ASME V vs EN ISO in NDT: Differences That Change Procedures, Reports and PED Approval

BY DAN YAMASHITA · ASNT/PCN LEVEL III · IWE · SIMPLENDT · JUN 8, 2026 · 13 MIN READ

One of the most underestimated conversions for anyone moving from ASME fabrication to the European market is the NDT program. The temptation is to take the radiographic, ultrasonic or penetrant testing procedure already running under ASME Section V, swap the normative reference to "EN ISO" and move on. It does not work — and the Notified Body or the RTPO spots it at the first review.

The reason is that Section V and the EN ISO NDT standards are not two accents of the same language: they are two different architectures. What changes is the separation between method and acceptance, the selection of the image quality indicator, the testing class, the interpretation times and — perhaps most importantly for the report — where the acceptance criterion comes from. This article walks through those differences method by method.

The structural difference: method and acceptance are separate in ISO

In the ASME world, Section V says how to perform the examination, and the acceptance criterion sits in the construction code (Section VIII Division 1 or 2, B31.3, etc.). A single construction code references Section V and defines what is acceptable.

In the EN ISO world, method and acceptance live in separate standards: one standard says how to test, another defines the acceptance level, and both connect to the weld quality level of ISO 5817 through ISO 17635 (the "umbrella" standard that guides the selection of method, technique and acceptance level). This separation is what most disorients people coming from ASME.

RT — radiography: the IQI is the detail that fails most procedures

In radiography, ASME Section V Article 2 and EN ISO 17636-1 reach similar results by different routes. ISO works with two testing classes: class A (basic techniques) and class B (improved techniques, with higher sensitivity — in practice, the general case required by most specifications and pressure equipment applications). ASME does not use this nomenclature — it defines technique requirements (geometric unsharpness, source-to-film distance) without the A/B label.

The point that fails most hastily converted procedures is IQI selection (image quality indicator). In ASME, the thickness used to select the penetrameter/IQI is the nominal single-wall thickness plus the estimated weld reinforcement when present. In ISO 17636-1, the IQI is selected from the nominal thickness t or the penetrated thickness w of the wall(s), depending on the technique — without adding the reinforcement as the normal selection basis — and in the double wall double image (DWDI) technique, w = 2t is used. For the same joint, this can lead to different IQI wires or holes.

Practical note: class B is the general case — it is what most specifications and pressure equipment applications require, and what qualification to ISO 15614-1 normally presupposes. Use class A only when the product standard, the contract or the third party explicitly allows it.

Film system: where ASME radiographs fail before the first indication

ISO 11699-1 classifies film systems into six classes, from C1 (highest quality) to C6 — and ISO 17636-1 ties the film system class to the testing class: for class B, a C4 or better system is typically required in the usual combinations of source and steel thickness. ASME, in turn, imposes no system class: film choice is effectively free in practice, because sensitivity is demonstrated by the IQI on every exposure — films are classified per ASTM E1815, but Section V does not tie film class to technique.

The practical consequence: manufacturers working under ASME usually shoot routine work on Class II film (C5 — e.g. D7), reserving Class I for thinner sections. When radiographs shot that way are offered for an ISO/PED project at class B, the film system already falls short — the radiograph is rejected outright, before any indication is even evaluated. It is one of the quickest, and most ignored, checks when receiving a package of "reusable" radiographs.

Same weld, two IQIs: a 13 mm SWSI joint

To make the difference in selection basis concrete, consider a steel butt joint, single wall single image (SWSI), nominal thickness 13 mm, with the IQI on the source side:

• ASME V, Art. 2 (Table T-276): the selection basis is 13 mm plus the estimated weld reinforcement — at ~1.5 mm per face, ≈ 16 mm — falling in the range over 12.7 up to 19.0 mm: hole No. 20 (essential hole 2T) or essential wire No. 8 (0.41 mm, SE-747).
• ISO 17636-1, class B (Table B.3 — single wall, IQI on the source side): the basis is the nominal thickness t = 13 mm, with no reinforcement added — range over 12 up to 20 mm: wire W13 (0.20 mm).

The wire ISO requires is less than half the diameter of the one ASME requires — 0.20 mm versus 0.41 mm. A radiograph comfortably accepted under ASME may simply not resolve the wire that ISO class B demands. Combined with the film system class, this is why "reusing the film from the ASME project" rarely survives third-party review.

UT — ultrasonic testing: explicit testing levels

In ultrasonic testing, EN ISO 17640 introduces something that ASME Section V Article 4 does not label the same way: testing levels (A, B, C and D), in increasing order of rigor — the higher the level, the greater the required coverage, sensitivity and documentation. The testing level is chosen according to the intended quality level. Acceptance comes from ISO 11666, and the correspondence is direct: ISO 5817 quality level B → ISO 11666 acceptance level 2.

For advanced techniques, the separation continues: Phased Array has its method in ISO 13588 and its acceptance in ISO 19285; TOFD has its method in ISO 10863 and its acceptance in ISO 15626. Anyone already running PAUT under ASME (see the article on phased array in lieu of radiography) can reuse the technique, but needs to re-anchor the scan plan and the criteria in the EN ISO framework.

PT and MT — surface methods: times and criteria are not the same

In the surface methods, the differences show up in the times and, once again, in the acceptance. In penetrant testing, ASME Section V Article 6 sets the penetration time per Table T-672 (varying with material, product form and discontinuity type) and interpretation between 10 and 60 minutes after the developer — beyond 60 minutes, the interpretation is no longer valid. EN ISO 3452-1 uses a penetration time normally between 5 and 60 minutes (not less than the manufacturer's recommendation) and a development time typically on the order of 10 to 30 minutes. They are distinct windows, and the procedure must state the correct reference — not mix the two schools.

In magnetic particle testing, Section V Article 7 gives way to EN ISO 17638 for the technique, with acceptance per ISO 23278. In both surface methods, EN ISO acceptance leads to the "X" suffix for quality level B, as we will see next.

The acceptance criterion: the difference that changes the report

Here is the heart of the conversion. In ASME, you compare the indication against the construction code criterion. In EN ISO, you start from the ISO 5817 quality level (B, C or D) and translate it, through ISO 17635, into the acceptance level for each method. The same quality level becomes different numbers depending on the test — there is no single "acceptance level".

Correspondence per ISO 17635. The "X" suffix (MT/PT) means every detected linear indication is evaluated against level 1, the stricter one. For RT, ISO 10675-1 (Tables 2 and 3) confirms B → level 1; for UT, ISO 11666 (Table 1) confirms B → level 2.

The practical consequence is direct: in procedure qualification to ISO 15614-1, especially at Level 2, the path normally leads to ISO 5817 quality level B, with the exceptions stated in the standard itself — so on the NDT report of a PED/CE project you typically work with RT level 1, UT level 2 and MT/PT level 2X. That mapping must be explicit in the procedure and in the report.

Personnel and approval: who signs the report

Changing the method and acceptance standards is not enough if the person interpreting the test is not qualified against the right yardstick. In Europe, the normal route is certification to EN ISO 9712 — central certification, by a body independent of the employer — unlike the employer-based SNT-TC-1A logic. To interpret, evaluate against the criterion and take responsibility for the report, Level 2 (or 3) is required; Level 1 may perform tasks under instruction and supervision, but is not responsible for the accept/reject decision.

And there is the third-party layer: for Category III and IV equipment, the PED (Annex I, Section 3.1.3) requires NDT personnel qualification to be approved by an RTPO. SNT-TC-1A on its own does not satisfy this; personnel certified to other standards can be approved by an RTPO if equivalence to the ISO 9712 criteria is demonstrated, on an individual basis. The full detail is in the guide to PED, RTPO and CE marking.

In practice: converting an ASME V procedure to EN ISO

Putting it all together, converting an NDT procedure from the ASME world to EN ISO follows a clear sequence:

• Replace the standards chain: method to EN ISO 17636/17640/3452-1/17638 and acceptance derived from ISO 5817 via ISO 17635.
• Recalculate IQI selection (RT): from the applicable t/w of the wall(s), without adding reinforcement as the normal basis; DWDI w = 2t. Review class A/B, minimum density and film system (ISO 11699-1).
• Define the testing level (UT) per ISO 17640 and acceptance per ISO 11666; for PAUT/TOFD, use their own method and acceptance standards (ISO 13588/19285 and ISO 10863/15626).
• Adjust PT times: penetration and development per ISO 3452-1, instead of ASME's 10–60 min window.
• Remap the acceptance criterion per method (RT 1 / UT 2 / MT-PT 2X for ISO 5817 B) and state it in the report.
• Confirm personnel qualification to EN ISO 9712 — or an equivalent accepted individually by the RTPO — and, for Cat III/IV, the RTPO approval.

This is procedure engineering work, not translation. SimpleNDT does exactly this conversion: review and rewriting of NDT procedures to the EN ISO framework, remapping of acceptance criteria, and support for personnel qualification and approval — with PCN Level 3 (ISO 9712) certification and PED RTPO-approved NDT personnel status. See the NDT procedure services and PED compliance services.