SIST EN ISO 18081:2024

SLOVENSKI STANDARD
01-september-2024
Nadomešča:
SIST EN ISO 18081:2017
Neporušitvene preiskave - Akustična emisija - Preskušanje tesnosti z akustično
emisijo (ISO 18081:2024)
Non-destructive testing - Acoustic emission testing (AT) - Leak detection by means of
acoustic emission (ISO 18081:2024)
Zerstörungsfreie Prüfung - Schallemissionsprüfung - Dichtheitsprüfung mittels
Schallemission (ISO 18081:2024)
Essais non destructifs - Contrôle par émission acoustique - Détection de fuites par
émission acoustique (ISO 18081:2024)
Ta slovenski standard je istoveten z: EN ISO 18081:2024
ICS:
17.140.99 Drugi standardi v zvezi z Other standards related to
akustiko acoustics
19.100 Neporušitveno preskušanje Non-destructive testing
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 18081
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2024
EUROPÄISCHE NORM
ICS 19.100 Supersedes EN ISO 18081:2016
English Version
Non-destructive testing - Acoustic emission testing (AT) -
Leak detection by means of acoustic emission (ISO
18081:2024)
Essais non destructifs - Essais d'émission acoustique - Zerstörungsfreie Prüfung - Schallemissionsprüfung -
Détection de fuites par émission acoustique (ISO Dichtheitsprüfung mittels Schallemission (ISO
18081:2024) 18081:2024)
This European Standard was approved by CEN on 28 June 2024.

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
© 2024 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18081:2024 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 18081:2024) has been prepared by Technical Committee ISO/TC 135 "Non-
destructive testing" in collaboration with Technical Committee CEN/TC 138 “Non-destructive testing”
the secretariat of which is held by AFNOR.
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 January 2025, and conflicting national standards shall
be withdrawn at the latest by January 2025.
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 supersedes EN ISO 18081:2016.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations 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.
Endorsement notice
The text of ISO 18081:2024 has been approved by CEN as EN ISO 18081:2024 without any modification.

International
Standard
ISO 18081
Second edition
Non-destructive testing — Acoustic
2024-07
emission testing (AT) — Leak
detection by means of acoustic
emission
Essais non destructifs — Essais d’émission acoustique —
Détection de fuites par émission acoustique
Reference number
ISO 18081:2024(en) © ISO 2024
ISO 18081:2024(en)
© ISO 2024
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 18081:2024(en)
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Qualification of test personnel . 2
5 Principle of acoustic emission testing . 2
5.1 The acoustic emission phenomenon .2
5.2 Influence of different media and different phases .2
5.3 Influence of pressure differences .3
5.4 Influence of geometry of the leak path .4
5.5 Influence of wave propagation .4
6 Applications . 5
7 Testing equipment . 5
7.1 General requirements .5
7.2 Sensors . .5
7.2.1 Typical frequency ranges (band widths) .5
7.2.2 Mounting technique .6
7.2.3 Temperature range, wave guide .6
7.2.4 Intrinsic safety .6
7.2.5 Immersed sensors .6
7.2.6 Integral electronics (amplifier, RMS converter, ASL converter, band pass) .6
7.3 Portable and non-portable AE instruments .7
7.4 Single and multi-channel AT instruments .7
7.4.1 Single-channel instruments .7
7.4.2 Multi-channel instruments .7
7.5 Determination of features (RMS, ASL vs. hit or continuous AE vs. burst AE) .7
7.6 System verification using artificial leak noise sources .7
8 Test procedure for leak detection . 8
8.1 Mounting of sensors .8
8.2 Additional features to be determined .9
8.3 Background noise.9
8.3.1 General .9
8.3.2 Environmental noise .9
8.3.3 Process noise .9
8.4 Data acquisition .9
9 Location procedures . 10
9.1 General .10
9.2 Single-sensor location based on AE wave attenuation .10
9.3 Multi-sensor location based on Δt values (linear, planar) .11
9.3.1 Threshold level and peak level timing technique.11
9.3.2 Cross-correlation technique .11
10 Data presentation .12
10.1 Numerical data presentation (level meter) . 12
10.2 Parametric dependent function . 12
10.3 Frequency spectrum . 13
11 Data interpretation .13
11.1 Leak validation . 13
11.1.1 On-site (during test) and off-site (post analysis) . 13
11.1.2 Correlation with pressure. 13
11.1.3 Rejection of false indications . 13
11.2 Leakage rate estimation .14

iii
ISO 18081:2024(en)
11.3 Demand for follow-up actions .14
12 Quality management documents .15
12.1 Test procedure . 15
12.2 Test instruction . 15
13 Test documentation and reporting .16
13.1 Test documentation . .16
13.2 Test report .16
Annex A (informative) Example applications of leak detection .18
Bibliography .31

iv
ISO 18081:2024(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 135, Non-destructive testing, Subcommittee
SC 9, Acoustic emission testing, in collaboration with the European Committee for Standardization (CEN)
Technical Committee CEN/TC 138, Non-destructive testing, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 18081:2016), which has been technically
revised.
The main changes are as follows:
— Figure 1 has been improved;
— term “AT equipment” has been replaced by “AE instrument” in the whole document;
— term “system” has been replaced by “instrument” in the whole document;
— Figure 2 showing an adjustable air jet has been added;
— Formula (1) has been corrected;
— Table 2 “Leakage grading and the influence of leak flow dynamic on AE activity“ has been added;
— editorial corrections throughout the document.
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.

v
International Standard ISO 18081:2024(en)
Non-destructive testing — Acoustic emission testing (AT) —
Leak detection by means of acoustic emission
1 Scope
This document specifies the general principles required for leak detection by acoustic emission testing (AT).
It is addressed to the application of the methodology on structures and components, where a leak flow as a
result of pressure differences appears and generates acoustic emission (AE).
It describes phenomena of the AE generation and influence of the nature of fluids, shape of the gap, wave
propagation and environment.
The different application techniques, instrumentation and presentation of AE results are discussed. Also
included are guidelines for the preparation of application documents which describe specific requirements
for the application of the acoustic emission testing.
Annex A gives procedures for some leak-testing applications.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 9712, Non-destructive testing — Qualification and certification of NDT personnel
ISO 12716, Non-destructive testing — Acoustic emission inspection — Vocabulary
ISO/TS 18173, Non-destructive testing — General terms and definitions
EN 1330-1, Non-destructive testing — Terminology — Part 1: General terms
EN 1330-2, Non-destructive testing — Terminology — Part 2: Terms common to the non-destructive testing methods
EN 1330-9, Non-destructive testing — Terminology — Part 9: Terms used in acoustic emission testing
EN 13477-1, Non-destructive testing — Acoustic emission — Equipment characterisation — Part 1: Equipment
description
EN 13477-2, Non-destructive testing — Acoustic emission — Equipment characterisation — Part 2: Verification
of operating characteristics
EN 13554, Non-destructive testing — Acoustic emission testing — General principles
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 12716, ISO/TS 18173, EN 1330-1,
EN 1330-2 and EN 1330-9 apply.
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/

ISO 18081:2024(en)
NOTE The definitions of leak, leakage rate, leak tightness are those defined in ISO 20484.
4 Qualification of test personnel
It is assumed that acoustic emission testing is performed by qualified and capable personnel. In order to
prove this qualification, it is recommended to certify the personnel in accordance with ISO 9712.
5 Principle of acoustic emission testing
5.1 The acoustic emission phenomenon
See Figure 1.
Key
1 fluid d main dimension of leak orifice
2 AE sensor I wall thickness
P pressure on side of fluid U leaking fluid
P pressure on side of sensor
Figure 1 — Schematic principle of acoustic emission and its detection
The continuous acoustic emission in the case of a leak, in a frequency range, looks like an apparent increase
in background noise, depending on pressure.
5.2 Influence of different media and different phases
The detectability of the leak depends on the fluid type and its physical properties. These will contribute to
the dynamic behaviour of the leak flow (laminar, turbulent) (see Table 1).

ISO 18081:2024(en)
Table 1 — Influence of the different parameters on the AE activity
Sub-
Parameter Higher activity Lower activity
clause
gas
Test media liquid
two phase
5.2 Viscosity low high
Type of flow turbulent laminar
Fluid velocity high low
5.3 Pressure difference high low
Shape of leak crack like hole
5.4 Length of leak path long short
Surface of leak path rough smooth
In contrast to turbulent flow, the laminar flow in general does not produce detectable acoustic emission
signals.
Acoustic emission in conjunction with a leakage is generated by the following:
— turbulent flow of the escaping gas or liquid;
— fluid friction in the leak path;
— cavitations, during two-phase flow (gas coming out of solution) through a leaking orifice;
— the pressure surge generated when a leakage flow starts or stops;
— backwash of particles against the surface of equipment being monitored;
— gaseous or liquid jet (verification source);
— pulsating bubbles;
— explosion of bubbles;
— shock-bubbles on the walls;
— vaporization of the liquid (flashing).
The frequency content of cavitation may comprise from several kHz to several MHz.
Cavitation results in a burst emission whose energy is at least one order of magnitude higher than that
caused by turbulence.
The relative content in gas or air strongly influences the early stage of cavitation.
The acoustic waves generated by leaks can propagate by the walls of the structure as well as through any
fluids inside.
Acoustic waves are generated by vibration at ultrasonic frequencies of the molecules of the fluid. The
vibrations are produced by turbulence and occur in the transition between a laminar and a turbulent flow
within the leak path and as these molecules escape from an orifice.
The acoustic waves produced by the above-mentioned factors are used for leak detection and location.
5.3 Influence of pressure differences
The pressure difference is the primary factor affecting leak rate. However, the presence of leak paths can
depend on a threshold value of fluid temperature or pressure.

ISO 18081:2024(en)
Pressure-dependent leaks and temperature-dependent leaks have been observed, but in extremely
limited number.
Pressure-dependent or temperature-dependent leaks denote a condition where no leakage exists until
a threshold pressure or temperature is reached. At this point, the leakage appears suddenly and can be
detectable.
When the pressure or temperature is reversed, the leakage follows the prescribed course to the critical
point at which leakage drops to zero.
Temperature and pressure are not normally applied in the course of leak testing for the purpose of locating
such leaks. Instead, they are used to force existing discontinuities to open, so as to start or increase the
leakage rate to point of detection.
An example of this effect is the reversible leakages at seals below the service temperature and/or service
pressure.
Sound waves emitted by a leak will normally have a characteristic frequency spectrum depending on the
pressure difference and shape of the leak path.
Therefore, the detectability of the leak depends on the frequency response of the sensor and this shall be
taken into account when selecting the instrumentation.
5.4 Influence of geometry of the leak path
The AE intensity from a natural complex leak path (e.g. pinhole corrosion, fatigue or stress corrosion cracks)
is generally greater than that produced by leakage from an artificial source, such as a drilled hole used for
verification.
The main parameters defining the complexity are the cross section, length and surface roughness of the
leak path.
5.5 Influence of wave propagation
Acoustic emission signals are the response of a sensor to sound waves generated in solid media. These waves
are similar to the elastic waves propagated in gasses and fluids but are more complex because solid media
are also capable of transmit shear force.
Waves that encounter a change in media in which they are propagating may change directions or reflect. In
additions to reflection, the interface causes the waves to diverge from its original line of flight or refract in
the second medium. Also, the mode of the wave can be changed in the reflection and/or refraction process.
Incident waves upon an interface between two media will reflect or refract such that directions of the
incident, reflected and refracted waves all lie in the same plane. This plane is defined by the line along which
the incident wave is propagating and the normal to the interface.
The following factors are important to acoustic emission testing:
a) wave propagation has the most significant influence on the form of the detected signals;
b) wave velocity is key to computed source location;
c) wave attenuation governs the maximum sensor spacing that is acceptable for effective detection.
The wave propagation influences the received waveform in the following ways:
d) reflections, refractions and mode conversions on the way from source to sensor result in many different
propagation paths of different lengths;
e) multiple propagation paths on the way from source to sensor, even in the absence of reflecting
boundaries, can be caused by the structure itself, for example, spiral paths on a cylinder;

ISO 18081:2024(en)
f) separation of different wave components (different modes, different frequencies) travelling at different
velocities;
g) wave attenuation (volumetric dispersion, absorption, as well as attenuation due to the effects given in
5.5 d) and 5.5 f)).
The wave attenuation is influenced by liquids inside a structure or pipe, which will assist in the propagation
of acoustic waves, while liquids (inside and outside) have a tendency to reduce the detectability of the
acoustic waves.
This effect will depend on the ratio of the acoustic impedances of the different materials.
The sound wave inside will be used normally for the detection of AE sources over long distances because of
the low sound attenuation of most liquids.
6 Applications
Acoustic emission testing (AT) provides many possibilities to detect leaks from pressurized and atmospheric
equipment in industry and research fields.
AT is used in following areas:
a) pressure vessels;
b) pipe and piping systems;
c) above ground storage tanks;
d) underground storage tanks;
e) boiler drums;
f) boiler tubes;
g) autoclaves;
h) heat exchangers;
i) containments;
j) valves;
k) safety valves;
l) pumps;
m) vacuum systems.
7 Testing equipment
7.1 General requirements
The testing equipment (hard and software) shall be in accordance with the requirements of EN 13477-1 and
EN 13477-2.
7.2 Sensors
7.2.1 Typical frequency ranges (band widths)
The optimum frequency range for leak detection depends very much on the application, the fluid type,
pressure difference at the leak, the leak rate, and the sensor to source distance and more.

ISO 18081:2024(en)
For example, the optimum frequency range for tank floor leak detection of atmospheric tanks is around 20 kHz
to 80 kHz, because the source to sensor distance can be large and at these frequencies the attenuation is low.
The preferred frequency range for high pressure piping leak detection may go up to 500 kHz for optimum
signal-to-noise ratio in presence of background noise.
Leak detection at pipes for low pressure (e.g. water supply) is typically performed at frequencies down to 5 kHz.
a) Usually, a sensor shall be in direct contact to a test object.
b) Then a coupling agent shall be used between the sensor and the test object for optimum and stable wave
transfer.
c) Durability, consistency and chemical composition of the couplant shall comply with the duration of the
monitoring, the temperature range and the corrosion resistance of the test object.
7.2.2 Mounting technique
The mounting method is influenced by the duration of the monitoring.
For a temporary installation on a ferromagnetic test object, a magnetic holder may be the preferred
mounting tool.
For permanent installations, sensors may be fastened by metallic clamps or bonded to the test object using a
suitable adhering coupling.
7.2.3 Temperature range, wave guide
The operating temperature range of the AE sensor shall meet the surface temperature conditions of the test
object, otherwise waveguides shall be used between sensor and test object.
7.2.4 Intrinsic safety
If the sensor is to be installed in a potentially explosive atmosphere, the sensor shall be intrinsically safe and
ATEX Directive 2014/34/EU (Equipment) and 1999/92/EC (Workplace) can apply for the classified hazard
at the location where it is to be used. See also EN 60079-0, EN 60079-11 and EN 60079-14 for explosion-
proof installations.
7.2.5 Immersed sensors
a) If the sensor is to be immersed in a liquid, the sensor's IP-code (defined in EN 60529) shall be specified
to at least IP68.
b) Sensors and other immersed accessories shall be tight for the maximum possible pressure of the liquid.
7.2.6 Integral electronics (amplifier, RMS converter, ASL converter, band pass)
Passive sensors and sensors with an integrated pre-amplifier of suitable bandwidth are available.
Sensors with built-in electronics are less susceptible to electromagnetic disturbances, due to the elimination
of a sensor-to-pre-amplifier cable.
These sensors are usually a little larger in size and weight and have a more limited temperature range due
to internal electronics, e.g. 80 °C.
Sensors may also include a signal-to-RMS converter, a signal-to-ASL converter and/or a limit-comparator
with digital output.
ISO 18081:2024(en)
7.3 Portable and non-portable AE instruments
An instrument for leak detection by acoustic emission designed for portable use contains usually one or a
few channels.
The choice of a portable AT instrument is generally based on several factors, such as cost, size of the device,
test duration, environmental conditions (e.g. hazardous areas), and availability of external power.
Portable AE instruments are used for on-site leak testing of limited areas.
Non-portable instruments are used for testing of large structures or for permanent in-service monitoring of
leaks in critical applications.
7.4 Single and multi-channel AT instruments
7.4.1 Single-channel instruments
Single-channel instruments are mainly used for a point-by-point search strategy, the sensor being moved to
areas of interest over the structure.
These instruments typically acquire and store RMS, ASL, signal amplitude and signal waveform data for
determination of time and frequency features.
7.4.2 Multi-channel instruments
Multi-channel instruments are mainly used for large structures where the sensor positions are fixed and
one of the location procedures in 9.3 may be applied.
Also, permanently installed instruments for continuous remote in-service monitoring, for leak detection in
the piping network of nuclear plants, are often used with multi-channel configurations.
7.5 Determination of features (RMS, ASL vs. hit or continuous AE vs. burst AE)
Simple instruments determine continuously as a function over time the ASL (the arithmetic average of the
logarithm of the rectified AE signal over a specified period of time) and/or RMS (the square root of the
average of squared AE signal over a specified period of time) and/or average of the maximum value of the
signal amplitude within a specified period of time, and display the results.
On some of the instruments the resulting functions over time can be shown for each channel numerically or
graphically and be compared against static or computed alarm levels so alarm conditions may automatically
trigger an alarm.
More sophisticated instruments can also acquire and store waveform data for determination of time
differences by Δt-measurement or by cross-correlation method.
7.6 System verification using artificial leak noise sources
An artificial leak noise source shall be used for system verification. Figure 2 shows an example.
A setup using an air jet or a test block/pipe with a drilled hole passing a controlled flow of gas or liquid may
be used to determine the dependency of stimulation signal amplitude versus stimulated flow of gas or liquid
and signal amplitude measured at a certain distance from emitter.
A well reproducible artificial leak noise source, like a passive sensor stimulated by an electrical signal,
such as white noise or a sinusoidal signal of a certain frequency from a function generator, may be used for
periodic system verification.
ISO 18081:2024(en)
Key
1 coupling for compressed air
2 metallic block
3 nozzle with adjustable air jet
Figure 2 — Adjustable air jet
8 Test procedure for leak detection
8.1 Mounting of sensors
a) For aboveground structures, surface-mounted AE sensors with fixed positions shall be attached with
direct contact to the test object or via acoustic waveguides.
b) For leak testing of underground pressure equipment utilities such as waveguides (e.g. on vessels) or pigs
(e.g. in pipelines) may be applied.
c) The mounting technique and coupling materials shall be selected dependent on temperature and
duration of measurement (see 7.6).
d) The quality of sensor coupling may be enhanced by special shoes that conform to the diameter/curvature
of the test object.
e) With leak detection pigs for buried pipelines, the AT sensors shall be mounted on the pig and
measurements are usually made during the pig run (see A.2).
f) The corresponding position of the pig shall be determined on the basis of an encoder and/or acoustic
markers positioned on the outside of the pipe.
g) The sensors shall be positioned so as to ensure leak location based on appropriate location procedure
(see Clause 9) and to achieve the required location accuracy.
h) Their positions on the structure shall be taken into consideration welds, changes of shape that affect
flow characteristics, shadowing effects of nozzles and ancillary attachments.
i) Prior to testing, wave propagation and attenuation measurements, using a Hsu-Nielsen source or
artificial leak noise sources (see 7.2), shall be performed on the test object in order to determine the
effective wave velocity and to calculate the maximum allowed sensor distance needed for leak detection
with specified sensitivity.
The maximum sensor spacing for detection and location of leaks is influenced by many factors, such as
surface covering by coating, cladding or insulation, background noise level, pressure on the test object, type
of fluid, type of leak.
ISO 18081:2024(en)
8.2 Additional features to be determined
In its simplest form leak detection will comprise measurement of the RMS/ASL at each defined sensor
position as a function of time for estimation of approximate location of the source.
In addition, pressure is measured as a function of time and the occurrence of a change in RMS/ASL, can be
correlated to a change of pressure.
It is recommended that the RMS/ASL is determined as a function of increasing or decreasing pressure for
verification purposes.
For more complex situations or improved diagnosis, other features may be determined, such as the following:
a) crest factor;
b) arrival time;
c) maximum value of signal amplitude;
d) signal waveform;
e) frequency spectrum;
f) related external parameters, e.g. pipe or valve temperature, pressure difference at the valve.
8.3 Background noise
8.3.1 General
The background noise is usually a combination of environmental and process noise.
8.3.2 Environmental noise
Sometimes it is unavoidable that environmental noise, even airborne noise, is picked up in addition to the
sound of interest. This can be noise from e.g. weather conditions, road traffic, rail, airplanes or birds.
In such cases, it can be helpful to add a sensor (guard) to monitor the airborne noise (waterborne in subsea
environment) to identify and disregard the environmental noise.
8.3.3 Process noise
Process noise will be created from the in-service conditions of the tested structure, e.g. product flow noise.
The influence of the process noise may be reduced by
— choosing an appropriate test period,
— isolating from the noise sources, and
— using more sophisticated analysis methods, filtering, pattern recognition.
8.4 Data acquisition
Data acquisition in its simplest form involves point measurements of one variable (e.g. RMS, ASL, or signal
amplitude) in a search mode to detect and locate a leak.
a) Whenever the equipment allows, the results of all measurements as well as the test parameters shall
be stored.
b) When more advanced equipment is used, the necessary signal parameters shall be acquired and
recorded continuously or periodically.

ISO 18081:2024(en)
c) The duration of the acquisition shall be chosen taking into account the values and fluctuation of the
background noise.
9 Location procedures
9.1 General
The AE signals caused by a fluid leak are usually continuously superposed by transients reflecting the
nature of the fluid dynamics, leak path, structural response and wave propagation path in the containment
structure.
Attention shall be paid to attenuation (e.g. by coatings, wrappings, insulations) of acoustic waves and possible
multiple wave paths (metallic wall or liquid fluid) between source and sensor location to get reliable results.
Various strategies for leak location have been developed.
In general, none of the strategies yields highly accurate location, but for industrial applications even an
approximate location can be very economic.
9.2 Single-sensor location based on AE wave attenuation
This strategy uses the attenuation of the AE waves in the containment structure. Close to the source the
signal levels will be higher than further away from the source. The position of the leak is assigned to the
measurement position with the highest signal level, e.g. RMS, ASL or average of the maximum value of the
signal amplitude.
Often a single-sensor hand-held device is used to make the tests at different positions on a structure. In this
case tests shall be performed over a longer time span or repeatedly per position in order to identify possible
fluctuations in the AE signals that can affect localization.
A variant of the above is the technique of “acoustic field mapping” where point-by-point tests are made
following a grid pattern.
A further application of this technique is the difference method with a two-point access and the leak in
between.
a) The calculation shall be performed using the difference of signal levels at the access points A and B.
b) If the difference is zero
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