The Image Quality Indicator — universally known in the field as the IQI, and also called a penetrameter — is the single most important quality control device in industrial radiographic testing. Every radiograph produced under ASME Section V Article 2 must demonstrate that the radiographic technique achieved the minimum required sensitivity, and the IQI is the tool that provides that demonstration. Get IQI selection or placement wrong, and every radiograph taken with that incorrect technique is invalid — requiring re-exposure, re-inspection, and the delays and costs that follow.

Despite its critical importance, IQI selection is one of the topics where radiographers and welding inspectors most frequently make errors in the field. The errors take several forms: selecting the wrong IQI number for the material thickness, placing the IQI on the wrong side of the weld, forgetting the F marker for film-side placements, failing to place IQIs at both ends of long weld exposures, and misidentifying the 2T hole requirement. These errors are not matters of academic principle — they have real consequences: rejected radiograph packages during ASME Code inspection, failed surveillance audits by the Authorised Inspector, and in the worst cases, pressure components in service that have not been adequately examined.

This article provides a complete, code-referenced guide to IQI selection and use under ASME Section V Article 2. Every rule is stated with its paragraph reference, every selection table is explained with practical worked examples, every common error is identified and corrected, and a 30-question timed quiz tests your knowledge across the complete subject.

What is an IQI and Why is it Required? T-260

An Image Quality Indicator is a small reference device, made from a material radiographically similar to the object being examined, that is placed on the object during radiographic exposure. When the radiograph is processed, the image of the IQI appears on the film. The analyst examines this IQI image to verify that the radiographic technique produced an image of sufficient quality — that small details of specified size are clearly visible.

What the IQI Does NOT Do

This is the single most important conceptual point about IQIs, and it is frequently misunderstood by trainees and even experienced practitioners:

The IQI as Technique Validation

ASME Section V T-261 states the purpose clearly: IQIs are required to provide visual evidence that the radiographic examination technique produces the required radiographic sensitivity. If the required IQI image is not visible on the radiograph, the technique has not met the code minimum sensitivity requirement and the radiograph is invalid — the examination must be repeated with an improved technique.

Hole-type vs Wire-type IQI T-233, T-276, T-283

ASME Section V Article 2 recognises two distinct types of IQI, each with its own design standard, selection table, and sensitivity acceptance criterion. Both types are acceptable under the code; the choice is at the discretion of the examination procedure, provided the correct selection table is used for the chosen type.

IQI Selection — Tables T-276 and T-283 T-276, T-283

IQI selection is a code-mandated process, not an engineering judgement call. The applicable ASME Section V table specifies exactly which IQI designation number (for hole-type) or essential wire number (for wire-type) must be used for each range of nominal single-wall radiographic thickness. Using a thinner IQI than required produces an artificially good sensitivity result — the IQI appears on the film more easily than warranted. Using a thicker IQI than required is overly conservative and will cause unnecessarily frequent failed technique validations.

Hole-type IQI — Table T-276 (Representative Values)

Wire-type IQI — Table T-283 (Representative Values)

Radiographic Sensitivity and the 2-2T Rule T-274

The concept of radiographic sensitivity is expressed in terms of the IQI used. The standard ASME Section V sensitivity expression is stated as a ratio: the IQI designation number as a percentage of the nominal radiographic thickness, followed by the hole that must be visible.

The “2-2T” designation is the standard minimum sensitivity requirement for ASME pressure vessel and pressure piping radiography. Better sensitivity than 2-2T is always acceptable and preferred — it means the film quality is better than the minimum required. The code only establishes a minimum floor; better technique that achieves 1-2T or 2-1T sensitivity is entirely acceptable and may be required by some Owner specifications for critical weld examinations.

IQI Placement Rules T-277

ASME Section V T-277 specifies exactly where the IQI must be placed on the part. The placement rules are designed to ensure that the IQI image on the film represents the worst-case image quality that the technique can achieve over the weld area being examined — if the IQI is visible in the worst position, it will also be visible in all better positions within that exposure.

T-277.1 — General Placement Rules

• The IQI shall be placed on the source side of the part being examined — between the radiation source and the part. This is the preferred standard position.
• The IQI shall be placed on the weld or adjacent to the weld within the area of interest being radiographed. The IQI must be within the area of diagnostic interest shown on the radiograph.
• The IQI shall be placed with the body of the IQI (for hole-type) or the wire set (for wire-type) within the area being examined — not on the edge of the film holder or outside the area of interest.
• The lead identification numbers and letters on the IQI must be visible on the radiograph and must not obscure the area being examined for discontinuities.
• For hole-type IQIs, the IQI identification number must be on the end of the IQI away from the weld, so the IQI numbers do not overlap with the weld image.

T-277.1(a) — IQI on Weld vs Adjacent Material

The IQI may be placed directly on the weld surface (if the weld surface condition permits consistent probe coupling) or adjacent to the weld on the base material at the same nominal thickness as the weld being examined. If the weld cap creates a significant thickness variation that would affect the IQI image, the IQI is typically placed on the base metal adjacent to the weld rather than on the weld cap.

T-277.2 — IQI Placement for Welds with Backing Strips

When a weld is made with a permanent backing strip (backing bar), the backing strip thickness must be included in the nominal radiographic thickness used for IQI selection. If the backing strip is present during radiographic examination, it adds to the total material thickness that the radiation must penetrate, which requires a thicker (higher-numbered) IQI than would be needed without the backing strip.

Source-side vs Film-side IQI Placement T-277.1(b)

Source-side placement is the standard and preferred position for all IQIs in ASME Section V radiographic examination. However, there are practical situations — particularly when radiographing the inside of vessels or pipe bends from outside — where access to the source side is restricted and the IQI must be placed on the film side. ASME Section V provides specific rules for both situations.

Film-side Placement Requirements T-277.1(b)

When source-side IQI placement is impractical (documented in the examination record), film-side placement is permitted under the following specific conditions:

1. Lead letter F marker: A lead letter “F” must be placed adjacent to the IQI on the part, so that the letter F appears on the radiograph next to the IQI image. This permanently records on the film that a film-side IQI was used.
2. One designation number better: For film-side placement, the IQI selected must be one designation number better (thinner) than the table requirement for the nominal radiographic thickness. If the source-side table requires IQI No. 20, film-side placement requires IQI No. 17 (the next thinner standard IQI).
3. Justification documented: The examination record must document why source-side placement was impractical — it is not acceptable to use film-side placement simply for convenience.

Number of IQIs Required per Exposure T-277.2

ASME Section V T-277.2 specifies the minimum number of IQIs that must be used based on the length of weld being radiographed in a single exposure:

IQI Geometry — Dimensions in Detail T-233, ASTM E1025, E747

Hole-type IQI Dimensional Requirements

Per ASTM E1025 (referenced by ASME Section V), hole-type IQIs must meet precise dimensional requirements:

• Thickness tolerance: ±5% of the designated thickness
• Length and width: Minimum 25 mm (1 in) long and 10 mm (3/8 in) wide for most IQI numbers; larger IQIs have proportionally larger dimensions
• Hole diameters: 1T hole, 2T hole, and 4T hole — each drilled perpendicular to the plaque face with diameter tolerance ±5%
• Identification numbers: Lead numbers (or numbers of the same material as the IQI) that are 1T tall, permanently attached and visible on the radiograph
• Material: Must be radiographically equivalent to or more difficult to penetrate than the material being examined — never use a more penetrable (lighter) material IQI on a denser workpiece

IQI Material Groups T-276.1

ASME Section V Table T-276.1 assigns materials to IQI material groups based on radiographic equivalence. The IQI material must be from the same group as or a more absorbing group than the material being examined:

Weld Reinforcement and IQI Selection T-274

The nominal radiographic thickness used for IQI selection is the single-wall thickness of the material being examined. ASME Section V T-274.1 specifies how weld reinforcement affects the IQI selection calculation:

Special Cases and Exceptions

Radiographic Shims T-277.3

When the IQI is placed on a curved surface (such as a pipe curvature) and there is a gap between the IQI and the weld surface that would create a step change in radiographic thickness, a radiographic shim may be placed under the IQI to bring it to the same radiographic level as the weld being examined. The shim material must have the same radiographic equivalence as the weld material and must be sized to ensure the IQI sits at the correct radiographic level.

Single-Film Technique for Double-Wall Exposures

For double-wall radiographic techniques (where the radiation passes through both walls of a pipe and only one wall is being examined on the film), the IQI is placed on the source-side wall adjacent to the weld being examined. The nominal radiographic thickness for IQI selection is based only on the single wall containing the weld being evaluated — not the total double-wall thickness the radiation must penetrate.

Inaccessible Welds — Substrate Technique

When the weld being radiographed is in a location where the IQI cannot physically be placed on the weld itself (such as inside a closed vessel with no internal access), a separate substrate block of the same nominal radiographic thickness as the weld is radiographed simultaneously with the production exposure. The IQI is placed on this substrate, and the IQI image from the substrate exposure demonstrates the technique sensitivity. The examination record must document this alternative placement and reference the substrate qualification radiograph.

Acceptance Criteria for IQI Images T-274.2, T-285

A radiograph meets the IQI sensitivity requirement when the following conditions are all simultaneously satisfied:

1. The IQI designation number is clearly visible on the radiograph and matches the required designation from Table T-276 (hole-type) or the required essential wire from Table T-283 (wire-type)
2. For hole-type IQIs: the 2T hole is clearly visible (standard 2-2T requirement) — both its outline and its density difference from the background must be discernible
3. For wire-type IQIs: the essential wire is clearly visible — its complete image across the width of the IQI must be distinguishable from the background
4. The film density in the area of interest is within the specified range (typically 1.8 to 4.0 for X-ray film, or as specified in the applicable code edition)
5. For film-side placement: the lead F marker is visible adjacent to the IQI image
6. The IQI image does not overlap the weld or any area being examined for discontinuities (IQI identification numbers must be clear of the examination area)

Common Errors in IQI Selection and Use

IQI Material Groups T-276.1

The IQI must be made from a material radiographically similar to or more dense than the material being examined. ASME Section V Table T-276.1 groups materials by radiographic equivalence. A steel IQI (Group 3) is acceptable for use on austenitic stainless steel (Group 4) because steel is more absorbing than austenitic stainless at typical X-ray energies — if the IQI is visible on the stainless steel radiograph, the sensitivity through the denser IQI material confirms adequate technique. The reverse is not permitted: a lighter material IQI (aluminium) cannot be used on steel because the aluminium IQI would appear too readily on the steel radiograph, creating a false impression of good sensitivity that the technique has not actually achieved through the steel material.

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