oSIST prEN ISO 11666:2026

• Abstract:

  ISO 11666:2018 specifies two ultrasonic acceptance levels known as acceptance level 2 (AL 2) and acceptance level 3 (AL 3) for full penetration welded joints in ferritic steels, which correspond to ISO 5817:2014, quality levels B and C. An acceptance level corresponding to ISO 5817:2014, quality level D is not included in this document, as ultrasonic testing is generally not requested for this weld quality.; These acceptance levels are applicable to testing carried out in accordance with ISO 17640.; ISO 11666:2018 applies to the testing of full penetration ferritic steel welds, with thicknesses from 8 mm to 100 mm. It can also be used for other types of welds, materials and thicknesses, provided the tests have been performed with necessary consideration of the geometry and acoustic properties of the component, and an adequate sensitivity can be employed to enable the acceptance levels of this document to be applied. The nominal frequency of probes used in this document is between 2 MHz and 5 MHz, unless attenuation or requirements for higher resolution call for other frequencies. It is important to consider the use of these acceptance levels in conjunction with frequencies outside this range carefully.

• Status: Not Published
• Public Enquiry End Date: 01-Nov-2026
• ICS: 25.160.40 - Welded joints and welds
• Technical Committee: VAR - Welding
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 10-Sep-2026
• Due Date: 28-Jan-2027
• Ref Project: prEN ISO 11666 - Non-destructive testing of welds - Ultrasonic testing - Acceptance levels (ISO/DIS 11666:2026)

• Revises: SIST EN ISO 11666:2018 - Non-destructive testing of welds - Ultrasonic testing - Acceptance levels (ISO 11666:2018)
• Effective Date: 01-Oct-2026

oSIST prEN ISO 11666:2026 is the SIST publication of the ISO draft standard on ultrasonic testing of welds. It specifies two ultrasonic acceptance levels, AL 2 and AL 3, for full penetration welded joints in ferritic steels, for testing carried out in accordance with ISO 17640. Engineers, inspectors and quality managers use it to decide whether ultrasonic indications in these welds are acceptable, whether they need further testing, and how to judge grouped or cumulative indications.

What does oSIST prEN ISO 11666:2026 specify?

oSIST prEN ISO 11666:2026 sets out acceptance levels for ultrasonic examination of welded joints, with the main focus on full penetration fusion-welded joints in ferritic steels. It links the ultrasonic result to weld quality levels in ISO 5817, so the acceptance decision can be tied to a defined weld quality target.

The document applies to welds examined in accordance with ISO 17640. It is intended for weld thicknesses from 8 mm to 100 mm, and it can also be used for other weld types, materials and thicknesses when the geometry and acoustic properties are taken into account and the sensitivity is suitable for the component being tested. The nominal probe frequency is in the 2 MHz to 5 MHz range unless attenuation or higher resolution requires another frequency.

Clause 1 covers scope, Clause 2 lists normative references, Clause 3 gives terms and definitions, Clause 4 explains how to determine discontinuity length, Clause 5 sets sensitivity levels, and Clause 6 gives the acceptance rules. The document also has two normative annexes that support the practical use of the main clauses.

Annex	What it covers
Annex A	Levels for the different sensitivity-setting techniques and thickness ranges
Annex B	Fixed-amplitude level technique for determining discontinuity length

What are the key requirements of oSIST prEN ISO 11666:2026?

oSIST prEN ISO 11666:2026 gives the decision rules for ultrasonic indications in ferritic steel welds. In practice, the operator calibrates sensitivity, measures indication length, and then checks the indication against the applicable acceptance level, including any rules for grouping or cumulative length.

Acceptance basis and applicability

Clause 6.1 links the acceptance levels to ISO 5817 quality levels B and C and states that they apply to all testing levels and all techniques defined in ISO 17640, including straight-beam probes. This matters because the same acceptance logic is used even when the inspection method changes.

When discontinuity characterization is required, ISO 23279 is applied. In that case, planar discontinuities are not acceptable, while non-planar discontinuities are judged against the acceptance levels in this document. That means the contract or inspection specification needs to say whether characterization is part of the assessment.

Measuring discontinuity length

Clause 4 defines discontinuity length by the distance along the weld where the echo amplitude remains above the evaluation level, using the fixed-amplitude level technique in Annex B. This is important because acceptance depends not just on signal height but also on how far the indication extends.

Annex B shows how to scan across an indication, note the probe positions where the echo falls to the evaluation level, and take the distance between those positions as the measured length. In practice, this gives a repeatable way to measure an indication before it is compared with the acceptance rules.

Sensitivity setting and reference levels

Clause 5 requires sensitivity setting according to ISO 17640 and names four techniques:

• a distance-amplitude curve (DAC) or time-corrected gain (TCG) using side-drilled holes
• a distance-gain-size (DGS) system using disk-shaped reflectors
• a reference notch for the thinner range and steeper beam angles
• the tandem technique using a flat-bottomed hole reference reflector

Clause 5 also defines the reference level, evaluation level, recording level and acceptance levels used in the assessment. Annex A ties these levels to the selected technique and thickness range. In practice, the test procedure must match the calibration method, the joint thickness and the probe arrangement, otherwise the acceptance comparison is not valid.

Indications from longitudinal and transverse discontinuities

Clause 6.2 covers indications from longitudinal discontinuities. If an indication is below the acceptance level but extends beyond the stated length threshold, it has to be subjected to further testing with additional beam angles and, where relevant, the tandem technique. This prevents a long, weak indication from being accepted without extra checking.

Clause 6.3 applies the same acceptance levels to transverse discontinuities when detection of transverse discontinuities is specified. In practice, the same acceptance framework is used, but only if the inspection requirement includes transverse flaw detection.

Grouping and cumulative length

Clause 6.4 allows nearby individually acceptable discontinuities to be treated as a single group when their separation meets the stated spacing conditions. The group is then assessed by its combined length and the larger of the two amplitudes. This matters because several small indications close together can behave like one larger flaw.

Clause 6.5 limits the cumulated length of individually acceptable discontinuities above the recording level within each weld-length section based on six times the thickness. The permitted cumulative length is tighter for AL 2 than for AL 3. In practice, even if each indication is individually acceptable, the overall distribution of indications in a weld section still has to stay within the document’s limit.

What terms does oSIST prEN ISO 11666:2026 define?

• Acceptance level 2 (AL 2) - the stricter of the two ultrasonic acceptance levels in this document, linked to ISO 5817 quality level B.
• Acceptance level 3 (AL 3) - the second ultrasonic acceptance level, linked to ISO 5817 quality level C.
• Distance-amplitude curve (DAC) - a calibration curve used as the reference in one of the sensitivity-setting techniques.
• Time-corrected gain (TCG) - a gain-correction method used with side-drilled holes to set sensitivity consistently over depth.
• Distance-gain-size (DGS) system - a sensitivity-setting system based on disk-shaped reflectors.
• Fixed-amplitude level technique - the method in Annex B for measuring discontinuity length from the positions where the echo falls to the evaluation level.
• Reference level - the baseline signal level used to set and compare the other assessment levels.
• Recording level - the level above which an indication is recorded for further assessment.

Who uses oSIST prEN ISO 11666:2026?

oSIST prEN ISO 11666:2026 is used by people who perform, specify or review ultrasonic testing of welds. That includes ultrasonic testing operators, welding inspectors, NDT engineers, quality managers, and buyers or specifiers who need a clear acceptance rule for welded joints.

It is relevant in welding and fabrication work where ferritic steel fusion welds are checked by ultrasonic testing. It is also useful when an inspection procedure, purchase specification or quality plan needs to state how indications are measured, grouped and accepted.

What changed in oSIST prEN ISO 11666:2026 from the previous edition?

oSIST prEN ISO 11666:2026 is the third edition and cancels and replaces ISO 11666:2018.

The foreword states these main changes:

• the document has been editorially revised
• the normative reference ISO 5817 has been dated
• Clause 5 and Clause 6.5 have been described in more detail

Which standards are used with oSIST prEN ISO 11666:2026?

• ISO 5817 - provides the weld quality levels that are linked to AL 2 and AL 3.
• ISO 17640 - provides the ultrasonic testing techniques, testing levels and assessment framework used by this document.
• ISO 23279 - is used when discontinuity characterization is required, especially for distinguishing planar from non-planar discontinuities.
• ISO 5577 - supplies the terms and definitions used in Clause 3.
• ISO 17635 - is cited in the bibliography and gives the general relationship between acceptance levels, testing levels and quality levels.

What does the oSIST prEN ISO 11666:2026 document contain?

The document contains clauses for scope, normative references, terms and definitions, discontinuity length, sensitivity levels and acceptance levels. Annex A provides the level diagrams and acceptance tables for the different techniques and thickness ranges, while Annex B explains the fixed-amplitude level technique used to measure discontinuity length.

The annex figures show how reference, evaluation, recording and acceptance levels relate for the different sensitivity-setting techniques. The tables in Annex A organize the acceptance rules by technique and weld thickness, and they are the main working reference during inspection and reporting.

SLOVENSKI STANDARD
01-oktober-2026
Neporušitveno preskušanje zvarnih spojev - Ultrazvočno preskušanje - Stopnje
sprejemljivosti (ISO/DIS 11666:2026)
Non-destructive testing of welds - Ultrasonic testing - Acceptance levels (ISO/DIS
11666:2026)
Zerstörungsfreie Prüfung von Schweißverbindungen - Ultraschallprüfung -
Zulässigkeitsgrenzen (ISO/DIS 11666:2026)
Essais non destructifs des assemblages soudés - Contrôle par ultrasons - Niveaux
d'acceptation (ISO/DIS 11666:2026)
Ta slovenski standard je istoveten z: prEN ISO 11666
ICS:
25.160.40 Varjeni spoji in vari Welded joints and welds
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 11666
ISO/TC 44/SC 5
Non-destructive testing of welds —
Secretariat: AFNOR
Ultrasonic testing — Acceptance
Voting begins on:
levels
2026-09-07
Essais non destructifs des assemblages soudés — Contrôle par
Voting terminates on:
ultrasons — Niveaux d'acceptation
2026-11-30
ICS: 25.160.40
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
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This document has not been edited by the ISO Central Secretariat.
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Reference number
ISO/DIS 11666:2026(en)
DRAFT
ISO/DIS 11666:2026(en)
International
Standard
ISO/DIS 11666
ISO/TC 44/SC 5
Non-destructive testing of welds —
Secretariat: AFNOR
Ultrasonic testing — Acceptance
Voting begins on:
levels
Essais non destructifs des assemblages soudés — Contrôle par
Voting terminates on:
ultrasons — Niveaux d'acceptation
ICS: 25.160.40
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document has not been edited by the ISO Central Secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2026
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
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Published in Switzerland Reference number
ISO/DIS 11666:2026(en)
ii
ISO/DIS 11666:2026(en)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Determination of discontinuity length . 1
5 Sensitivity levels . 2
6 Acceptance levels. 2
6.1 General .2
6.2 Indications from longitudinal discontinuities.3
6.3 Indications from transverse discontinuities .3
6.4 Grouping of discontinuities .3
6.5 Cumulated length of acceptable discontinuities . .4
Annex A (normative) Levels . 6
Annex B (normative) Fixed-amplitude level technique .18
Bibliography .21

iii
ISO/DIS 11666:2026(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,
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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).
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For an explanation on 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 the following URL:
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 44, Welding and allied processes, Subcommittee
SC 5, Testing and inspection of welds.
This third edition cancels and replaces the second edition (ISO 11666:2018), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— this document has been editorially revised;
— the normative reference ISO 5817 has been dated;
— Clause 5 and 6.5 have been described in more detail.
Requests for official interpretations of any aspect of this document should be directed to the Secretariat
of ISO/TC 44/SC 5 via your national standards body. A complete listing of these bodies can be found at
www.iso.org.
iv
DRAFT International Standard ISO/DIS 11666:2026(en)
Non-destructive testing of welds — Ultrasonic testing —
Acceptance levels
1 Scope
This document specifies two ultrasonic acceptance levels known as acceptance level 2 (AL 2) and acceptance
level 3 (AL 3) for full penetration welded joints in ferritic steels, in accordance with the quality levels of
ISO 5817.
These acceptance levels are applicable to testing carried out in accordance with ISO 17640.
This document applies to the testing of fusion-welded joints in metallic materials with low sound attenuation
(especially that due to scatter) with thicknesses from 8 mm to 100 mm. It is primarily intended for use on
full penetration welded joints where both the welded and parent materials are ferritic. It can also be used
for other types of welds, materials and thicknesses, provided the tests have been performed with necessary
consideration of the geometry and acoustic properties of the component, and an adequate sensitivity can be
employed to enable the acceptance levels of this document to be applied.
The nominal frequency of probes used in this document is between 2 MHz and 5 MHz, unless attenuation
or requirements for higher resolution call for other frequencies. It is important to consider the use of these
acceptance levels in conjunction with frequencies outside this range carefully.
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 5817, Welding — Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) —
Quality levels for imperfections
ISO 17640, Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels, and assessment
ISO 23279, Non-destructive testing of welds — Ultrasonic testing — Characterization of discontinuities in welds
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5577 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Determination of discontinuity length
The length of a discontinuity shall be determined by measuring the distance along the length over which the
echo amplitude is above the evaluation level, using the fixed-amplitude level technique specified in Annex B.

ISO/DIS 11666:2026(en)
5 Sensitivity levels
a) The sensitivity setting has been performed by one of the following techniques accordingly to ISO 17640.
Technique 1: the reference is a distance-amplitude curve (DAC) or a time-corrected gain (TCG) for side-
drilled holes of diameter 3 mm.
Technique 2: the references for transverse and longitudinal waves using the distance-gain-size (DGS) system
based on the diameter of disk-shaped reflectors (D ).
DSR
Technique 3: using a reference notch. The reference notch shall be rectangular, 1 mm wide with a depth of
1 mm. This
...