An image quality indicator is a deliberately imperfect artefact of known geometry placed on the part so the radiograph proves its own sensitivity. If the specified feature is visible on the film, the technique is demonstrated capable of showing a discontinuity of comparable dimension. It says nothing about whether the weld is sound – it says the radiograph is good enough to judge.

Two families are in use. A hole-type (plaque) IQI to ASTM E1025 is a thin plate of the same radiographic material group as the part, containing 1T, 2T and 4T holes where T is the plate thickness. The ASME designation number is that thickness in thousandths of an inch, so a number 17 IQI is 0.017 in thick and its 2T hole is 0.034 in across. The standard quality level is 2-2T: an IQI 2 percent of the part thickness with the 2T hole visible. A wire-type IQI to ASTM E747 or ISO 19232-1 is a set of parallel wires of graded diameter in a plastic envelope, and the essential wire is the thinnest that must be resolved.

ASME BPVC Section V, Article 2, Table T-276 fixes both selections against nominal single-wall material thickness, with separate columns for source-side and film-side placement. Source-side is the default; a film-side IQI is only permitted where source-side access is impossible, and it must then be marked with a lead letter F. The film-side requirement is one row less demanding because the IQI sits in a less magnified, sharper part of the beam.

Because IQI sizes come in discrete steps, achieved sensitivity in percent varies across a thickness band and is systematically worse on thin sections, where the smallest practical IQI is still a large fraction of the part. That is expected, not a technique fault.

Worked example

Nominal single-wall thickness	12.7 mm
IQI placement	source
ISO wire number required (from ISO 17636-1 Table 4)	w11
ASME hole-type IQI designation	17
Hole IQI thickness	0.432 mm
Essential 2T hole diameter	0.864 mm
Hole IQI sensitivity	3.4 %
ASME/ASTM E747 essential wire number	7
Essential wire diameter	0.32 mm
Wire IQI sensitivity	2.52 %
ISO 19232-1 wire diameter selected	0.32 mm
ISO wire sensitivity	2.52 %

12.7 mm is 0.500 in, the top of the ASME 'over 0.375 through 0.500 in' band, so source-side calls for hole designation 17 and essential wire 7. The number 17 IQI is 17 x 0.0254 = 0.432 mm thick with a 2T hole of 0.864 mm, giving 0.432/12.7 = 3.40 percent. ASTM E747 wire 7 is 0.0126 in = 0.32 mm, giving 0.32/12.7 x 100 = 2.52 percent. ISO wire W11 is also 0.32 mm.

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