EN ISO 18081:2016

SLOVENSKI STANDARD
01-marec-2017
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Non-destructive testing - Acoustic emission testing (AT) - Leak detection by means of
acoustic emission (ISO 18081:2016)
Zerstörungsfreie Prüfung - Schallemission - Dichtheitsprüfung mittels Schallemission
(ISO 18081:2016)
Essais non destructif - Émission acoustique - Détection de fuite par émission acoustique
(ISO 18081:2016)
Ta slovenski standard je istoveten z: EN ISO 18081:2016
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
June 2016
EUROPÄISCHE NORM
ICS 19.100
English Version
Non-destructive testing - Acoustic emission testing (AT) -
Leak detection by means of acoustic emission (ISO
18081:2016)
Essais non destructifs - Contrôle par émission Zerstörungsfreie Prüfung - Schallemissionsprüfung -
acoustique - Détection de fuites par émission Dichtheitsprüfung mittels Schallemission (ISO
acoustique (ISO 18081:2016) 18081:2016)
This European Standard was approved by CEN on 22 April 2016.

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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, 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: Avenue Marnix 17, B-1000 Brussels
© 2016 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18081:2016 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
European foreword
This document (EN ISO 18081:2016) has been prepared by Technical Committee CEN/TC 138 “Non-
destructive testing” the secretariat of which is held by AFNOR, in collaboration with Technical
Committee ISO/TC 135 “Non-destructive testing”.
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 December 2016, and conflicting national standards
shall be withdrawn at the latest by December 2016.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent
rights.
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, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 18081:2016 has been approved by CEN as EN ISO 18081:2016 without any modification.

INTERNATIONAL ISO
STANDARD 18081
First edition
2016-06-01
Non-destructive testing — Acoustic
emission testing (AT) — Leak
detection by means of acoustic
emission
Essais non destructifs — Contrôle par émission acoustique —
Détection de fuites par émission acoustique
Reference number
ISO 18081:2016(E)
©
ISO 2016
ISO 18081:2016(E)
© ISO 2016, Published in Switzerland
All rights reserved. Unless otherwise specified, 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
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2016 – All rights reserved

ISO 18081:2016(E)
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Personnel qualification . 2
5 Principle of acoustic emission method . 2
5.1 The AE phenomenon. 2
5.2 Influence of different media and different phases . 3
5.3 Influence of pressure differences . 4
5.4 Influence of geometry of the leak path . 4
5.5 Influence of wave propagation . 4
6 Applications . 5
7 Instrumentation . 5
7.1 General requirements . 5
7.2 Sensors . 5
7.2.1 Typical frequency ranges (band widths) . 5
7.2.2 Mounting method . 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 AT instruments . 6
7.4 Single and multichannel AT equipment . 6
7.4.1 Single-channel systems . 6
7.4.2 Multi-channel systems . 6
7.5 Measuring features (RMS, ASL vs. hit or continuous AE vs. burst AE) . 7
7.6 Verification using artificial leak noise sources . 7
8 Test steps for leak detection . 7
8.1 Sensor application . 7
8.2 Measured features . . 8
8.3 Background noise . 8
8.3.1 Environmental noise . 8
8.3.2 Process noise . 8
8.4 Data acquisition . 8
9 Location procedures . 9
9.1 General considerations . 9
9.2 Single sensor location based on AE wave attenuation. 9
9.3 Multi-sensor location based on Δt values (linear, planar) . 9
9.3.1 Threshold level and peak level timing method. 9
9.3.2 Cross correlation method .10
9.4 Wave type and wave mode based location .11
10 Data presentation .11
10.1 Numerical data presentation (level-meter) .11
10.2 Parametric dependent function (e.g. pressure) .11
10.3 Frequency spectrum .12
11 Data interpretation .12
11.1 Leak validation .12
11.1.1 On-site (during test) and off-site (post analysis) .12
11.1.2 Correlation with pressure .12
11.1.3 Rejection of false indications .12
ISO 18081:2016(E)
11.2 Leakage rate estimation .13
11.3 Demands on follow-up actions .13
12 Quality management documents .13
12.1 Test procedure .13
12.2 Test instruction .13
13 Test documentation and reporting .14
13.1 Test documentation .14
13.2 Test report .15
Annex A (normative) Examples of leak detection .16
Bibliography .28
iv © ISO 2016 – All rights reserved

ISO 18081:2016(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
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 documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
ISO 18081 was prepared by the European Committee for Standardization (CEN) Technical Committee
CEN/TC 138, Non-destructive testing, in collaboration with ISO Technical Committee TC 135, Non-
destructive testing, Subcommittee SC 9, Acoustic emission testing, in accordance with the agreement on
technical cooperation between ISO and CEN (Vienna Agreement).
INTERNATIONAL STANDARD ISO 18081:2016(E)
Non-destructive testing — Acoustic emission testing (AT)
— Leak detection by means of acoustic emission
1 Scope
This International Standard 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 methods, instrumentation and presentation of AE results is discussed.
Also included are guidelines for the preparation of application documents which describe specific
requirements for the application of the AE method.
Different application examples are given.
Unless otherwise specified in the referencing documents, the minimum requirements of this
International Standard are applicable.
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/IEC 17025, General requirements for the competence of testing and calibration laboratories
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
EN 60529, Degrees of protection provided by enclosures (IP Code)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 1330-1, EN 1330-2 and
EN 1330-9 and the following apply.
NOTE The definitions of leak, leakage rate, leak tight are those defined in EN 1330-8.
ISO 18081:2016(E)
4 Personnel qualification
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 method
5.1 The AE phenomenon
See Figure 1.
Key
1 fluid
2 AE 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.
2 © ISO 2016 – All rights reserved

ISO 18081:2016(E)
Table 1 — Influence of the different parameters on the AE activity
Parameter Higher activity Lower activity
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
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).
In contrast to turbulent flow, the laminar flow does not in general, produce detectable acoustic emission
signals.
Acoustic signals in conjunction with a leakage are 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 system 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.
ISO 18081:2016(E)
5.3 Influence of pressure differences
The pressure difference is the primary factor affecting leak rate. However, the presence of leak
paths may depend on a threshold value of fluid temperature or pressure. 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 threshold
pressure or temperature is reached. At this point, the leakage appears suddenly and may 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.
AE 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 a standard 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 sound waves propagated in air and other fluids but are more complex because
solid media are also capable of resisting 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 wave to diverge from its original line of flight
or refract in the second medium. Also the mode of the wave may be changed in the reflection and/or
refraction process.
An incident wave 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 below factors are important to AE technology:
a) wave propagation has the most significant influence on the form of the detected signal;
b) wave velocity is key to computed source location;
c) sound attenuation governs the maximum sensor spacing that is acceptable for effective detection.
The wave propagation influences the received waveform in the following ways:
— reflections, refractions and mode conversions on the way from source to sensor result in many
different propagation paths of different lengths;
— multiple propagation paths on the way from source to sensor, even in the absence of reflecting
boundaries may be caused by the structure itself. For example, spiral paths on a cylinder;
— separation of different wave components (different modes, different frequencies) travelling at
different velocities;
4 © ISO 2016 – All rights reserved

ISO 18081:2016(E)
— sound attenuation (volumetric dispersion, absorption, as well as attenuation due to the first and
third effects listed above).
The sound 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
detectable signal of the propagation of the acoustic waves. This effect will depend on the ratio of the
acoustic impedances of the different materials. The AE wave inside will be used normally for the
detection of AE sources over long distances because of the low wave sound attenuation for most liquids.
6 Applications
Acoustic emission testing provides many possibilities to detect leaks from pressurized equipment in
industry and research fields. AT is used in following areas:
a) pressure vessels;
b) pipe and piping systems;
c) storage tanks;
d) boiler drums;
e) boiler tubes;
f) autoclaves;
g) heat exchangers;
h) containments;
i) valves;
j) safety valves;
k) pumps;
l) vacuum systems.
7 Instrumentation
7.1 General requirements
Instrumentation components (hard and software) shall conform to 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. 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 400 kHz
for optimum signal-to-noise ratio in presence of disturbing sources. Leak detection at pipes for low
pressure (e.g. water supply) is usually performed at or below 5 kHz.
Usually, a sensor is in direct contact to a test object. Then a coupling agent must be used between the
sensor and the test object for optimum and stable wave transfer. Durability, consistency, and chemical
ISO 18081:2016(E)
composition of the coupling agent must comply with the duration of the monitoring, the temperature
range and the corrosion resistance of the test object.
7.2.2 Mounting method
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 might 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 should usually be ATEX conformant in accordance with the classified hazard at the location
where it is to be used. See EN 60079-0, EN 60079-11 and EN 60079-14 for explosion-proof installations.
7.2.5 Immersed sensors
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. Sensor 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 integral 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.
Sensors may also include a signal-to-RMS converter, a signal-to-ASL converter and/or a limit-comparator
with digital output.
7.3 Portable and non-portable AT instruments
An acoustic emission leak detection instrument designed for portable use contains usually one or a
few channels. The choice of a portable device is generally based on several factors, such as cost, test
duration, hazard and availability of external power.
Portable devices are used for valve leak detection.
7.4 Single and multichannel AT equipment
7.4.1 Single-channel systems
Single-channel systems are usually used for a point-by-point search mode, the sensor being moved to
areas of interest over the structure.
7.4.2 Multi-channel systems
Multi-channel systems are mainly used for large structures where the sensor positions are fixed and
one of the location procedures in 9.3 may be applied.
6 © ISO 2016 – All rights reserved

ISO 18081:2016(E)
Also, permanently installed instruments for continuous remote structural health monitoring, for leak
detection in the piping network of nuclear plants, are often used in multi-channel configurations.
7.5 Measuring features (RMS, ASL vs. hit or continuous AE vs. burst AE)
Simple instruments measure 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 peak amplitudes 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 may also acquire and store waveform data for determination of time
differences by Δ-t-measurement or cross correlation method.
7.6 Verification using artificial leak noise sources
An artificial leak noise source should be used for system verification.
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 amplitude versus stimulated flow of gas or
liquid and amplitude measured at a certain distance from emitter.
A well reproducible artificial leak noise source, like a passive sensor stimulated by electrical wave, such
as white noise or a sinusoidal signal of a certain frequency from a function generator, may be used for
periodic system verification.
8 Test steps for leak detection
8.1 Sensor application
For aboveground structures, surface-mounted AE sensors with fixed positions are attached in direct
contact to the test object or via acoustic waveguides. The mounting method and useful coupling
materials mainly depend on temperature and duration of measurement (see 7.1).
The quality of sensor coupling can be enhanced by special shoes that conform to the diameter/curvature
of the tested structure.
With leak detection pigs for buried pipelines, the AE sensors are mounted on the pig and measurements
are usually made during the pig run (see A.2). The corresponding position of the pig can be measured
on the basis of an encoder and/or acoustic markers positioned on the outside of the pipe.
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. Their positions on the structure shall take
into consideration welds, changes of shape that affect flow characteristics, shadowing effects of nozzles
and ancillary attachments, etc.
For preparing the periodic system verification, appropriate locations for artificial stimulation shall be
defined at the test object and the response of certain sensors in various distances to the stimulation
shall be determined and periodically verified.
Prior to the test, wave propagation and attenuation measurements, using a Hsu-Nielsen source or
artificial leak noise sources (see 7.5), shall be performed on the structure in order to determine
the effective wave velocity and to calculate the maximum allowed sensor distance needed for leak
detection with predefined 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, test object pressure, type of fluid, type of leak, etc.
ISO 18081:2016(E)
8.2 Measured features
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 the RMS/ASL is measured as a function of increasing or
decreasing pressure for verification purposes.
For more complex situations for improved diagnosis, other features may be measured, such as the
following:
— crest factor;
— arrival time;
— wave form;
— frequency spectrum;
— related external parameters (e.g. temperature).
8.3 Background noise
The background noise is usually a combination of environmental and process noise.
8.3.1 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 weather conditions, road traffic, rail, airplanes, birds, etc.
In such cases, it might be helpful to add a sensor (guard) to monitor the airborne noise (waterborne in
subsea environment) to identify and disregard the environmental noise.
8.3.2 Process noise
Process noise will be created from the in-service conditions of the tested structure. Its influence might
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
peak amplitude) in a search mode to detect and locate a leak. Whenever the equipment allows, the
results of all measurements as well as the test parameters shall be stored.
When more advanced equipment is used, the necessary signal parameters shall be acquired and
recorded continuously or periodically.
The duration of the acquisition shall be chosen taking into account the values and fluctuation of the
background noise measurements.
8 © ISO 2016 – All rights reserved

ISO 18081:2016(E)
9 Location procedures
9.1 General considerations
The AE signals from waves caused by a fluid leak are usually continuous superposed by transients
reflecting the nature of the fluid dynamics, leak path, structural response and wave propagation path
in the containment structure.
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. Near 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 RMS or ASL.
Often a single sensor hand-held device is used to make the measurements at different positions on a
structure. In this case measurements shall be taken over a longer time span or repeatedly per position
in order to identify possible fluctuations in the AE signal that could affect localization.
A variant of the above is the method of “acoustic field mapping” where point by point measurements are
made following a grid pattern and reported as ISO-amplitude level mapping.
A further application of this methodology is the amplitude difference method with a two-point access.
The calculation can be performed using the amplitude difference at the access points A and B. If the
difference is zero, the source must be on half distance between A and B. At a linear structure with
access points A and B, the source location X can be calculated using Formula (1):
s
05, ∗−UU
()
AB
XX=∗05, − X + (1)
()
sA B
α
where
X is the X-Location of source;
s
X is the X-location of access point A;
A
X is the X-location of access point B;
B
U is the signal level at access point A in dB ;
A AE
U is the signal level at access point B in dB ;
B AE
α is the attenuation constant in dB/m
α must either be known or determined by a measurement at a third access point at a known distance
from A and B.
9.3 Multi-sensor location based on Δt values (linear, planar)
9.3.1 Threshold level and peak level timing method
In this strategy, the attenuation curve is known and several sensors in a location scheme are used to
locate the source from Δt values.
ISO 18081:2016(E)
Because the signals are more or less continuous in nature, this method relies on the presence of
superimposed transients on the signals. The arrival times are measured using threshold level and/or
burst signal maximum amplitude.
The result of the threshold level method can be improved by adjusting the threshold per channel based
on the amplitude distribution or the known wave attenuation.
An example of the use of this strategy is planar location on an above ground storage tank floor (see A.4).
9.3.2 Cross correlation method
Correlation commonly refers to a broad class of statistical relationships involving dependence. Cross-
correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them.
Although, it is commonly used in order to search for a shorter duration pattern within a long duration
signal, it can be used for other linear measurements. It also has applications in pattern recognition.
In the field of AE, cross correlation has been used to find the time-frequency-pattern of a burst in a
continuous waveform record. The time-lag can be determined between two channels and used for
location calculation. The cross-correlation is defined as:
∞
*
f ∗ gt = fgττ×+tdτ (2)
()() () ()
∫
∞
where
f* denotes the complex conjugate of f.
Similarly, for discrete functions, the cross-correlation is defined as:
∞
     
fn∗ g = fm* ×+gn m (3)
()
∑
     
m=−∞
The cross-correlation is similar in nature to the convolution of two functions.
In an auto-correlation, which is the cross-correlation of a signal with itself, there will always be a peak
at a lag of zero unless the signal is a trivial zero signal. Therefore, it can be used to dig out a signal from
high background noise.
The correlation is always used to include a standardizing factor in such way that correlations have
values between −1 and +1, and the term cross-correlation is used for referring to the correlation corr
(X, Y) between two (random) variables X and Y.
As an example, consider two real functions f and g differing only by an unknown shift along the x-axis.
One can use the cross-correlation to find how much g has to be shifted along the x-axis to make it
identical to f. The formula essentially slides the g -function along the x-axis, calculating the integral of
their product at each position. When the functions match, the value of fg∗ is maximized.
()
For the application in sense of leak detection, the cross-correlation is useful for determining the time
lag between two signals coming from the same
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