SIST EN ISO 16828:2025

SLOVENSKI STANDARD
01-oktober-2025
Neporušitvene preiskave - Ultrazvočne preiskave - Metoda zvočne poti za
odkrivanje in ocenjevanje velikosti nezveznosti (ISO 16828:2025)
Non-destructive testing - Ultrasonic testing - Time-of-flight diffraction technique for
detection and sizing of discontinuities (ISO 16828:2025)
Zerstörungsfreie Prüfung - Ultraschallprüfung - Beugungslaufzeittechnik zum Auffinden
und Ausmessen von Inhomogenitäten (ISO 16828:2025)
Essais non destructifs - Contrôle par ultrasons - Technique de diffraction du temps de vol
pour la détection et le dimensionnement des discontinuités (ISO 16828:2025)
Ta slovenski standard je istoveten z: EN ISO 16828:2025
ICS:
19.100 Neporušitveno preskušanje Non-destructive testing
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 16828
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2025
EUROPÄISCHE NORM
ICS 19.100 Supersedes EN ISO 16828:2014
English Version
Non-destructive testing - Ultrasonic testing - Time-of-flight
diffraction technique for detection and sizing of
discontinuities (ISO 16828:2025)
Essais non destructifs - Contrôle par ultrasons - Zerstörungsfreie Prüfung - Ultraschallprüfung -
Technique de diffraction du temps de vol pour la Beugungslaufzeittechnik zum Auffinden und
détection et le dimensionnement des discontinuités Ausmessen von Inhomogenitäten (ISO 16828:2025)
(ISO 16828:2025)
This European Standard was approved by CEN on 11 July 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 ISO 16828:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 16828:2025) 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 2026, and conflicting national standards shall
be withdrawn at the latest by January 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.
This document supersedes EN ISO 16828:2014.
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 16828:2025 has been approved by CEN as EN ISO 16828:2025 without any modification.

International
Standard
ISO 16828
Second edition
Non-destructive testing —
2025-07
Ultrasonic testing — Time-of-flight
diffraction technique for detection
and sizing of discontinuities
Essais non destructifs — Contrôle par ultrasons — Technique
de diffraction du temps de vol pour la détection et le
dimensionnement des discontinuités
Reference number
ISO 16828:2025(en) © ISO 2025
ISO 16828:2025(en)
© ISO 2025
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 16828:2025(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units. 2
5 General . 3
5.1 Principle of the technique .3
5.2 Requirements for surface condition and couplant .5
5.3 Materials and process type .5
6 Qualification of test personnel . 5
7 Requirements for test equipment. 6
7.1 General .6
7.2 Instrument and display .6
7.3 Probes .8
7.4 Scanning .8
8 TOFD setup procedures . 9
8.1 General .9
8.2 Probe selection and probe separation .9
8.2.1 Probe selection.9
8.2.2 Probe separation .10
8.3 Time window setting .10
8.4 Sensitivity setting .10
8.5 Scan increment setting .10
8.6 Setting of scanning speed .11
8.7 Checking of system performance.11
9 Interpretation and analysis of data .11
9.1 Basic analysis of discontinuities .11
9.1.1 General .11
9.1.2 Characterization of discontinuities . 12
9.1.3 Estimation of discontinuity position . 12
9.1.4 Estimation of discontinuity length . 13
9.1.5 Estimation of discontinuity depth and height. 13
9.2 Detailed analysis of discontinuities .14
9.2.1 General .14
9.2.2 Additional scans .14
9.2.3 Additional algorithms . 15
10 Detection and sizing in complex geometries .16
11 Limitations of the TOFD technique .16
11.1 General .16
11.2 Accuracy and resolution .17
11.2.1 General .17
11.2.2 Inaccuracy in the lateral position.17
11.2.3 Timing inaccuracy .17
11.2.4 Inaccuracy in sound velocity .17
11.2.5 Inaccuracy in probe centre separation.17
11.2.6 Spatial resolution .18
11.3 Obscured zones .18
12 TOFD testing without data recording. 19

iii
ISO 16828:2025(en)
13 Test procedure . 19
14 Test report . 19
Annex A (informative) Reference blocks .20
Bibliography .21

iv
ISO 16828:2025(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 3, Ultrasonic 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 16828:2012), which has been technically
revised.
The main changes are as follows:
— title revised by removing “as a method”;
— clarifications of abbreviations and symbols;
— figures have been updated;
— formulae have been corrected;
— term "dead zone" replaced by "obscured zone".
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
ISO 16828:2025(en)
Introduction
The following standards on ultrasonic testing developed by ISO/TC 135 are related.
ISO 16810, Non-destructive testing — Ultrasonic testing — General principles
ISO 16811, Non-destructive testing — Ultrasonic testing — Sensitivity and range setting
ISO 16823, Non-destructive testing — Ultrasonic testing — Through-transmission technique
ISO 16826, Non-destructive testing — Ultrasonic testing — Testing for discontinuities perpendicular to the surface
ISO 16827, Non-destructive testing — Ultrasonic testing — Characterization and sizing of discontinuities

vi
International Standard ISO 16828:2025(en)
Non-destructive testing — Ultrasonic testing — Time-of-
flight diffraction technique for detection and sizing of
discontinuities
1 Scope
This document specifies the general principles for the application of the time-of-flight diffraction (TOFD)
technique for both detection and sizing of discontinuities in low-alloyed carbon steel components.
This document also applies to other types of materials, provided the application of the TOFD technique is
performed with necessary consideration of geometry, acoustical properties of the materials, and the test
sensitivity.
Although this document is applicable, in general terms, for discontinuities in materials and applications
covered by ISO 16810, it contains references to the application on welds. This approach has been chosen for
reasons of clarity as to the probe positions and directions of scanning.
Unless otherwise specified in the referencing documents, the minimum requirements specified in this
document apply.
Unless explicitly stated otherwise, this document is applicable to the following categories of test objects as
specified in ISO 16811:
— category 1, without restrictions;
— categories 2 and 3, specified restrictions apply (see Clause 10);
— categories 4 and 5 require special procedures, which are also addressed (see Clause 10).
NOTE Techniques for the use of TOFD for weld testing are described in ISO 10863 and the related acceptance
criteria are given in ISO 15626.
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.
ISO 5577, Non-destructive testing — Ultrasonic testing — Vocabulary
ISO 9712, Non-destructive testing — Qualification and certification of NDT personnel
ISO 16810, Non-destructive testing — Ultrasonic testing — General principles
ISO 22232-1, Non-destructive testing — Characterization and verification of ultrasonic test equipment — Part
1: Instruments
ISO 22232-2, Non-destructive testing — Characterization and verification of ultrasonic test equipment — Part
2: Probes
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5577 and the following apply.

ISO 16828:2025(en)
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/
3.1
scanning surface obscured zone
scanning surface dead zone
zone where indications may be obscured due to the presence of the lateral wave (3.6)
3.2
back wall obscured zone
back wall dead zone
zone where signals may be obscured due to the presence of the back wall echo
3.3
perpendicular scan
scan perpendicular to the ultrasonic beam direction
Note 1 to entry: Refer to Figure 4.
3.4
parallel scan
scan parallel to the ultrasonic beam direction
Note 1 to entry: Refer to Figure 5.
3.5
time-of-flight diffraction setup
TOFD setup
probe arrangement defined by probe characteristics and probe centre separation (3.7)
Note 1 to entry: Probe characteristics are e.g. frequency, transducer size, beam angle, wave mode.
3.6
lateral wave
longitudinal wave traveling the shortest path from transmitter probe to receiver probe
3.7
probe centre separation
PCS
distance between the index points of transmitter and receiver probe
Note 1 to entry: The PCS for two probes located on a curved surface is the straight-line, geometric separation between
the two probe index points and not the distance measured along the surface.
4 Symbols and units
A list of the symbols and units used in this document is given in Table 1.
Table 1 — Symbols and units
Symbol Unit Meaning
D mm depth of the scanning surface obscured zone
ds
D mm depth of the back wall obscured zone
dw
d mm depth of a discontinuity tip below the scanning surface
δd mm error in depth
R mm spatial resolution
ISO 16828:2025(en)
TTabablele 1 1 ((ccoonnttiinnueuedd))
Symbol Unit Meaning
s mm half the distance between the index points of the two ultrasonic probes (half the PCS)
δs mm inaccuracy in half the probe centre separation
t µs time of flight from the transmitter to the receiver
Δt µs time-of-flight difference between the lateral wave and a second ultrasonic signal
δt µs inaccuracy in time of flight
t µs time of flight at depth d
d
t µs duration of the ultrasonic pulse measured at 10 % of the peak amplitude
p
t µs time of flight of the back wall echo
w
W mm wall thickness
coordinate parallel to the scanning surface and parallel to a predetermined reference line. For
X mm weld testing this reference line should coincide with the weld. The origin of the axes may be
defined as best suits the test object (see Figure 1)
Δx mm discontinuity length
coordinate parallel to the scanning surface, perpendicular to the predetermined reference line
Y mm
(see Figure 1)
δy mm inaccuracy in lateral position
Z mm coordinate perpendicular to the scanning surface (see Figure 1)
Δz mm discontinuity height
v mm/µs sound velocity
δv mm/µs inaccuracy in sound velocity
Key
X, Y, Z main coordinates (see Table 1)
Figure 1 — Definition of coordinates
5 General
5.1 Principle of the technique
The TOFD technique relies on the interaction of ultrasonic waves with the tips of discontinuities. This
interaction results in diffracted waves over a large angular range. Detection of the diffracted waves makes it
possible to establish the presence of the discontinuity.

ISO 16828:2025(en)
The time of flight of the recorded signals is a measure for the height of the discontinuity, thus enabling sizing
of the discontinuity.
The dimension of the discontinuity is always determined from the time of flight of the diffracted signals.
The signal amplitude is not used in size estimation.
Key
d
1 transmitter probe Discontinuity.
e
2 receiver probe Lower tip path.
a f
Lateral wave path. Back wall echo path.
b
Upper tip path.
Figure 2 — Basic TOFD configuration
The basic configuration for the TOFD technique consists of separate ultrasonic transmitter and receiver
probes (see Figure 2).
Longitudinal wave probes with wide-angle beams are typically used since the diffraction of ultrasonic
waves is only weakly dependent on the orientation of the discontinuity tip. This enables the testing of a
certain volume in one scan.
However, restrictions apply to the size of the volume that can be tested during a single scan (see 8.2).
The first signal to arrive at the receiver probe after emission of an ultrasonic pulse is usually the lateral
wave which travels just beneath the scanning surface of the test object.
In the absence of discontinuities, the second signal to arrive at the receiver probe is the back wall echo.
These two signals are typically used for reference purposes. If mode conversion is neglected, any signals
caused by discontinuities in the material should arrive between the lateral wave and the back wall echo,
since the latter two correspond, respectively, to the shortest and longest paths between transmitter and
receiver probe. For similar reasons the diffracted signal generated at the upper tip of a discontinuity will
arrive before the signal generated at the lower tip of the discontinuity. A typical pattern of indications
(A-scan presentation) is shown in Figure 3 for the configuration shown in Figure 2.
The height of the discontinuity can be deduced from the difference in time of flight of the two diffracted
signals (see 9.1.5).
Note the phase reversal between the lateral wave and the back wall echo, and between echoes of the upper
and lower tip of the discontinuity.
Where access to both surfaces of the test object is possible and discontinuities are distributed throughout
the wall thickness, scanning from both surfaces will improve the overall precision, particularly in regard to
discontinuities near the surfaces.

ISO 16828:2025(en)
Key
b
T time Upper tip signal.
c
A amplitude Lower tip signal.
a d
Lateral wave. Back wall echo.
Figure 3 — Schematic A-scan presentation of an embedded discontinuity
5.2 Requirements for surface condition and couplant
The surface condition shall meet at least the requirements stated in ISO 16810.
Since the diffracted signals can be weak, the degradation of signal quality due to poor surface condition will
have a severe impact on the reliability of the test.
a) Different coupling media can be used, but their type shall be compatible with the materials to be tested.
Examples of coupling media are: water (possibly containing an agent e.g. wetting, anti-freeze, corrosion
inhibitor), contact paste, oil, grease, and cellulose paste containing water.
b) The characteristics of the coupling medium shall remain constant throughout the testing.
c) The coupling medium shall be suitable for the temperature range in which it will be used.
5.3 Materials and process type
Due to the relatively low signal amplitudes that are used in the TOFD technique, the technique can be applied
routinely on materials with relatively low levels of attenuation and scatter for ultrasonic waves. In general,
application on unalloyed and low-alloyed carbon steel components and welds is possible, but also on fine-
grained austenitic steels and aluminium.
Coarse-grained materials and materials with significant anisotropy however, such as cast iron, austenitic
weld materials and high-nickel alloys, will require additional validation and additional data processing.
By mutual agreement, a representative test block with artificial and/or natural discontinuities can be used
to confirm testability.
Note that diffraction characteristics of artificial discontinuities can differ significantly from those of real
discontinuities.
6 Qualification of test personnel
Personnel performing testing with the TOFD technique shall be qualified in accordance with ISO 9712, and
shall have received additional training and examination on the use of the TOFD technique on the products to
be tested as specified.
ISO 16828:2025(en)
7 Requirements for test equipment
7.1 General
The test equipment used for the TOFD technique shall be in accordance with the requirements of ISO 22232-1
and ISO 22232-2.
It is recommended to verify the setup and the accuracy of the time-to-depth conversion according to 8.7.
7.2 Instrument and display
a) The receiver bandwidth shall, as a minimum, range between 0,5 and 2 times the nominal probe frequency
at −6 dB, unless specific materials and product classes require a larger bandwidth. Appropriate band
filters can be used;
b) the transmitter pulse can either be unipolar or bipolar. The pulse duration shall not exceed 0, 5 times
the period corresponding to the nominal probe frequency;
c) unrectified signals shall be digitized with a sampling rate of at least six times the nominal probe
frequency;
d) to select an appropriate portion of the time base within which signals (A-scan data) are digitized, a
window with programmable position and length shall be present;
e) window start shall be programmable between 0 µs and 200 µs from the transmitter pulse, window
length shall be programmable between 5 µs and 100 µs; in this way, the appropriate signals (lateral
or creeping wave, back wall signal, one or more mode converted signals as described in 8.3) can be
selected to be digitized and displayed;
f) digitized A-scan data should be displayed in amplitude related grey or single-colour levels, plotted
adjacently to form a B-scan presentation. See Figure 4 and Figure 5 for typical B-scan presentations of
perpendicular and parallel scans, respectively;
g) the number of grey or single-colour scales should at least be 64;
h) for archiving purposes, the equipment shall be capable of storing all A-scan or B-scan data (as
appropriate) on a storage medium such as hard disc or solid-state memory;
i) for reporting purposes, it shall be capable of making hard copies or screen captures of A-scan or B-scan
presentations (as appropriate);
j) the equipment should be capable of performing signal averaging;
k) in order to achieve the relatively high-gain settings required for typical TOFD signals, a pre-amplifier
may be used, which should have a flat response over the frequency range of interest;
l) this pre-amplifier shall be positioned as close as possible to the receiving probe;
m) additional requirements regarding features for basic and advanced analysis of discontinuities are
described in Clause 9.
ISO 16828:2025(en)
a) Typical TOFD setup b) Corresponding B-scan presentation
Key
1 reference line 6 lateral wave
2 direction of probe displacement (X-direction) 7 discontinuity upper tip
perpendicular to the propagation of the ultrasound
3 transmitter probe 8 discontinuity lower tip
4 receiver probe 9 back wall reflection
5 time of flight (through-wall extent)
Figure 4 — Typical perpendicular scan in weld testing
a) Typical TOFD setup b) Corresponding B-scan presentation

ISO 16828:2025(en)
Key
1 reference line 6 lateral wave
2 direction of probe displacement parallel to the 7 discontinuity upper tip
propagation of the ultrasound (Y-direction)
3 transmitter probe 8 discontinuity lower tip
4 receiver probe 9 back wall reflection
5 time of flight (through-wall extent)
Figure 5 — Typical parallel scan in weld testing
7.3 Probes
a) The ultrasonic probes used for the TOFD technique shall initially be in accordance with ISO 22232-2;
b) number of probes: 2 (transmitter and receiver);
c) type: any suitable probe (see 8.2);
d) wave mode: usually longitudinal wave; the use of transverse wave probes is more complex but may be
agreed upon in special cases;
e) both probes shall have the same centre frequency within a tolerance of ±20 %; for details on probe
frequency selection see 8.2;
f) the pulse duration of the lateral wave shall not exceed two cycles, measured at 10 % of the peak
amplitude;
g) the pulse repetition frequency shall be set such that no interference occurs between signals caused by
successive transmission pulses.
7.4 Scanning
a) Scanning mechanisms shall be used to maintain a constant distance and alignment between the index
points of the two probes.
b) Scanning mechanisms shall provide the ultrasonic equipment with probe position information in order
to enable the generation of position-related B-scan presentations.
Information on probe position can be provided by means of e.g. incremental, magnetic or optical
encoders, or potentiometers.
c) Scanning mechanisms in TOFD can either be motor or manually driven.
They shall be guided by means of a suitable guiding mechanism (e. g. steel band, belt, automatic track
following systems, guiding wheels).
d) Guiding with respect to a reference line (e.g. the centre line of a weld) shall be kept within a tolerance of
±10 % of the probe centre separation (PCS).
e) For general applications, combinations of ultrasonic equipment and scanning mechanisms shall be
capable of acquiring and digitizing signals with a rate of at least one A-scan per 1 millimetre scan length.
f) Data acquisition and scanning mechanism movement shall be synchronized for this purpose.

ISO 16828:2025(en)
8 TOFD setup procedures
8.1 General
Probe selection and probe configuration are important TOFD setup parameters. They largely determine the
overall accuracy, the signal-to-noise ratio and the coverage of the region of interest of the TOFD technique.
The setup procedure described in this clause intends to ensure:
— sufficient system gain and signal-to-noise ratio to detect the diffracted signals of interest;
— acceptable spatial resolution and adequate coverage of the region of interest;
— efficient use of the dynamic range of the test system.
8.2 Probe selection and probe separation
8.2.1 Probe selection
In this subclause typical probe arrangements are given for TOFD testing in order to achieve good detection
capabilities on both thin and thick test objects.
Note that these arrangements are not mandatory and that the exact requirements to achieve a specification
should be checked.
a) For steel thicknesses up to 70 mm, a single pair of probes can be used.
b) The recommended probe selection parameters to achieve sufficient resolution and
adequate coverage are shown in Table 2 for three different ranges of wall thicknesses.
Table 2 — Recommended probe parameters for a single TOFD setup on steel thicknesses up to 70 mm
Wall thickness, W Centre frequency Transducer diameter Nominal probe angle
mm MHz mm °
< 10 10 to 15 2 to 6 50 to 70
10 ≤ W < 30 5 to 10 2 to 6 50 to 60
30 ≤ W < 70 2 to 5 6 to 12 45 to 60
c) For thicknesses greater than 70 mm, the wall thickness shall be divided into more than one test zone,
each zone covering a different depth region.
Table 3 shows the recommended centre frequencies, transducer sizes and nominal beam angles to
achieve sufficient resolution and adequate coverage for thick materials from 70 mm up to 300 mm.
d) These zones can be tested simultaneously or separately.
Table 3 — Recommended probe parameters for multiple TOFD setups on steel thicknesses from
70 mm up to 300 mm
Depth region, d Centre frequency Transducer diameter Nominal probe angle
R
mm MHz mm °
< 30 5 to 10 2 to 6 50 to 70
30 ≤ d < 100 2 to 5 6 to 12 45 to 60
R
100 ≤ d ≤ 300 1 to 3 10 to 25 45 to 60
R
ISO 16828:2025(en)
8.2.2 Probe separation
The maximum diffraction efficiency occurs when the included angle is about 120°.
a) The probes should be arranged such that the (imagined) beam centre lines intersect at about this angle
in the depth region where discontinuities are anticipated/sought.
b) Deviations of more than −35° or +45° from this value may cause the diffracted echoes to be weak and
should not be used unless detection capabilities can be demonstrated.
8.3 Time window setting
a) The time window recorded shall at least cover the depth region of interest, as shown e.g. in Table 2 and
Table 3.
b) Ideally, the time window recorded should start at least 1 µs prior to the time of arrival of the lateral
wave, and should at least extend up to the first back wall echo.
c) Because mode converted echoes can be of use in identifying discontinuities, the recorded time window
should also include the time of arrival of the first mode-converted back wall echo.
d) Where a smaller time window is appropriate (e.g. to improve sizing accuracy), it will be necessary
to demonstrate that discontinuity detection capabilities are not impaired, for instance by using
representative discontinuities or diffracting artificial discontinuities in a reference block as described
in Annex A.
8.4 Sensitivity setting
The aim is to make sure that the signals from discontinuities are covered by the dynamic range of the
ultrasonic instrument and that the limiting noise is acoustic rather than electronic.
a) The probe separation and the time window shall be set to those values that will be used in the subsequent
testing.
b) The instrument settings (e.g. gain, electronic noise suppression) shall be adjusted such that the
electronic noise is reduced to a minimum. When testing the area near the surface, the electronic noise
prior to the arrival of the lateral wave should be at least 6 dB lower in amplitude than within the region
of the time base after the arrival of the lateral wave.
c) The amplitude within the region of the time-base after the arrival of the lateral wave should be set to
approximately 5 % of the amplitude scale.
In case of very low acoustic noise, e.g. for fine grained steel, or when examining an area in the volume or
near the back wall, the sensitivity setting and the acceptable signal-to-noise ratio shall be specified in
the test procedure.
The sensitivity setting can now be checked making use of representative discontinuities or diffracting
artificial discontinuities in a reference block as described in Annex A.
d) Side-drilled holes may be used provided that the signal from the bottom surface of the side-drilled hole
is used for sensitivity setting, and not the signal from the top surface of the side-drilled hole.
e) Different shapes of notches may be used provided they generate diffracted waves.
The results can be used to justify reducing the gain setting or give warning that the signal-to-noise ratio is
insufficient (e.g. less than 12 dB).
8.5 Scan increment setting
The scan increment setting is defined as the distance between adjacent recorded A-scans in the direction of
probe displacement.
ISO 16828:2025(en)
The scan increment setting shall be dependent on the wall thickness to be tested.
a) For thicknesses up to 10 mm, the scan increment shall be no more than 0,5 mm.
b) For thicknesses between 10 mm and 150 mm, the scan increment shall be no more than 1 mm.
c) Above 150 mm, the scan increment shall be no more than 2 mm.
8.6 Setting of scanning speed
The scanning speed shall be selected such that it is compatible with the requirements of 8.3, 8.4 and 8.5.
8.7 Checking of system performance
The system performance shall be checked before and after a test by recording and comparing an appropriate
number of representative A-scans.
For a given PCS, setting of time-to-depth conversion is best carried out using the lateral wave signal and the
back wall signal with the known material sound velocity.
This setting shall be verified on a suitable block of known thickness with an uncertainty of ±0,05 mm. At
least one depth determination shall be performed in the depth range of interest, typically by recording a
minimum of 20 A-scans.
The determined thickness or depth shall be within 0,2 mm of the actual or known thickness or depth. For
curved components geometrical corrections can be necessary.
9 Interpretation and analysis of data
9.1 Basic analysis of discontinuities
9.1.1 General
a) Reporting or acceptance criteria shall be agreed upon by the contracting parties prior to testing.
b) Reporting or acceptance criteria shall be written in a specification prior to testing.
c) Discontinuities detected by TOFD shall be characterized by at least their:
1) position in the test object (
...