prEN ISO 11666
(Main)Non-destructive testing of welds - Ultrasonic testing - Acceptance levels (ISO/DIS 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
- Publication Date
- 02-Mar-2028
- Technical Committee
- CEN/TC 121 - Welding
- Drafting Committee
- CEN/TC 121/WG 21 - Testing of welds
- Current Stage
- 4020 - Submission to enquiry - Enquiry
- Start Date
- 10-Sep-2026
- Completion Date
- 10-Sep-2026
prEN ISO 11666 is a draft CEN/ISO standard for non-destructive testing of welds by ultrasonic testing. It specifies acceptance levels 2 and 3 for full penetration welded joints in ferritic steels, so inspectors can judge whether ultrasonic indications are acceptable when testing welds to ISO 17640. In practice, it gives the rules for setting sensitivity, measuring indication length, grouping indications, and applying acceptance criteria.
What does prEN ISO 11666 specify?
prEN ISO 11666 specifies ultrasonic acceptance levels for welded joints, and it is centered on AL 2 and AL 3. These levels correspond to ISO 5817 quality levels B and C, which matters because they link ultrasonic findings to weld quality levels used in fabrication and inspection specifications.
The document applies mainly to full penetration fusion-welded joints in ferritic steels with low sound attenuation. It can also be used for other weld types, materials and thicknesses when the geometry and acoustic properties are taken into account and suitable sensitivity can be achieved.
The document is organized into:
- Clause 4 - determination of discontinuity length
- Clause 5 - sensitivity levels
- Clause 6 - acceptance levels, including grouping and cumulated length
- Annex A - levels
- Annex B - fixed-amplitude level technique
| Annex | What it covers |
|---|---|
| Annex A | The level system used for the different techniques and thickness ranges |
| Annex B | The fixed-amplitude level technique for measuring discontinuity length |
What are the key requirements of prEN ISO 11666?
prEN ISO 11666 sets out how to measure ultrasonic indications, how to set sensitivity, and how to decide acceptance. The practical effect is that a user needs a consistent testing method and a clear rule set for when an indication is acceptable, when it needs further testing, and when several indications must be treated together.
How is discontinuity length determined?
Clause 4 says to measure discontinuity length by scanning along the indication and taking the distance over which the echo amplitude stays above the evaluation level. Annex B gives the fixed-amplitude level technique used for that measurement.
In practice, this means the length decision is based on a repeatable probe movement and a defined signal threshold, not on visual judgment alone. That helps different operators reach comparable results.
How is sensitivity set?
Clause 5 links the acceptance system to the sensitivity-setting technique used in ISO 17640. The document allows several techniques, including distance-amplitude curve (DAC), time-corrected gain (TCG), distance-gain-size (DGS), a reference notch, and the tandem technique.
This matters because the acceptance levels depend on the chosen reference and technique. A lab or site needs to use the correct technique for the weld thickness, beam angle and inspection arrangement, otherwise the acceptance decision is not comparable with the standard.
What acceptance logic applies?
Clause 6.1 ties the acceptance levels to the quality levels and testing levels used with ISO 5817 and ISO 17640. It also states that the levels apply across the testing levels and techniques defined in ISO 17640, including straight-beam probe testing.
When characterization is required in line with ISO 23279, planar discontinuities are not acceptable. If characterization is not required, the acceptance levels apply to all discontinuities. That distinction matters in specifications because it changes whether the inspector must identify the discontinuity type before making the acceptance decision.
How are indications from longitudinal discontinuities handled?
Clause 6.2 says the document uses the indication length, the maximum echo amplitude, and the technique-specific level settings in Annex A. If an indication is below the acceptance level but long enough above the evaluation level, further testing is required with additional beam angle(s), and possibly the tandem technique if specified.
In practice, long but weaker indications are not ignored. They trigger a second look so the inspector can confirm whether the indication is really acceptable.
How are transverse discontinuities treated?
Clause 6.3 applies the same acceptance levels to transverse discontinuities when detection of transverse discontinuities is specified. That matters because some inspection plans focus mainly on longitudinal defects, while others also require transverse-discontinuity evaluation.
How are grouped and accumulated indications assessed?
Clause 6.4 says several individually acceptable indications can be treated as one group when their separation meets the stated distance conditions. The combined length and the largest amplitude in the group are then checked against the applicable acceptance level.
Clause 6.5 adds a cumulated-length rule for a defined weld-length section. It limits the total length of acceptable indications above the recording level, with a stricter allowance for AL 2 than for AL 3. This prevents a weld from passing with many small indications spread along the same section.
What terms does prEN ISO 11666 define?
prEN ISO 11666 uses a small set of terms that drive the whole acceptance decision. The most important are the level terms and the sensitivity-setting methods.
- Acceptance level 2 (AL 2) - the acceptance level linked to ISO 5817 quality level B.
- Acceptance level 3 (AL 3) - the acceptance level linked to ISO 5817 quality level C.
- Reference level - the signal level used as the basis for the other levels.
- Evaluation level - the threshold used to judge indication length and compare indications.
- Recording level - the threshold above which an indication is recorded for assessment.
- Distance-amplitude curve (DAC) - a sensitivity-setting method based on reference reflectors and signal distance.
- Time-corrected gain (TCG) - a sensitivity-setting method that compensates signal level with sound path.
- Distance-gain-size (DGS) - a sensitivity-setting system based on reflector size and distance.
Who uses prEN ISO 11666?
prEN ISO 11666 is used by ultrasonic testing personnel who inspect welded joints in ferritic steels. It is also used by welding quality staff, NDT coordinators, and inspectors who need a shared acceptance basis for weld examinations.
Manufacturers and fabricators use it when they write inspection plans for welded parts, and buyers use it when they specify acceptance criteria in procurement documents. Quality managers and conformity assessment teams use it to review whether a weld meets the agreed ultrasonic acceptance level.
What changed in prEN ISO 11666 from the previous edition?
prEN ISO 11666 cancels and replaces ISO 11666:2018. The document says the edition has been technically revised.
The stated changes are:
- editorial revision
- ISO 5817 made a dated normative reference
- Clause 5 described in more detail
- Clause 6.5 described in more detail
Which standards are used with prEN ISO 11666?
prEN ISO 11666 refers to three normative standards in its body, and it also points to ISO 17635 for the relationship between acceptance, testing and quality levels.
- ISO 5817 - provides the weld quality levels that AL 2 and AL 3 map to.
- ISO 17640 - provides the ultrasonic testing techniques, testing levels, and assessment framework used by this document.
- ISO 23279 - provides discontinuity characterization when characterization is required.
- ISO 17635 - gives the general rules for metallic materials and the wider relationship between weld acceptance and testing levels.
What does the prEN ISO 11666 document contain?
prEN ISO 11666 contains a worked acceptance system rather than a general theory of ultrasonic testing. It includes a length measurement method in Clause 4, sensitivity-setting techniques in Clause 5, and acceptance logic in Clause 6 for longitudinal and transverse indications, grouping, and cumulated length.
Annex A provides the level diagrams and the acceptance tables for the different techniques and thickness ranges. Annex B gives the fixed-amplitude level technique with a figure showing how the beam is scanned and how the length is taken from the positions where the signal reaches the evaluation level.
The document also includes figures for grouped discontinuities and for cumulated length along a weld section. These figures matter in practice because they show how to apply the text when several indications appear close together or along the same weld length.
Relations
- Effective Date
- 20-Nov-2024
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Frequently Asked Questions
prEN ISO 11666 is a draft published by the European Committee for Standardization (CEN). Its full title is "Non-destructive testing of welds - Ultrasonic testing - Acceptance levels (ISO/DIS 11666:2026)". This standard covers: 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.
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.
prEN ISO 11666 is classified under the following ICS (International Classification for Standards) categories: 25.160.40 - Welded joints and welds. The ICS classification helps identify the subject area and facilitates finding related standards.
prEN ISO 11666 has the following relationships with other standards: It is inter standard links to EN ISO 11666:2018. Understanding these relationships helps ensure you are using the most current and applicable version of the standard.
prEN ISO 11666 is available in PDF format for immediate download after purchase. The document can be added to your cart and obtained through the secure checkout process. Digital delivery ensures instant access to the complete standard document.
Standards Content (Sample)
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
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
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STANDARDS MAY ON OCCASION HAVE TO
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RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
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
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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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,
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 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
...