The 2-2T equivalent sensitivity level, meaning an Image Quality Indicator (IQI) thickness of 2% of the inspected material and a visible hole of 2× the IQI thickness, remains the default acceptance threshold across film and digital radiographic procedures in 2026, with codes such as ASME Section V, ASTM E1025, ASTM E747, and EN 462-1 anchoring the call-out [S1][S3][S4].
For a 25.4 mm (1 in.) steel weld the working IQI is therefore roughly 0.51 mm (0.020 in.) thick with a 1T, 2T, and 4T hole, and the 2T hole must be visible on the radiograph; minimum IQI thickness is conventionally 0.13 mm (0.005 in.) where the 2% rule would otherwise drop below that floor [S3][S4]. Codes that demand tighter than 2-2T, such as 1-1T for some nuclear and aerospace work or 2-1T where higher contrast is required, are achieved by selecting a thinner IQI set (wire set A/B) or by changing the radiographic technique rather than by changing the IQI itself [S2][S3].
What "2-2T" actually means and where the numbers come from
ASTM E1025 governs hole-type (placard) IQIs and groups shims into eight absorption classes so the IQI material matches the part material, while ASTM E747 covers wire-type IQIs in sets A through D with the set letter and material group marked on the carrier [S3]. The 2-2T notation is read as "two percent thickness, two-T hole": the first number is IQI thickness as a percentage of the region-of-interest thickness, the second is the smallest hole diameter (as a multiple of IQI thickness) that the radiograph must resolve [S3][S7]. A 2-4T call-out, common on thicker castings and ASME Section V nuclear components, means the same 2% thickness IQI is used but the 4T hole must be visible, which trades spatial-resolution demand for easier detectability on heavy sections [S3][S4]. The 0.005 in. (0.13 mm) minimum IQI thickness is a long-standing floor that prevents 2% on very thin stock from disappearing into the noise floor of the imaging system [S4].
Material match is not optional. The IQI must be made from, or be in the same absorption group as, the part being radiographed, which is why E1025 ships aluminum, steel, and other metal shims with notches or lead numbers that identify the group at a glance [S3]. EN 462-1 covers the European wire-type family (Fe1/Fe2, Al1/Al2, Cu1/Cu2, Ni1/Ni2) used when European or EN-based codes govern the job, and the wire diameter in millimetres runs from 0.05 mm up to 3.2 mm depending on set and material [S4].
Where 2-2T applies and where it does not
Welds in pressure-bearing pipework, ASME Section I and Section VIII pressure vessels, and most petroleum piping fall back to 2-2T as the default; API 1104 for pipeline girth welds and AWS D1.1 for structural welds both reference the same 2% / 2T family of sensitivity, while MIL-R-11471 (ORD), NAVSEA 250-1500-1, and T9074-AS-GIB-010/271 add their own IQI designation system for military and naval work [S4]. Codes that demand tighter than 2-2T typically call out 2-1T (higher contrast, common in aerospace) or 1-1T (very thin austenitic welds and select nuclear components); 4-2T and 2-4T are used where thicker, harder-to-penetrate sections make the 2T hole impractical to resolve [S3][S4]. Sensitivity requirements are also a function of the part's job: critical-service welds, nuclear ASME Section III components, and flight-critical aerospace parts are routinely pulled toward 2-1T, while general structural and process-pipe work stays at 2-2T [S3][S4][S7].
Film radiography versus computed and digital radiography sensitivity
Halmshaw's work on industrial radiography IQIs is explicit that image unsharpness has to change considerably before either wire or hole IQIs register a sensitivity change, so a 2-2T radiograph is necessary but not sufficient: a real defect of the same diameter and material loss may still be invisible because production discontinuities have gradual boundaries rather than the sharp edges of an IQI hole [S3][S6]. For computed radiography (CR) and digital radiography (DR), the Equivalent Penetrameter Sensitivity (EPS) approach was developed at the National Research Council Canada because a CR plate can resolve the required IQI hole while the underlying signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) are still too low for reliable defect detection [S5]. EPS uses a plate with a graded series of holes, counts the smallest hole visible at the working kV/mA/SDD setting, and gives a quantitative single-number sensitivity that tracks the CR system's intrinsic contrast sensitivity at a given exposure level rather than just confirming a single IQI is visible [S5]. In day-to-day practice, an inspector can expect EPS to read 1-1T or 1-2T on a well-tuned CR system, and EPS degrades faster than the visible IQI when the IP is worn, the scanner optics drift, or the exposure is shortened, which is precisely the failure mode EPS was built to catch [S5].
Selection between film, CR, and DR therefore changes the sensitivity check, not the sensitivity target: a 2-2T code call-out still has to be met, but the evidence on the radiograph is a film IQI image for film, an EPS-derived equivalent for CR, and either an IQI placement or an EPS plate for DR, depending on the governing code [S2][S5]. Process engineers specifying a new radiography bay in 2026 should plan for film IQIs per ASTM E1025/E747 plus EN 462-1 wire sets for European work, and an EPS test plate plus CR-scanner SNR/CNR baseline for the digital side, so that acceptance can be defended with both the visible IQI and a quantitative sensitivity number [S3][S4][S5].
Comparison of common IQI sensitivity levels
Reading the main options side by side, 1-1T is the tightest call-out, used for thin austenitic welds and selected nuclear components where the part is below roughly 6 mm and any defect signature must be maximised; 2-1T is the standard aerospace level, holding IQI thickness at 2% but demanding the smaller 1T hole be visible; 2-2T is the default for ASME Section V, AWS D1.1, API 1104, and most general industrial work; and 2-4T is used on heavy castings and thick-wall vessels where the 2T hole cannot be resolved but the technique is still controlled at 2% thickness [S3][S4][S7]. The trade-off is direct: a smaller second number drives harder spatial-resolution requirements on the imaging chain, while a smaller first number drives harder contrast/SNR requirements on the exposure; codes pick the pair that matches the failure mode they are trying to catch [S3][S7].
Quoted from the field
"The sensitivity of radiographic technique is usually represented by ability to visualize a percentage change of some parameters with respect to the test piece thickness, where smaller percentage means better radiographic sensitivity." [S5] That sentence, from the 2018 NRC EPS paper, is the cleanest one-line definition of what a sensitivity number means in 2026 practice, and it underpins both the 2-2T default and the EPS metric that sits behind it on CR systems [S5].
Limitations and failure modes of 2-2T
Three failure modes are worth flagging before any radiograph is accepted on a 2-2T call-out: the IQI is the wrong material group (aluminum IQI on a steel part silently breaks the comparison), the IQI is placed on a non-representative region of the part such that scatter or geometry differs from the area of interest, and the radiograph passes the 2-2T check but the actual discontinuities have gradual boundaries that the IQI's sharp holes overstate [S3][S6]. For CR and DR, the additional failure mode is a clean visible IQI masking poor SNR/CNR; the EPS test exists for exactly this case, and an inspection programme that skips EPS on CR is accepting the visible IQI as proof of technique quality when the imaging chain may be borderline [S5]. Halmshaw's 5-cited note that image unsharpness has to change considerably before wire or hole IQI sensitivity shifts is the reason a 2-2T radiograph can pass while real defects are missed, and is the reason high-reliability codes layer additional geometric unsharpness (Ug) and SNR/CNR checks on top of the IQI placement [S6].
Sourcing, standards, and what to track next
The governing standards to keep on hand for a 2026 IQI programme are ASTM E1025 (hole-type design and material groups), ASTM E747 (wire-type IQIs), ASTM E1742 (alternate hole-type), EN 462-1 (European wire-type), ASME Section V Article 2 (radiographic examination), AWS D1.1, API 1104, and for digital work the NRC Canada's EPS methodology as published in the 2018 ECNDT paper [S3][S4][S5][S7]. For readers cross-checking the ASTM E1025 hole-type IQI family or the wire-type IQI sensitivity scaling against related test methods, note that the 2-2T default is shared with the ASTM E747 wire sets used in European and aerospace work [S3][S4]. Trackable signals over the next reporting cycle are any revision of ASTM E1025/E747 that changes the 2% rule or the 0.005 in. minimum, and any update to ASME Section V Article 2 on EPS acceptance for CR/DR, both of which would shift the practical sensitivity floor across the industry [S3][S4][S5].
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