EN 18097:2025

SLOVENSKI STANDARD
01-februar-2026
Hidrometrija - Merjenje intenzivnosti padavin - Meroslovne zahteve in preskusne
metode za nelovne merilnike dežja
Hydrometry - Measurement of precipitation intensity - Metrological requirements and test
methods for non-catching type rain gauges
Hydrometrie - Messung der Niederschlagsintensität - Metrologische Anforderungen und
Prüfverfahren für nicht auffangende Niederschlagsmessgeräte
Hydrométrie - Mesurage de l'intensité des précipitations - Exigences métrologiques et
méthodes d'essai relatives aux pluviomètres non collecteurs
Ta slovenski standard je istoveten z: EN 18097:2025
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 18097
EUROPEAN STANDARD
NORME EUROPÉENNE
December 2025
EUROPÄISCHE NORM
ICS 07.060
English Version
Hydrometry - Measurement of precipitation intensity -
Metrological requirements and test methods for non-
catching type rain gauges
Hydrométrie - Mesurage de l'intensité des Hydrometrie - Messung der Niederschlagsintensität -
précipitations - Exigences métrologiques et méthodes Metrologische Anforderungen und Prüfverfahren für
d'essai relatives aux pluviomètres non collecteurs nicht auffangende Niederschlagsmessgeräte
This European Standard was approved by CEN on 26 October 2025.

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, Türkiye 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
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 18097:2025 E
worldwide for CEN national Members.

Content Page
European foreword . 3
Introduction . 4
1 Scope . 5
2 Normative references . 5
3 Terms, definitions and symbols . 5
3.1 Terms and definitions . 5
3.2 Symbols . 5
4 Measuring rainfall intensity with non-catching rain gauges . 7
4.1 General. 7
4.2 Optical . 7
4.3 Impact . 7
4.4 Radar . 8
5 Classification of non-catching rain gauges . 8
5.1 Characteristics of the calibration device . 8
5.2 Testing protocol . 8
Annex A (informative) Example of the use of the test method to calibrate a sample disdrometer in
the laboratory . 16
Bibliography . 20

European foreword
This document (EN 18097:2025) has been prepared by Technical Committee CEN/TC 318 “Hydrometry”,
the secretariat of which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by June 2026, and conflicting national standards shall be
withdrawn at the latest by June 2026.
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.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
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, Türkiye and the United
Kingdom.
Introduction
According to CEN/TR 17993:2023 “Calibration and accuracy of non-catching precipitation measurement
instruments” [10], although many attempts have been made and various approaches have been tested, no
fully traceable calibration procedure exists for most of the non-catching gauges (NCGs) available on the
market.
The document was prepared following a request for research development submitted by CEN/TC 318 to
EURAMET, the European Association of National Metrology Institutes in October 2017, through the
cooperation programme between STAIR (the joint CEN-CENELEC strategic Working Group supporting
standardisation in research and innovation) and EMPIR (the EURAMET’s European Metrology
Programme for Innovation and Research). This led to the approval and funding of the EURAMET pre-
normative Research Project “EMPIR 18NRM03 - INCIPIT Calibration and accuracy of non-catching
instruments to measure liquid/solid atmospheric precipitation” (Merlone et al., 2022 [1]), in which a
calibration procedure was developed and proposed for consideration as a basis for standardisation.

1 Scope
This document considers liquid atmospheric precipitation (rain) and defines the procedures and
equipment to perform laboratory tests, in steady-state conditions, for the calibration, check and
metrological confirmation of non-catching rainfall measurement instruments. This document is not
applicable to field performance.
It provides a classification of non-catching 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 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 17277:2019, Hydrometry — Measurement requirements and classification of rainfall intensity
measuring instruments
3 Terms, definitions and symbols
3.1 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
3.2 Symbols
RI -1 rainfall intensity
mm h
C  numerical factor for the conversion of the rate of rainfall from
R
-1 -1 6
[m s ] to [mm h ], equal to 3,6·10
D mm nominal/generic diameter of the sphere having the same volume of the drop
(equivolumetric spherical drop)
D mm minimum diameter of the equivolumetric spherical drop
min
D mm maximum diameter of the equivolumetric spherical drop
max
N -3 -1 number of drops per unit volume of air and unit drop size interval
m mm
V 3 drop volume
m
W -1 terminal velocity of the falling drop
T m s
W -1 generic fall velocity of the drop
m s
D mm equivolumetric spherical diameter of the generated reference drop
ref
D mm equivolumetric spherical diameter of the drop as measured by the instrument
meas
under test
W -1 reference fall velocity of the generated drop
ref
m s
W -1 fall velocity of the generated drop as measured by the instrument under test
meas
m s
mm mean value of the equivolumetric spherical diameter of a set of generated
µ
Dref
reference drops
mm standard deviation of the equivolumetric spherical diameter of a set of
σ
Dref
generated reference drops
-1 mean value of the fall velocity of a set of generated reference drops
µ
m s
Wref
-1 standard deviation of the fall velocity of a set of generated reference drops
σ
m s
Wref
mm mean value of the equivolumetric spherical diameter of a set of generated
µ
Dmeas
drops as measured by the instrument under test
mm standard deviation of the equivolumetric spherical diameter of a set of
σ
Dmeas
generated drops as measured by the instrument under test
-1 mean value of the fall velocity of a set of generated drops as measured by the
µ
m s
Wmeas
instrument under test
-1 standard deviation of the fall velocity of a set of generated drops as measured
σ
m s
Wmeas
by the instrument under test
-3 -1 scale parameter of the exponential form of the drop size distribution
N
m mm
Λ -1 slope parameter of the exponential form of the drop size distribution
mm
-1 rainfall intensity of the reconstructed target (reference) event
RI
mm h
ref
mm i-th mean reference diameter chosen for the discretisation of the target event
µ
D
i,ref
mm i-th reference drop size bin
∆µ
D
i,ref
-1 rainfall intensity of the target event as measured by the instrument under test
RI
mm h
meas
e
% generic percentage relative deviation
R
% interval embracing 80% of the percentage relative deviations calculated on a
set of generated drops
e
% 10° percentile of the percentage relative deviations calculated on a set of
q
generated drops
e % 90° percentile of the percentage relative deviations calculated on a set of
q
generated drops
k coverage factor, assumed equal to 1,28
% mean value of the percentage relative deviations of the equivolumetric drop
µ
eD
( )
diameter of a set of measured and generated reference drops
% mean value of the percentage relative deviations of the fall velocity of a set of
µ
eW
( )
measured and generated reference drops
% standard deviation of the percentage relative deviations of the equivolumetric
σ
eD
( )
drop diameter of a set of measured and generated reference drops
% standard deviation of the percentage relative deviations of the fall velocity of
σ
eW
( )
a set of measured and generated reference drops

4 Measuring rainfall intensity with non-catching rain gauges
4.1 General
The initial manual measurement methods used to study hydrometeor characteristics have evolved with
advances in technology and electronics. Nowadays, various techniques are used to determine the
characteristics of liquid/solid particles, such as devices to measure the displacement and mechanical
energy caused by raindrops/graupels hitting a surface, and optical detection, which measures the size,
shape, speed and diameter of hydrometeors as they pass through a light or laser beam, etc. These
instruments are called “distrometers” (or more commonly “disdrometers”) when they provide a measure
of the drop size distribution (DSD).
4.2 Optical
Optical disdrometers use visible or infrared light to detect falling hydrometeors. The instruments
available use different technical solutions but all of them present a similar structure. The instrument is
equipped with an infrared or visible light emitter/transmitter to illuminate a volume of the atmosphere
and with an optical sensor to detect the emitted light. The illuminated measurement volume is usually
defined by the shape of the lens and the relative position between the emitter and the receiver. As rain
drops pass through the sensing volume, the light changes intensity and scatters in various directions. This
change is detected by the sensor and allows the physical properties of the particle (e.g. the diameter and
fall velocity) to be derived. Scattering can be broadly defined as the deflection of radiation from its
original direction of propagation.
4.3 Impact
Impact disdrometers utilise the kinetic energy of the falling drops when impacting the exposed surface
of the sensor. A plastic or metal membrane is used at the measurement surface to detect the impact of
individual rain drops. In some systems, the mechanical movement of the membrane is converted into an
electrical signal by an attached moving magnet/coil system. In other solutions, the amplitude and the
frequency spectrum of the vibrations generated by rain drops hitting the membrane are detected and
analysed to determine the drop number and size. The impact method is therefore suitable for
determining the rainfall intensity and drop size distribution over a given time window.
Impact disdrometers can be divided into two categories: acoustic and displacement disdrometers. Both
types are commercially available and are used to measure liquid precipitation because the energy of the
raindrops is directly related to the mass and density of the water; snowflakes and hailstones, for example,
have completely different effects on the sensors, and could lead to an under- or overestimation of the
amount of precipitation.
4.4 Radar
Since the sixties, microwave-based technologies have been developed and improved in both the
communications and meteorological fields. Ground-based microwave radiometry has its traditional
applications in meteorology in the estimation of columnar profiles of vapour content, non-raining clouds
liquid water and temperature. Precipitation measurements using microwave sensors have emerged in
the last decades and are based on the reduction in signal power through the atmosphere during a
precipitation event. The attenuation and scattering of the sensor emissions are related to the
precipitation rate but also depend on the physics of the precipitation particles, such as the liquid or solid
phase and the different particles sizes, but the frequency of the emitted signal also plays a fundamental
role.
Microwave disdrometers use the Doppler effect to measure raindrops passing nearby. A falling drop
moving vertically towards the instrument produces a return signal as it enters the measurement volume,
the frequency of which is a function of the speed at which the drop crosses the equi-phase surfaces. These
surfaces are defined by the points where R1 + R2 = constant, where R1 is the distance between the
emitting antenna and the falling drop and R2 is the distance between the drop and the receiving antenna.
5 Classification of non-catching rain gauges
5.1 Characteristics of the calibration device
The measurable quantities for the non-catching rain gauges are the drop size and velocity. In order to
produce reference drops in a controlled environment, three examples of drop generators were developed
within the INCIPIT project. The first drop generator was developed by the Teknologisk Institut (DTI) in
Denmark, the second one was developed by the University of Genova (UNIGE) in Italy and the third one
by the FPS Economy, Metrology, National Standards (SMD) in Belgium.
The characterisation of the drop generators provides the reference drop size and velocity in a traceable
manner, together with their respective uncertainties. See Baire et al, 2022 [2] for a description of the
three sample drop generators and their characterisation.
5.2 Testing protocol
Some disdrometers provide independent drop size and fall velocity output information for each
individual drop detected, and these two parameters are used first for calibration. The integral value of
the rainfall intensity is then also used for comparison with the corresponding instrument output, which
is calculated as the weighted moment of the drop size distribution N(D), when the drop terminal velocity
W is used as the weight of each individual drop, as given in Formula (1) (see e.g. Ulbrich 1983 [3];
T
Nespor and Sevruk, 1998 [4]):
D
max
RI C N DV⋅⋅DW D dD (1)
( ) ( ) ( )
RT
∫
D
min
where
C 6
R is equal to 3,6·10 , the numerical factor for the conversion of the rate of rainfall from
-1 -1
[m s ] to [mm h ];
N(D) -3 -1
is the number of drops per unit volume of air and unit drop size interval [m mm ],
with the equivolumetric diameter D [mm] (D ≤ D ≤ D );
min max
V(D) 3 3
-9
is the drop volume, assumed equal to π ·10 ·D ⁄6 [m ];
W (D) -1
T is the drop terminal velocity [m s ].
=
For instruments that provide only rain intensity as an output (thus assuming a theoretical relationship
between the drop diameter and its terminal velocity), or that do not provide output information for
individual drops, such an integral parameter cannot be used alone for calibration according to the present
standard.
Raw data is used to calibrate each instrument, where the raw data is the dimensional (size) and kinematic
(fall velocity) information of each individual drop. Derived variables, such as kinetic energy and
precipitation intensity, cannot be used for calibration if raw data are not provided as an output of the
instrument. The effect of any filtering or interpretation software that is neither open (e.g. proprietary
software, undisclosed procedures), nor released for the purpose of the test shall be included in the
calibration results.
The calibration procedure described below is derived from the work of Chinchella (2022) [5].
Calibration of non-catching precipitation measurement instruments shall be performed by generating a
controlled set of water drops and letting them fall from a known height over the sensing area (or volume)
of the instrument under test. The drop size and fall velocity shall be determined by a comprehensive
characterization of the adopted drop generator and their uncertainty assessed and traced back to the
International System of Units (see e.g., Baire et al., 2022 [2]). This should be included in the calibration
certificate (see EN ISO/IEC
...