ISO/FDIS 23350

Hydrometry -- Catching-type liquid precipitation measuring gauges

Hydrométrie -- Appareils de mesure de précipitations liquides de type collecteur

General Information

• Status: Published
• 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/WG 7 - Hydrometry - Precipitation measuring devices
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 12-Mar-2021
• Completion Date: 12-Mar-2021

Standards Content (sample)

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
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ISO/FDIS 23350:2021(E)
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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

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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

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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).

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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.
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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].
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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

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

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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.
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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

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.
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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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ISO/FDIS 23350:2021(E)

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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ISO/FDIS 23350:2021(E)
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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:
0,1 mm h Uncertainty is seriously af­
5 % above
for 0,2–2 mm h fected by wind.
2 mm h ,
5 % for 2 % above Instruments can show signifi­
­1 ­1
> 2 mm h 10 mm h . cation of non­linear behaviour.
In field:
For < % above 0,2 mm h :
5 mm h
detection only (yes/no)
and
instrument time constant is
5 % above
significantly affected during
100 mm h .
solid precipitation using
catchment type of gauges.
Precipitation 0–24 h 60 s T n/a 60 s Threshold value of
duration (daily) 0,02 mm h .
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ISO/FDIS 23350:2021(E)
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:
 
Uu=+ u 2 (1)
cA B
where
is combined uncertainty;
is the uncertainty of type-A;
is the uncertainty of type-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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ISO/FDIS 23350:2021(E)
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

(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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ISO/FDIS 23350:2021(E)
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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ISO/FDIS 23350:2021(E)
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
...