Weld radiography acceptance levels: ISO 5817, ISO 10675 and who decides

"Quality level B to ISO 5817." The line arrives inside a specification, usually copied from an earlier contract, and the engineering office reads it as a general request to do good work. That single letter has already decided which technique class the weld radiography has to follow and which acceptance level the indications will be judged against. It has therefore also decided how long the part sits under the beam and how many shots end up as repeats. The bill shows up on the first batch, when the images the inspection room has been producing for years stop being enough and nobody can say which standard made that happen.

Three standards, three jobs that don't substitute for each other

The confusion seen most often in the specifications in circulation concerns which family a standard belongs to, before any detail. ISO 17636-1:2022 and ISO 17636-2:2022, the first for film and the second for digital detectors, say how the image is made and how you prove it is good. The scope of Part 2 explicitly excludes acceptance levels for the indications found on digital radiographs and points to the ISO 10675 series. That series says what gets rejected, with Part 1 of 2021 for steel, nickel, titanium and their alloys and Part 2, also 2021, for aluminum. Above both sits ISO 17635, the general standard that guides the choice of method, techniques and acceptance levels. The current text is the April 2025 edition, which replaced the 2016 edition that many specifications still cite.

Writing that a weld "is accepted to ISO 17636" is therefore a category error, because that standard does not contain the criterion, much as the competition rules don't contain the final standings. ISO 17635 also states its own boundaries. It applies to completed welds, excluding base material before welding and inspection between passes, and it covers steel, aluminum, nickel, titanium and their alloys. It asks for personnel qualified to ISO 9712 or equivalent and, above all, leaves the extent of testing to an application standard or the contract specification.

ISO 5817 and ISO 10675: the letter in the spec picks a row of three cells

The tables that tie quality level, technique class and acceptance level together sit outside the technique standard. They sit in Tables 2 and 3 of ISO 10675-1:2021, one for film radiography and one for CR and digital detectors. Quality level B carries technique class B and acceptance level 1, level C carries class B and level 2, and level D carries class A and level 3. For level C a note allows, on circumferential welds, the minimum number of exposures of class A. Every combination for film and digital radiography, plus aluminum, ultrasonic, penetrant and magnetic particle testing, is built into our ISO 5817 technique class and acceptance level tool, which also returns the line to paste into your inspection plan. It follows that asking for the strictest acceptance level while allowing the basic technique is a combination the standard doesn't offer, and a specification written that way puts the shop somewhere no written procedure can rescue.

The client picks the row when setting the quality level, or the product standard picks it on the client's behalf. The radiographer receives it already written and has to build the quotation around it. The scope of ISO 17635:2025 adds a warning worth summarizing. Acceptance levels are not a direct interpretation of the quality levels of ISO 5817 or ISO 10042; they relate to the overall quality of the batch of welds. The correspondence between the two therefore holds on a general basis, not indication by indication. Annex A, normative in the 2025 edition under the title “Rules and standards to be applied”, collects the correlations between quality levels, testing levels and acceptance levels for specific testing techniques. It applies unless the specification or the product standard makes a different choice, while the criteria for judging indications still come from the standard that sets the acceptance levels, ISO 10675 for radiography.

For aluminum the pair changes name and keeps its shape: quality levels come from ISO 10042:2018 and acceptance levels from Part 2 of ISO 10675, with the same three rows. It matters most in shops that run steel fabrication and light-alloy structures side by side, with two different specifications open on the same desk.

A single extra letter, then, changes the criterion. Acceptance levels 2 and 3 can be specified with an X suffix, which declares any indication longer than 25 mm unacceptable, whatever the level limits would allow. The client lays it over the chosen level as a length constraint, and it shifts the reject rate without touching the quality level letter. Anyone who receives a spec with an X next to the number is holding a stricter requirement than it looks.

In steel fabrication the designer picks the letter through the execution class

In steel structures the designer starts from the execution class, and the weld quality level follows from it. EN 1090-2:2018+A1:2024 ties fabrication, inspection and testing requirements to the EXC set in design. Visual examination is always required at 100%, while the extent of supplementary nondestructive testing comes from clause 12.4.2.2 and Table 24 and the minimum waiting times after welding from Table 23. The project specification can also name specific joints to test, with their own extent and method. On UK highway structures the proportion of testing is also adjusted case by case, down for automatic or robotized processes and up for higher steel grades, site welds and repairs.

For EXC3 joints the acceptance criterion for imperfections is quality level B of EN ISO 5817, and the testing method is chosen among those described in ISO 17635. These points, along with the UK rule on proportions, can be read in Guidance Note 6.02 of the Steel Bridge Group published by SCI, which cites the 2024 edition of the standard. The percentages have to be read in the standard itself.

In practice the chain in a fabrication shop starts two links above where the engineering office expects to find it. The execution class settles the extent of testing and the quality level; the quality level settles technique class and acceptance level. Only at the end does somebody check whether the in-house radiographic system can reach the class that has meanwhile been agreed by contract. Running alongside are the welding quality requirements of the ISO 3834 series, in the comprehensive, standard and elementary grades of Parts 2, 3 and 4, all in their 2021 editions. They frame how a shop shows it can produce that level as well as measure it.

What changes in the inspection room when the class goes from A to B

The two classes carry modest names, basic techniques and improved techniques, and ISO 17636-2 treats them as regulated alternatives. Class B is used where class A is not sensitive enough, and techniques of even higher sensitivity are allowed if the parties agree and specify all parameters. For digital work the choice of technique is agreed between the contracting parties in any case. At equal class, flaw visibility between film and digital counts as equivalent, and the proof runs through image quality indicators. Wires to ISO 19232-1 or step-and-hole IQIs to ISO 19232-2 cover sensitivity, and the duplex wire IQI of ISO 19232-5 covers unsharpness and basic spatial resolution. That last quantity entered the title of the standard with the 2018 edition, a sign that in digital work the thinnest visible wire no longer tells you on its own how good the image is.

One point speaks directly to whoever signs the specification. The informative Annex A of ISO 10675-1 notes that radiography readily detects volumetric imperfections in class A as in class B. Crack detectability depends instead on height, branching, opening and orientation to the beam, and class B or better raises it above class A. Where the defect that worries the designer is a crack, the class stops being contractual paperwork and becomes the difference between finding and not finding. How much focal spot size and distances weigh in this is covered in our article on focal spot, unsharpness and X-ray tube parameters.

The standard also provides the way out, and regulates it. Where a class B condition cannot be achieved for technical reasons, such as the available source or the source-to-object distance the part geometry imposes, the parties may work under class A conditions. The lost sensitivity is compensated with a higher minimum gray value and a higher normalized signal-to-noise ratio, and the test then counts as class B if the IQIs reach the required sensitivity. ISO 17636-2 formalizes three compensation principles, for reduced contrast, for insufficient detector sharpness and for the loss introduced by bad pixel correction. This belongs in a written procedure agreed between the parties, settled before the first failing part shows up on site.

A CR system's class belongs to the whole chain, and the imaging plate is one link

Computed radiography quotations keep repeating a phrase that looks like a specification: "class … imaging plate". ISO 16371-1:2011, called up as a dated reference by ISO 17636-2:2022, states in its scope that CR system classes do not refer to the imaging plates of a particular manufacturer. The class results from using a given plate together with the exposure conditions, in particular total exposure, the scanner type and the scanning parameters. Changing scanner or read-out parameters is enough to move it, and a datasheet quoting the plate alone proves nothing about the system that will enter the inspection room.

The same standard separates two kinds of test that commercial discussions often blur. Manufacturer tests need special tooling a user laboratory normally lacks; user tests are simpler and serve quick verification and long-term stability. A buyer should know which of the two produced the number in front of them.

Who qualified the system, and who verifies it over time

The division of duties is sharper still in the American standards many specifications call up alongside the ISO set. ASTM E2446-24 is the manufacturer's characterization. It needs specialized phantoms, states it is not intended for users and carries the criteria by which CR systems are classified into performance levels, so that systems from different suppliers become comparable. ASTM E2445/E2445M-20 for computed radiography and ASTM E2737-23 for digital detector arrays are user practices instead. They establish a baseline and monitor it over time, and both state that they are not an acceptance test of the system. The examination itself runs to ASTM E2033/E2033M-24 for CR and ASTM E2698-26 for DDAs, while image exchange and archiving use DICONDE, the ASTM E2339-21 format harmonized with medical DICOM.

For a buyer all of this reduces to one question for the supplier: which standard produced the number in the offer, and who ran it. Without that detail a "certified system" is a sales phrase, and values from the two families, manufacturer qualification and user verification, don't compare with each other.

The saving digital brings, and the line that eats it

On the move from film to digital, one of the most solid public references is still a paper by a research team at BAM, the German federal institute for materials research, published on NDT.net in 2007. Companies reported exposure times cut to as little as 5 to 25% of film exposures with CR and DDAs. The processing and interpretation cycle shrank to about a quarter in mobile testing and to under 5% in serial inspection of castings and welds, on top of savings on consumables and chemical waste.

In the same text the authors write the other half, the one quotations leave out. Digital does not by itself deliver film image quality, and contrast sensitivity depends on exposure conditions, with a real risk of misapplication. With digital detector arrays and multi-gain calibration, contrast sensitivity can improve up to ten times, but getting there takes exposures that can run longer than film. The time figures are company statements gathered by the authors, and the paper is several years old, so they read as orders of magnitude.

Then there is a line that hardly anyone budgets. With film, the checks covered processing and the densitometer; digital adds periodic verification of the detector, and that verification has an owner, a phantom, an interval and an archive of results to maintain. The cost is modest, but it takes consistency. When the verification lapses, you lose the ability to show, two years from now, that the system was in order on the day that joint went through.

Who can sign the report when the image is a file

ISO 9712:2021 covers the radiographic method among the certifiable methods and allows its system to apply to techniques within an established method, provided a complete scheme exists. The modules, though, are written by the certification bodies' schemes. In the BINDT PCN scheme, in a 2024 requirements document, radiographic techniques are listed separately: film, computed radiography with imaging plates, digital radiography with direct detectors, computed tomography and the interpretation-only variants. Those for direct detectors and tomography are stated as still in development, and for interpreting digital images the document asks for at least 12-bit images in DICONDE or TIFF format.

A misunderstanding follows that is worth defusing before signature. The abbreviation RT-D in ISO 17635:2025, with its variants for CR and DDA, designates the method, while personnel qualification depends on the certification body's scheme. Anyone committing to deliver reports on digital images should check with their own certification body which techniques are certifiable today, before promising them to a customer.

What to send us for an assessment

One practical rule falls out of the whole chain: whoever knows their row buys a system, whoever doesn't buys a detector and discovers the row afterwards. To assess your weld radiography we need these facts:

• quality level required and the document that imposes it (specification, product standard, execution class);
• material, thickness and joint geometry, including any aluminum case;
• technique class and acceptance level required, with any X suffix;
• defect of concern, volumetric or crack, because the class changes detectability;
• source available and distances the part imposes, to see whether class B is achievable or must be compensated;
• current system (film, CR or DR) and who performed its classification;
• data format and archiving requirements;
• volumes: joints or exposures per shift or per year.

You can send them by email to info@dipitech.com or on WhatsApp, attaching the specification or inspection procedure.

With those facts we help you choose the configuration that genuinely holds the line in your specification, among the digital industrial radiography solutions, with a tailored quote within 24 hours and no obligation. The choice between computed radiography and digital detector arrays is covered in CR or DR in industrial radiography and in the criteria for choosing a DR detector for weld inspection. How a pipe weld is actually radiographed, from exposure geometry and IQI to reading the defects, is shown in the typical cases of weld and casting radiography. Where the joint is too complex for a projection to settle it, the next step is industrial computed tomography.