Hydrometry - Measurement requirements and classification of rainfall intensity measuring instruments

This document considers liquid atmospheric precipitation and defines the procedures and equipment to perform laboratory and field tests, in steady-state conditions, for the calibration, check and metrological confirmation of liquid precipitation measurement instruments. It provides a classification of catching-type measurement instruments based on their laboratory performance. The classification does not relate to the physical principle used for the measurement, nor does it refer to the technical characteristics of the instrument assembly, but is solely based on the instrument calibration. Attribution of a given class to an instrument is not intended as a high/low ranking of its quality but rather as a quantitative standardized method to declare the achievable measurement accuracy in order to provide guidance on the suitability for a particular purpose, while meeting the user’s requirements.

Hydrometrie - Messbedingungen und Klassifizierung von Geräten zur Messung der Regenintensität

Dieses Dokument berücksichtigt flüssigen atmosphärischen Niederschlag und legt Verfahren und Ausrüstung zur Durchführung von Labor  und Feldprüfungen unter stationären Bedingungen für die Kalibrierung, Prüfung und metrologische Bestätigung von Geräten zur Messung von flüssigem Niederschlag fest. Es enthält eine Klassifizierung von Messgeräten mit Auffangvorrichtung, basierend auf ihrer Leistung im Labor. Die Klassifizierung bezieht sich weder auf das für die Messung verwendete physikalische Prinzip noch auf die technischen Eigenschaften der Gerätebaugruppe, sondern basiert alleine auf der Gerätekalibrierung. Die Zuordnung einer bestimmten Klasse zu einem Gerät ist nicht als Einstufung der Qualität in hochrangig oder niederrangig vorgesehen, sondern eher als quantitatives standardisiertes Verfahren, um die erreichbare Messgenauigkeit anzugeben und somit Leitlinien zur Eignung für einen bestimmten Zweck zu geben und gleichzeitig die Anforderungen des Nutzers zu erfüllen.

Hydrométrie - Exigences de mesure et classification des instruments de mesure d'intensité pluviométrique

Le présent document couvre les précipitations atmosphériques liquides et définit les modes opératoires et l’équipement permettant d’effectuer des essais en laboratoire et sur le terrain, dans des conditions stables, pour l’étalonnage, le contrôle et la confirmation métrologique des instruments de mesure des précipitations liquides. Il fournit une classification des pluviomètres collecteurs d’après leurs performances en laboratoire. La classification ne concerne ni le principe physique utilisé pour le mesurage, ni les caractéristiques techniques de l’ensemble de l’instrument, mais uniquement l’étalonnage de l’instrument. L’attribution d’une classe à un instrument n’est pas destinée à servir de classement de sa qualité mais plutôt de méthode quantitative normalisée pour déclarer l’exactitude de mesure atteignable afin de fournir des recommandations sur l’adéquation avec un objectif particulier, tout en satisfaisant aux exigences de l’utilisateur.

Hidrometrija - Merilne zahteve in razvrstitev instrumentov za merjenje moči padavin

Ta standard obravnava tekoče padavine in določa razvrstitev instrumentov za merjenje moči padavin s posodo za zbiranje padavin na podlagi usposobljenosti laboratorija. Standardizirani preskusi umerjanja so opisani za oceno točnosti teh naprav za merjenje količine padavin v laboratoriju in na terenu. Razvrstitev ni povezana s fizikalnim načelom, ki se uporablja za merjenje, prav tako se ne navezuje na tehnične lastnosti sestava merilnih instrumentov. Razvrstitev temelji izključno na točnosti umerjanja naprave za merjenje moči padavin.

General Information

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Published
Publication Date
15-Oct-2019
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
16-Oct-2019
Completion Date
16-Oct-2019

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SLOVENSKI STANDARD
SIST EN 17277:2019
01-december-2019
Hidrometrija - Merilne zahteve in razvrstitev instrumentov za merjenje moči
padavin
Hydrometry - Measurement requirements and classification of rainfall intensity
measuring instruments

Messung der Regenintensität - Messbedingungen und Klassifizierung für auffangende

Regenmesser
Hydrométrie - Exigences de mesure et classification des instruments de mesure
d'intensité pluviométrique
Ta slovenski standard je istoveten z: EN 17277:2019
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
SIST EN 17277:2019 en,fr,de

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

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SIST EN 17277:2019
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SIST EN 17277:2019
EN 17277
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2019
EUROPÄISCHE NORM
ICS 07.060
English Version
Hydrometry - Measurement requirements and
classification of rainfall intensity measuring instruments

Hydrométrie - Exigences de mesure et classification Messung der Regenintensität - Messbedingungen und

des instruments de mesure d'intensité pluviométrique Klassifizierung für auffangende Regenmesser

This European Standard was approved by CEN on 19 August 2019.

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

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

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

member.

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

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

Centre has the same status as the official versions.

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

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

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

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

© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 17277:2019 E

worldwide for CEN national Members.
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Contents Page

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

Introduction .................................................................................................................................................................... 4

1 Scope .................................................................................................................................................................... 5

2 Normative references .................................................................................................................................... 5

3 Terms and definitions ................................................................................................................................... 5

4 User requirements for RI measurements .............................................................................................. 8

5 Measurement of RI ......................................................................................................................................... 8

6 Classification of RI gauges ......................................................................................................................... 10

Bibliography ................................................................................................................................................................. 18

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European foreword

This document (EN 17277:2019 ) has been prepared by Technical Committee CEN/TC 318

“Hydrometry”, the Secretary of which is held by BSI.

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

identical text or by endorsement, at the latest by April 2020, and conflicting national standards shall be

withdrawn at the latest by April 2020.

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

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

This document has been developed from the following:

— CEN/TR 16469:2013 Measurement of the rainfall intensity: requirements, calibration methods

and field measurements,

— UNI 11452:2012 Hydrometry - Liquid precipitation intensity: measurements requirements and

calibration methods for catching-type gauges

— BS 7843-3:2012 Code of practice for the design and manufacture of storage and automatic

collecting rain gauges

— WMO Guide to Meteorological Instruments and Methods of Observation, WMO-n. 8, ed. 2014

(updated 2017). ISBN 978-92-63-10008-5.

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

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

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

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

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

United Kingdom.
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Introduction

Precipitation gauges are one of the basic components of world hydro-metrological networks. A

requirement for more accurate instruments is crucial for many applications including water resources

management, public safety and disaster mitigation.

This standard provides a consistent process for classification of catching type rainfall intensity gauges

in laboratory conditions.

This standard will allow users to buy and use a rainfall intensity gauge knowing that it will perform to a

specific class of performance before it is deployed to the field.
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1 Scope

This document considers liquid atmospheric precipitation and defines the procedures and equipment

to perform laboratory and field tests, in steady-state conditions, for the calibration, check and

metrological confirmation of liquid precipitation measurement instruments. It provides a classification

of catching-type measurement instruments based on their laboratory performance. The classification

does not relate to the physical principle used for the measurement, nor does it refer to the technical

characteristics of the instrument assembly, but is solely based on the instrument calibration.

Attribution of a given class to an instrument is not intended as a high/low ranking of its quality but

rather as a quantitative standardized method to declare the achievable measurement accuracy in order

to provide guidance on the suitability for a particular purpose, while meeting the user’s requirements.

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.

EN ISO 10012:2003, Measurement management systems - Requirements for measurement processes and

measuring equipment ISO 10012:2003)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at https://www.iso.org/obp
3.1
precipitation (snowfall and rainfall)

the liquid or solid product of the condensation of water vapour falling from clouds or deposited from air

onto the ground; it includes rain, hail, snow, dew, rime, hoar frost and fog precipitation

Note 1 to entry: The total amount of precipitation that reaches the ground in a stated period is defined “rainfall”

when precipitation is liquid and “snowfall” when the precipitation is snow.

Note 2 to entry: Rainfall (total amount of liquid precipitation) is expressed in terms of the vertical depth of water

(usually in millimetres, mm) to which it would cover a horizontal projection of the Earth’s surface.

Note 3 to entry: Snowfall (total amount of snow) is expressed in terms of the vertical depth of water equivalent to

which it would cover a horizontal projection of the Earth’s surface. Snowfall is also expressed by the depth of

fresh, newly fallen snow covering an even horizontal surface.
[SOURCE: WMO no.8 “CIMO Guide” Part I Chap. 6 new edition 2014]
3.2
rainfall intensity

the amount of liquid precipitation (rainfall) collected per unit time interval; due to its variability from

minute to minute, RI is measured or derived (from the measurement of the amount) over 1 minute time

intervals and the measurement units are vertical depth of water per hour, usually in millimetres per

hour or mm h
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Note 1 to entry: The RI is derived or measured directly using only rainfall intensity gauges (see definition 3.4).

[SOURCE: WMO no.8 “CIMO Guide” Part I Chap. 6 new edition 2014]
3.3
catching type rain gauge

rain gauge which collects precipitation through an orifice, often a funnel, of well-defined size and

measures its water equivalent, volume, mass or weight that has been accumulated in a certain amount

of time

Note 1 to entry: This type of gauge includes storage, level monitoring, tipping bucket and weighing rain gauges.

These are the most common type of recording rain gauge in use in operational networks at the time of preparing

this text.
3.4
rainfall intensity gauge
RI gauge

automatic recording rain gauge which measures RI at a resolution of at least one minute

3.5
delay time of the output of a RI gauge

possible time delay between the output signal of a RI gauge and the time when the measurement was

performed

Note 1 to entry: This delay is usually due to internal calculations of the rain gauge.

Note 2 to entry: The internal calculation of the rainfall intensity in some rain gauges can cause a delay of the

output data message (e.g. 1 min) that can easily be shifted automatically to the correct time without any

degradation in measurement accuracy. This is typical of software corrected tipping bucket rain gauges through

embedded electronic chips or interfaces. The delay time should not be confused with the time constant. If real-

time output is not needed, software induced delay times are less critical than longer time constants or any other

effects, because delay times can easily be corrected to retrieve the original RI information.

[SOURCE: WMO IOM – 99]
3.6
measurand
quantity intended to be measured
[SOURCE: VIM 3rd edition, JCGM 200:2012]
3.7
measurement uncertainty

non-negative parameter characterizing the dispersion of the quantity values being attributed to a

measurand, based on the information used
[SOURCE: VIM 3rd edition, JCGM 200:2012]

Note 1 to entry: The parameter may be, for example, a standard deviation called standard measurement

uncertainty (or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.

Note 2 to entry: Measurement uncertainty comprises, in general, many components. Some of these may be

evaluated by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values

from series of measurements and can be characterized by standard deviations. The other components, which may

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be evaluated by Type B evaluation of measurement uncertainty, can also be characterized by standard deviations,

evaluated from probability density functions based on experience or other information.

Instrumental measurement uncertainty (VIM 3rd edition, JCGM 200:2012): component of measurement

uncertainty arising from a measuring instrument or measuring system in use

Instrumental uncertainty is used in a Type B evaluation of measurement uncertainty

Achievable measurement uncertainty (WMO no. 8, Part I Annex 1.B): it is intended as the measurement

uncertainty achievable in field and/or operational conditions
3.8
non-catching rain gauge
rain gauge where the rain is not collected in a container/vessel

Note 1 to entry: The rainfall intensity or amount is either determined by a contact-less measurement using

optical or radar techniques or by an impact measurement. This type of gauge includes optical disdrometers,

impact disdrometers, microwave radar disdrometers, optical/capacitive sensors.
3.9
resolution

smallest change in a quantity being measured that causes a perceptible change in the corresponding

indication
[SOURCE: VIM 3rd edition, JCGM 200:2012]
3.10
step function or unit step function

input signal that switches on at a specified time and stays switched on indefinitely for determining the

response (output) of a dynamic instrument system
[SOURCE: CEN/TR 16469:2013]
3.11
step response

time-varying response of an instrument system to a step function (heaviside step function)

[SOURCE: CEN/TR 16469:2013]
3.12
step response time

duration between the instant when an input quantity value of a measuring instrument or measuring

system is subjected to an abrupt change between two specified constant quantity values and the instant

when a corresponding indication settles within specified limits around its final steady value

[SOURCE: VIM 3rd edition, JCGM 200:2012]
3.13
time constant

rise time characterizing the response of an instrument classified as a system of first order response (the

way the system responds is approximated by a first order differential equation)

Note 1 to entry: It represents the time that the step response of an instrument system takes to reach the (1–

1/e)•100[%] approximately 63 % of the final or asymptotic value.
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[SOURCE: CEN/TR 16469:2013]
4 User requirements for RI measurements

This standard defines three classes of RI gauges according to their calibration. The standard will

describe the three classes, the laboratory calibration methods and the requirements for checking the

calibration in the field. The user shall determine what class of rain gauge to use for any given purpose,

based on the local hydro-geological and meteorological conditions. The network/instrument manager

shall declare the classification at the applicable RI ranges. Data from unclassified rain gauges shall be

used with caution.
5 Measurement of RI
5.1 General

Rainfall intensity (RI) is defined as the amount of liquid precipitation (rainfall) collected per unit time

interval. Due to its variability from minute to minute, there is an agreement of measuring RI over 1 min

time intervals and then RI in mm/hour is derived from the measurements taken in 1 min. RI is

measured directly using rainfall intensity gauges, for instance, using a gauge and measuring the flow of

the captured water, or the increase in collected water as a function of time. A number of measurement

techniques for the determination of the amount of precipitation are based on these direct intensity

measurements by integrating the measured intensity over a certain time interval.

Traditionally, the volume of liquid precipitation received by a collector through an orifice of known

surface area in a given period of time is assumed as the reference quantity, namely the rainfall amount.

Under the restrictive hypothesis that rainfall is constant over the accumulation period, a derived

quantity – the rainfall rate or intensity – can be easily calculated. The shorter the time interval used for

the calculation, the nearer to the real rate of precipitation reaching the ground. This approximate

measure of the rainfall intensity has been accepted for a long time as sufficiently accurate to meet the

requirements of both scientific and technical applications. Reasons for this are on the one hand that

most traditional applications in hydrology operate at the basin scale, thus dealing with a process of

rainfall aggregation on large space and time scales, while on the other hand the available technology of

measurement instruments, especially in terms of data storage and transmission capabilities, was lower

than is currently available.

Rainfall data requirements have become tighter and applications increasingly require enhanced quality

in RI measurements. The interpretation of rainfall patterns, rainfall event models and forecasting

efforts, everyday meteorological and engineering applications, etc., are all based on the analysis of

rainfall intensity arrays that are recorded at very fine intervals in time. The importance of RI

measurement is dramatically increased and very high values of RI are recorded, due to the shortening

of the reference period.

It is worth noting that the time scales required for calculation of RI at the ground are now much shorter

than in traditional applications. The design and management of urban drainage systems, flash flood

forecasting and mitigation, transport safety measures, and in general most of the applications where

rainfall data are sought in real-time, call for enhanced resolution in time (and space), even down to the

scale of one minute in many cases (1-MIN RI).
5.2 RI measurement accuracy

According to [17], the WMO “CIMO Guide” (Annex 1.E), the following values of expanded uncertainty

apply for precipitation intensity (liquid) measurements, in laboratory (calibration in constant flow

conditions) and in field conditions:
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Table 1 — Uncertainty of precipitation measurements according to WMO
Under constant flow conditions in laboratory 5 % above 2 mm/h
2 % above 10 mm/h
In field conditions 5 mm/h, and
5 % above 100 mm/h

The definitions introduced by the WMO and the corresponding values of the maximum acceptable

measurement uncertainties are adopted by this standard and, therefore, they shall be taken into

consideration for any catching type RI gauge.

The compliance to this standard does not include further sources of instrumental errors such as

sampling errors in tipping-bucket rain gauges.
5.3 Types of rain gauge

Rain gauges can be categorized in two main groups: (a) catching, and (b) non-catching types of rainfall

intensity measurement instruments ([16]). Gauges of the first group collect precipitation through an

orifice of well-defined size and measure its water equivalent volume, mass or weight that has been

accumulated in a certain amount of time. At present, catching type gauges are widely used in

operational hydro-meteorological networks to measure rainfall amount and intensity. Instruments of

the second group determine the rainfall amount or intensity either by a contactless measurement using

optical or radar techniques or by an impact measurement. A standardized procedure for the calibration

of non-catching rain gauges is not yet available.
Catching type rain gauges can be characterized as follows:
— they can be calibrated in the laboratory;

— they are able to measure RI within sampling time intervals ranging from a few seconds to several

minutes;
— they have finite resolution ranging from 0,001 mm to 1 mm;
— they have reasonably good reproducibility and long-term stability;
— they are widely used in operational practice and are cost effective;

— they are prone to wind-induced catching losses (depending on appropriate wind shielding);

— they are prone to wetting and evaporation losses, especially in low RI;

— regular maintenance, annual calibration and servicing, is needed to obtain high quality

measurements.

The majority of catching type gauges used in operational networks are weighing gauges (WGs) and

tipping bucket rain gauges (TBRGs) (see [16] for details).

In weighing gauges, precipitation is collected and continuously weighed. The WGs are those

instruments where the volume of water is derived by using the gravitational acceleration and the

density of water. These rain gauges do not use any moving mechanical parts in the weighing

mechanism, only elastic deformation occurs. Therefore, mechanical degradation and consequently the

need for maintenance are significantly reduced. The weighing is accomplished by various methods, e.g.

a frequency measurement of a string suspension, a strain gauge, or load cells measuring collected

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precipitation as change of measured weight increase by method of Wheatstone bridge. The digitized

output signal is generally averaged and filtered.

A tipping bucket rain gauge uses a metallic or plastic twin bucket balance to measure the incoming

water in portions of equal weight. When one bucket is full, its centre of mass is outside the pivot and the

balance tips, dumping the collected water and bringing the other bucket into position to collect. The

water mass content of the b
...

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