid precipitation,
primarily for the purpose of hydrological and meteorological observation. This document is applicable
to both non-recording and recording catching-type precipitation gauges for the measurement of liquid
precipitation. This document covers design criteria for the gauges and elements to be considered in
their construction. This document does not include specification, design and installation conditions.
NOTE Since the measurement of solid precipitation is outside the scope, the disadvantages of solid
precipitation apply to all gauges listed in Annex A. The measurement of solid precipitation and associated
disadvantages are well documented in Reference [1].
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
BS 7843-3, Acquisition and management of meteorological precipitation data from a gauge network —
Part 3: Code of practice for the design and manufacture of storage and automatic collecting raingauges
3 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 772 and BS 7843-3 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 http:// www .electropedia .org/
4 Instrument specifications
4.1 General
Catching-type liquid precipitation measuring gauges are classified as non-recording and recording
types. The non-recording type has ordinary and storage gauges. The recording type has weighing,
tipping-bucket and floating types. Catching-type liquid precipitation measuring gauges have a
-1
resolution of 0,1 mm to 1 mm, and can measure rainfall intensities of 0,1 to 1 000 mm⋅h or higher.
Measurement errors can occur according to installation conditions, the measurement environment,
solid precipitation and rainfall intensity.
A catching-type liquid precipitation measuring gauge shall be a circular orifice and shall be formed
perpendicular to the outer slope and the inner surface with sharp edges. The diameter of the edge
of the collecting device should not differ by more than 0,2 % of the stated diameter in any direction.
Furthermore, the collector should be designed to prevent rain from splashing in and out. This can be
achieved if the vertical wall is sufficiently deep and the slope of the funnel is sufficiently steep (at least
45 %). The container should have a narrow entrance and be sufficiently protected from radiation to
1
© ISO 2021 – All rights reserved

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ISO 23350:2021(E)
minimize the loss of water by evaporation. The surface of the funnel shall consist of a stable, durable
material such that the water drop is not retained by surface tension, is freely moved towards the orifice
and is passed to the measuring mechanism.
NOTE This document specifies the functional requirements of catching-type liquid precipitation measuring
gauges. Consideration of the siting and exposure of gauges is also important. More specifically, the installation
conditions, measurement environment and solid precipitation (see Reference [1] and BS 7843-3).
4.2 General gauge specifications
The gauge manufacturer should provide general specifications that can impact the measurement of
precipitation which include, but are not limited to:
a) range;
b) resolution;
c) accuracy;
d) size of orifice area;
e) standardized communication interfaces and protocols for data transmission;
f) power supply and consumption;
g) height of gauge (including rim height placement to avoid interference from deposited snow or other
materials near the gauge);
i) gauge materials.
Requirements for uncertainty, range and resolution for precipitation measurements should be
considered. It should be noted that, in some countries, trace observations are officially given a value of
zero, thus resulting in a biased underestimate of the seasonal precipititation. This problem is minimized
with weighing-type gauges since even very small amounts of precipitation will accumulate over time.
4.3 Environment
4.3.1 General
Liquid precipitation measuring gauges shall operate within the ranges of temperature in 4.3.2 and the
ranges of relative humidity in 4.3.3. Wind should be taken into consideration in areas that are prone to
inclement weather conditions.
4.3.2 Temperature
Catching-type liquid precipitation measuring gauges should function in their intended manner in the
following environmental conditions.
Operating temperature for non-recording type and recording type gauges:
— maximum temperature: +60 °C;
— minimum temperature (catching-type liquid precipitation measuring gauges, excluding heating or
weighing gauge without antifreeze): 0 °C;
— minimum temperature (catching-type liquid precipitation measuring gauge, with heating or
weighing gauge with antifreeze): –40 °C.
2
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ISO 23350:2021(E)
4.3.3 Relative humidity
Catching-type liquid precipitation measuring gauges shall operate within the following relative
humidity range.
Operating relative humidity for non-recording type and recording type gauges:
— maximum relative humidity: 100 % RH;
— minimum relative humidity: 0 % RH;
5 Recording
5.1 Recording format
For recording and non-recording type liquid precipitation gauges, the time stamp of each record to be
stored in a data storage gauge or manual recording shall be at least eight digits for the year, month and
day; and eight digits for the hour, minute, second and millisecond (e.g. 2019/12/31 09:00:00:00). For
-1
rainfall accumulation, there shall be at least five digits (e.g. 000,00 mm⋅h ), not including the decimal
place, for the liquid precipitation amount.
5.2 Recording interval
The recording interval for catching-type liquid precipitation measuring gauges shall be specified by the
network operator to ensure regional and climatological constraints are taken into consideration that
could affect the accuracy and precision of recorded measurements.
For recording type gauges, the measurement record of liquid precipitation should be transmitted to
the recording device instantaneously or at a specified time according to the gauge user. The recording
intervals are commonly hourly or every minute depending on gauge design and network purpose.
Non-recording type liquid precipitation gauges should be read manually at a specified interval to acquire
the data. Ordinary and storage gauges should be emptied in line with network operating principles,
commonly daily or monthly depending on gauge design and network purpose. Liquid precipitation is
reported per unit of time. The manufacturer or network operator should specify the volume of liquid
precipitation and time interval recorded in its most granular form.
6 Environmental protection and housing
The catching-type liquid precipitation measuring gauge should be constructed in a material that
is durable and can endure natural weathering without changes to its surface characteristics. Sound
watertight seams should be used throughout. The collecting funnel should fit firmly over the top of
the measurement mechanism but should be removable without undue force. The gauge should be
designed to minimize ingress by small animals and insects. The outer housing of the gauge should
also prevent water from entering the measuring component of the gauge, apart from through the inlet
funnel. Electrical connections for power supplies and outgoing signals should be provided using water-
resistant plugs or terminals, suitable for the environmental conditions in which the gauge is deployed.
[6]
Any marking identifying connectors should be weather-resistant and remain legible for the expected
lifetime of the catching-type liquid precipitation measuring gauge.
3
© ISO 2021 – All rights reserved

---------------------- Page: 8 ----------------------
ISO 23350:2021(E)
7 Installation
The manufacturer shall provide clear instructions for the installation of catching-type liquid
precipitation measuring gauges. As mentioned in 4.1, the position and exposure of raingauges should be
taken into consideration.
NOTE Measurements will be influenced by rim height, either through individual gauge design or local
deployment to allow for factors such as snow depth where close to ground level deployment is not practical. All
gauge types are expected to be positioned to ensure operation within the manufacturer’s design.
The rim of the gauge should be installed parallel to the ground surface. The design should maintain
plane of rim level to within ±2° of the base plane. Failure to ensure that the rim is level will introduce
a systematic error into the measurement. The measurement mechanism of the gauge, for example, a
tipping bucket mechanism, should be level to ensure there are no systematic errors.
The gauge should be installed securely to ensure that it remains stable. Impacts such as frost heave and
changes in ground conditions should be taken into account.
The gauge should be placed to ensure that the surrounding surface minimizes splashback that can
result in an over-catch. A surface of gravel or short grass is typical; and surfaces such as concrete should
be avoided.
8 Estimation of measurement uncertainty
8.1 General
The uncertainty of a value derived from primary measurements can be due to:
a) signal noise which affects the value being measured (variation by the precipitation intensity);
b) resolution of the measurement process;
c) other factors (wetting, splashing, evaporation, wind induced under-catch, volume-mass
conversions, time interval, etc.).
Type A and Type B are two methods of measurement uncertainty estimation for relating the dispersion
of values to the probability of “closeness” to the mean value (see Reference [7]).
NOTE Although installation and environmental factors are major sources of uncertainties, they are outside
the scope of this document.
8.2 Factors of measurement uncertainty
In practice, ther
...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 23350
ISO/TC 113/SC 5
Hydrometry — Catching-type liquid
Secretariat: ANSI
precipitation measuring gauges
Voting begins on:
2021­03­12
Voting terminates on:
2021­05­07
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 23350:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
©
NATIONAL REGULATIONS. ISO 2021

---------------------- Page: 1 ----------------------
ISO/FDIS 23350:2021(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
CH­1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved

---------------------- Page: 2 ----------------------
ISO/FDIS 23350:2021(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Instrument specifications . 1
4.1 General . 1
4.2 General gauge specifications . 2
4.3 Environment . 2
4.3.1 General. 2
4.3.2 Temperature . 2
4.3.3 Relative humidity . 3
5 Recording . 3
5.1 Recording format . 3
5.2 Recording interval . 3
6 Environmental protection and housing. 3
7 Installation . 4
8 Estimation of measurement uncertainty . 4
8.1 General . 4
8.2 Factors of measurement uncertainty . 4
8.3 Type-A estimation . 7
8.4 Type-B estimation . 7
8.5 Combined uncertainty . 7
Annex A (Informative) Catching-type liquid precipitation measuring gauges .8
Annex B (Informative) Uncertainties of catching-type liquid precipitation measuring gauges.15
Bibliography .21
© ISO 2021 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO/FDIS 23350:2021(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non­governmental, 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.
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 2021 – All rights reserved

---------------------- Page: 4 ----------------------
ISO/FDIS 23350:2021(E)

Introduction
Rainfall totals and intensity rates are measured at frequencies ranging from seconds to months, and the
data are required by the user on timescales ranging from immediate to several weeks. This document
covers instrumentation that measures rainfall accumulation and rainfall intensity (RI) by collecting
the volume of water that falls on a fixed area. The measurement unit and period of rainfall should be
decided on gauge design and network purpose by the gauge user.
Unlike many other meteorological instruments, there is no absolute physical standard against which
a raingauge (hereafter called catching-type liquid precipitation measuring gauges) can be compared.
Many different types, shapes and sizes of catching-type liquid precipitation measuring gauges are
acceptable for the measurement of rainfall and rainfall depth, each reflecting a specific requirement.
Most consist of a circular collecting device, delineating the fixed area of the sample, and a funnel leading
into a storage reservoir or measuring system, or both. Some types of automatic gauges do not require
a funnel. Since various sizes and shapes of orifice and gauge heights are used in different countries, the
measurements are not strictly comparable.
This document provides general information on the functions of catching-type liquid precipitation
gauges. The annexes in this document provide guidance on the types of catching-type liquid precipitation
measuring gauges currently available (see Annex A) and the measurement uncertainty associated with
them (see Annex B). Although advances in measurement electronic technologies and smart instruments
have led to the development of optical and hybrid type liquid precipitation measuring gauges, they are
not included in this document.
NOTE 1 This document specifies only the general functions of catching-type liquid precipitation measuring
gauges. For in-depth and extensive information on precipitation measuring gauges, see References [1] and [2].
NOTE 2 For detailed information on measurement unit and period of rainfall, see Reference [1].
NOTE 3 For detailed information on the design of a reference raingauge pit as well as installation, see
Reference [3].
© ISO 2021 – All rights reserved v

---------------------- Page: 5 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 23350:2021(E)
Hydrometry — Catching-type liquid precipitation
measuring gauges
1 Scope
This document specifies the typical requirements of instrumentation for measuring liquid precipitation,
primarily for the purpose of hydrological and meteorological observation. This document is applicable
to both non-recording and recording catching-type precipitation gauges for the measurement of liquid
precipitation. This document covers design criteria for the gauges and elements to be considered in
their construction. This document does not include specification, design and installation conditions.
NOTE 1 Since the measurement of solid precipitation is outside the scope, the disadvantages of solid
precipitation apply to all gauges listed in Annex A. The measurement of solid precipitation and associated
disadvantages are well documented in Reference [1].
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
BS 7843­3, Acquisition and management of meteorological precipitation data from a gauge network —
Part 3: Code of practice for the design and manufacture of storage and automatic collecting raingauges
3 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 772 and BS 7843-3 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 http:// www .electropedia .org/
4 Instrument specifications
4.1 General
Catching-type liquid precipitation measuring gauges are classified as non-recording and recording
types. The non-recording type has ordinary and storage gauges. The recording type has weighing,
tipping-bucket and floating types. Catching-type liquid precipitation measuring gauges have a
­1
resolution of 0,1 mm to 1 mm, and can measure rainfall intensities of 0,1 to 1 000 mm⋅h or higher.
Measurement errors can occur according to installation conditions, the measurement environment,
solid precipitation and rainfall intensity.
A catching-type liquid precipitation measuring gauge shall be of circular orifice and shall be formed
perpendicular to the outer slope and the inner surface with sharp edges. The diameter of the edge
of the collecting device should not differ by more than 0,2 % of the stated diameter in any direction.
Furthermore, the collector should be designed to prevent rain from splashing in and out. This can be
achieved if the vertical wall is sufficiently deep and the slope of the funnel is sufficiently steep (at least
45 %). The container should have a narrow entrance and be sufficiently protected from radiation in
© ISO 2021 – All rights reserved 1

---------------------- Page: 6 ----------------------
ISO/FDIS 23350:2021(E)

minimize the loss of water by evaporation. The surface of the funnel shall consist of a stable, durable
material such that the water drop is not retained by surface tension, is freely moved towards the orifice
and is passed to the measuring mechanism.
NOTE This document specifies the functional requirements of catching-type liquid precipitation measuring
gauges. Consideration of the siting and exposure of gauges is also important. More specifically, the installation
conditions, measurement environment and solid precipitation (see References [1] and BS 7843­3).
4.2 General gauge specifications
The gauge manufacturer should provide general specifications that can impact the measurement of
precipitation which include, but are not limited to:
a) range;
b) resolution;
c) accuracy;
d) size of orifice area;
e) standardized communication interfaces and protocols for data transmission;
f) power supply and consumption;
g) height of gauge (including rim height placement to avoid interference from deposited snow or other
materials near gauge);
i) gauge materials.
Requirements for uncertainty, range and resolution for precipitation measurements should be
considered. It should be be noted that, in some countries, trace observations are officially given a
value of zero, thus resulting in a biased underestimate of the seasonal precipititation. This problem is
minimized with weighing-type gauges since even very small amounts of precipitation will accumulate
over time.
4.3 Environment
4.3.1 General
Liquid precipitation measuring gauges shall operate within the ranges of temperature in 4.3.2 and the
ranges of relative humidity in 4.3.3. Wind should be taken into consideration in areas that are prone to
inclement weather conditions.
4.3.2 Temperature
Catching-type liquid precipitation measuring gauges should function in their intended manner in the
following environmental conditions.
Operating temperature for non-recording type and recording type gauges:
— maximum temperature: +60 °C;
— minimum temperature (catching-type liquid precipitation measuring gauges, excluding heating or
weighing gauge without antifreeze): 0 °C;
— minimum temperature (catching-type liquid precipitation measuring gauge, with heating or
weighing gauge with antifreeze): –40 °C.
2 © ISO 2021 – All rights reserved

---------------------- Page: 7 ----------------------
ISO/FDIS 23350:2021(E)

4.3.3 Relative humidity
Catching-type liquid precipitation measuring gauges shall operate within the following relative
humidity range.
Operating relative humidity for non-recording type and recording type gauges:
— maximum relative humidity: 100 % RH;
— minimum relative humidity: 0 % RH;
5 Recording
5.1 Recording format
For recording and non-recording type liquid precipitation gauges, the time stamp of each record to be
stored in a data storage gauge or manual recording shall be at least eight digits for the year, month and
day; and eight digits for the hour, minute, second and millisecond (e.g. 2019/12/31 09:00:00:00). For
­1
rainfall accumulation, there shall be at least five digits (e.g. 000,000 mm⋅h ), not including the decimal
place, for the liquid precipitation amount.
5.2 Recording interval
The recording interval for catching-type liquid precipitation measuring gauges shall be specified by the
network operator to ensure regional and climatological constraints are taken into consideration that
could affect the accuracy and precision of recorded measurements.
For recording type gauges, the measurement record of liquid precipitation should be transmitted to
the recording device instantaneously or at a specified time according to the gauge user. The recording
intervals are commonly hourly or every minute depending on gauge design and network purpose.
Non-recording type liquid precipitation gauges should be read manually at a specified interval to acquire
the data. Ordinary and storage gauges should be emptied in line with network operating principles,
commonly daily or monthly depending on gauge design and network purpose. Liquid precipitation is
reported per unit of time. The manufacturer or network operator should specify the volume of liquid
precipitation and time interval recorded in its most granular form.
6 Environmental protection and housing
The catching-type liquid precipitation measuring gauge should be constructed in a material that
is durable and can endure natural weathering without changes to its surface characteristics. Sound
watertight seams should be used throughout. The collecting funnel should fit firmly over the top of
the measurement mechanism but should be removable without undue force. The gauge should be
designed to minimize ingress by small animals and insects. The outer housing of the gauge should
also prevent water from entering the measuring component of the gauge, apart from through the inlet
funnel. Electrical connections for power supplies and outgoing signals should be provided using water­
resistant plugs or terminals, suitable for the environmental conditions in which the gauge is deployed.
[6]
Any marking identifying connectors should be weather-resistant and remain legible for the expected
lifetime of the catching-type liquid precipitation measuring gauge.
© ISO 2021 – All rights reserved 3

---------------------- Page: 8 ----------------------
ISO/FDIS 23350:2021(E)

7 Installation
The manufacturer shall provide clear instructions for the installation of catching-type liquid
precipitation measuring gauges. As mentioned in 4.1, the sitting and exposure of raingauges should be
taken into consideration.
NOTE Measurements will be influenced by rim height, either through individual gauge design or local
deployment to allow for factors such as snow depth where close to ground level deployment is not practical. All
gauge types are expected to be sited to ensure operation within the manufacturer’s design.
The rim of the gauge should be installed parallel to the ground surface. The design should maintain
plane of rim level to within ±2° of the base plane. Failure to ensure that the rim is level will introduce a
systematic error into measurement. The measurement mechanism of the gauge, for example, a tipping
bucket mechanism, should be level to ensure there are no systematic errors.
The gauge should be installed securely to ensure that it remains stable. Impacts such as frost heave and
changes in ground conditions should be taken into account.
The gauge should be sited to ensure that the surrounding surface minimizes splashback that can result
in an over-catch. A surface of gravel or short grass is typical; and surfaces such as concrete should be
avoided.
8 Estimation of measurement uncertainty
8.1 General
The uncertainty of a value derived from primary measurements can be due to:
a) signal noise which affects the value being measured (variation by the precipitation intensity);
b) resolution of the measurement process;
c) other factors (wetting, splashing, evaporation, wind induced under­catch, volume­mass
conversions, time interval, etc.).
Type A and Type B are two methods of measurement uncertainty estimation for relating the dispersion
of values to the probability of “closeness” to the mean value (see Reference [7]).
NOTE Although installation and environmental factors are major sources of uncertainties, they are outside
the scope of this document.
8.2 Factors of measurement uncertainty
In practice, there are many possible sources of uncertainty in a measurement, including:
a) incomplete definition of the measurement;
b) imperfect realization of the definition of the measurement;
c) nonrepresentative sampling – the possibility that the sample measured is not representative of the
defined measurement;
d) inadequate knowledge of the effects of environmental conditions on the measurement or imperfect
measurement of environmental conditions;
e) personal bias in reading analogue instruments;
f) finite instrument resolution or discrimination threshold;
g) inexact values of measurement standards and reference materials;
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h) inexact values of constants and other parameters obtained from external sources and used in the
data­reduction algorithm;
i) approximations and assumptions incorporated in the measurement method and procedure;
j) variations in repeated observations of the measurand under apparently identical conditions.
Table 1 provides an example of operational measurement uncertainty requirements and instrument
performance requirements.
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6 © ISO 2021 – All rights reserved
Table 1 — Operational measurement uncertainty requirements and
instrument performance requirements
1 2 3 4 5 6 7 8 9
Variable Range Reported Mode of Required Instrument Output Achievable Remarks
(precipitation) resolution measurement/ measurement time averaging measurement
observation uncertainty constant time uncertainty
Amount 0 mm–500 mm 0,1 mm T 0,1 mm for n/a n/a The larger of Quantity based on daily
(daily) ≤ 5 mm 5 % or 0,1 mm amounts.
2 % for > 5 mm Measurement uncertainty
depends on aerodynamic
collection efficieny of gauges
and evaporative losses in
heated gauges.
Depth of snow 0 m–25 m 1 cm I 1 cm for ≤ 20 cm < 10 s 1 min 1 cm Average depth over an area
representative of the observ­
5 % for > 20 cm
ing site.
Thickness of not specified 1 cm I 1 cm for ≤ 20 cm
ice accretion on
10 % for > 20 cm
ships
­1 ­1
Precipitation 0,02 mm h – 0,1 mm h I (trace): n/a for < 30 s 1 min Under constant Uncertainty values for liquid
­1 ­1
intensity 2 000 mm h 0,02–0,2 mm h flow conditions precipitation only.
in laboratory:
­1
0,1 mm h Uncertainty is seriously af­
5 % above
­1
for 0,2–2 mm h fected by wind.
­1
2 mm h ,
5 % for 2 % above Instruments can show signifi­
­1 ­1
> 2 mm h 10 mm h . cation of non­linear behaviour.
­1
In field:
For < % above 0,2 mm h :
­1
5 mm h
detection only (yes/no)
and
instrument time constant is
5 % above
significantly affected during
­1
100 mm h .
solid precipitation using
catchment type of gauges.
Precipitation 0–24 h 60 s T n/a 60 s Threshold value of
­1
duration (daily) 0,02 mm h .

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8.3 Type-A estimation
Type-A estimation is a method of evaluation of uncertainty by the statistical analysis of series of
observations. It stems from manual reading and evaporation and wetting of the instrument.
8.4 Type-B estimation
Type-B estimation is a method of evaluation of uncertainty by means other than the statistical analysis
of series of observations. It stems from resolution, aging and random error of the instrument.
8.5 Combined uncertainty
To determine the uncertainty of the derived value, U, it is necessary to combine the uncertainties of all
primary measurements, u, thus:
1
22
 
Uu=+ u 2 (1)
cA B
where
U
is combined uncertainty;
c
2
is the uncertainty of type-A;
u
A
2
is the uncertainty of type-B.
u
B
Formula (1) illustrates the method when combining the uncertainty of a reference precipitation datum
value. Other components of measurement uncertainty are added by inclusion of their squared value
within the brackets.
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Annex A
(Informative)

Catching-type liquid precipitation measuring gauges
A.1 Non-recording gauges
A.1.1 Ordinary gauge
A.1.1.1 Explanation
The ordinary gauge is composed of the collector orifice, a storage with a funnel shape and a bucket
2
(Figure A.1). The size of the opening of the collector is important. A receiving area of 1 000 cm is used
2 2
in some countries, but an area of 200 cm to 500 cm will probably be most convenient. The area of the
receiver can be made to equal .01 of the area of the collector. Storing rain or solid precipitation during
the observation hours, the graduated cylinder shall be used to measure the accumulated precipitation.
Key
1 funnel
2 measuring gauge
Figure A.1 — Ordinary gauge
A.1.1.2 Materials
An ordinary gauge is made of various types of metal or plastic materials with proper thermal expansion
coefficient. They should be manufactured to minimize errors due to wetting loss and evaporation.
A.1.1.3 Advantages
An ordinary gauge is easy to install and measure and is available to measure precipitation accurately at
a low cost.
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A.1.1.4 Disadvantages
Ordinary gauges must be read manually and cannot determine the exact start and end of the
precipitation occurrence. The timing cannot be determined between individual events within the
recording period. Uncertainty factors exist due to measurement time and scale indication. Evaporation
can result in under-reading, particularly if measurements are taken less frequently than daily.
A.1.1.5 Uncertainties
The uncertainties of ordinary gauges are detailed in Annex B.
A.1.2 Storage gauge
A.1.2.1 Explanation
An observer of a storage gauge can either measures the weight or read the graduated cylinder of a bucket
after collecting rainwater in the bucket for a certain period, such as measuring the total precipitation in
a remote area.
The gauge is composed of a collecting part with a funnel and a storage part that can hold enough
precipitation between scheduled readings (Figure A.2). Typically, proper oil of around 5 mm shall be
put in a measuring bottle and a removable inner can for overspill to prevent the evaporation of water.
This oil shall have a property to pass through collected water freely to prevent evaporation.
When a storage gauge is located far from the observer, the gauge requires special care in terms
of operation and management. For example, the installation of a video camera can provide valuable
information about the conditions at a gauging site. The extent of ice cover, periods of backwater due to
ice, etc., can be documented by a camera.
Key
1 funnel
2 bucket
Figure A.2 — Storage gauge
A.1.2.2 Materials
Storage gauges are made of various types of metal or plastic materials with proper thermal expansion
coefficient. They should be manufactured to minimize the loss of water by the wet error and evaporation.
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A.1.2.3 Advantages
This type of gauge is easy to install and measure, and is available to accurately measure precipitation
at a low cost.
A.1.2.4 Disadvantages
As it is not possible to measure in real-time and there exist many cases of poor accessibility, the gauge
requires monitoring to check if the the funnel is plugged. In addition, this type of gauge requires oil or
antifreeze to prevent evaporation, but such substances can cause environmental pollution.
Storage gauges must be read manually and cannot determine the exact start and end of the precipitation
occurrence. The timing cannot be determined between individual events wi
...