ISO/FDIS 23350

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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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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This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 5,

Any feedback or questions on this document should be directed to the user’s national standards body. A

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

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

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

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

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

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.

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

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:

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,

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

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

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).

The gauge manufacturer should provide general specifications that can impact the measurement of

g) height of gauge (including rim height placement to avoid interference from deposited snow or other

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

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

Catching-type liquid precipitation measuring gauges should function in their intended manner in the

— minimum temperature (catching-type liquid precipitation measuring gauges, excluding heating or

— minimum temperature (catching-type liquid precipitation measuring gauge, with heating or

Catching-type liquid precipitation measuring gauges shall operate within the following relative

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

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

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

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.

Any marking identifying connectors should be weather-resistant and remain legible for the expected

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

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

The gauge should be installed securely to ensure that it remains stable. Impacts such as frost heave and

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

a) signal noise which affects the value being measured (variation by the precipitation intensity);

c) other factors (wetting, splashing, evaporation, wind induced under­catch, volume­mass

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

In practice, there are many possible sources of uncertainty in a measurement, including:

c) nonrepresentative sampling – the possibility that the sample measured is not representative of the

d) inadequate knowledge of the effects of environmental conditions on the measurement or imperfect

h) inexact values of constants and other parameters obtained from external sources and used in the

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

Amount 0 mm–500 mm 0,1 mm T 0,1 mm for n/a n/a The larger of Quantity based on daily

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

Precipitation 0,02 mm h – 0,1 mm h I (trace): n/a for < 30 s 1 min Under constant Uncertainty values for liquid

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.

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.

To determine the uncertainty of the derived value, U, it is necessary to combine the uncertainties of all

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

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

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.

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.

An ordinary gauge is easy to install and measure and is available to measure precipitation accurately at

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.

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

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

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.

This type of gauge is easy to install and measure, and is available to accurately measure precipitation

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