IQI selection

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.

Hole IQI thickness (mm) = designation x 0.001 in x 25.4 = designation x 0.0254
Essential hole diameter = 2 x IQI thickness (2T)
Hole sensitivity % = 100 x IQI thickness / part thickness
Wire sensitivity % = 100 x essential wire diameter / part thickness