Radiographic sensitivity

Radiographic sensitivity expresses the smallest detectable feature as a percentage of the penetrated thickness: sensitivity % = 100 x d / T, where d is the diameter of the finest wire resolved or the thickness of the hole-type IQI whose essential hole is visible. A 2 percent sensitivity on 25 mm of steel means a 0.5 mm feature was demonstrably visible.

Lower numbers are better, and 2 percent is the conventional benchmark that the ASME 2-2T quality level is built around. Achieving it depends on three independent things: contrast, set by beam energy, film class and processing; definition, set by geometric unsharpness and detector resolution; and noise, set by film grain or detector signal-to-noise. Any one of them can be the limiting factor, and improving the wrong one changes nothing.

For hole-type IQIs, ASTM E1025 defines the equivalent penetrameter sensitivity, EPS = 100 x sqrt(T_iqi x h / 2) / x, where h is the hole diameter and x the specimen thickness. It lets a 1T, 2T or 4T hole reading be put on a common scale. For the standard 2T hole the square root collapses to the IQI thickness and EPS reduces to the simple thickness ratio.

Sensitivity is a property of the radiograph, not of the weld. It is recorded on the report as evidence that the technique was capable of finding what the code requires, and a radiograph that fails its IQI requirement is rejected without ever being interpreted.

Wire:  sensitivity % = 100 x wire diameter / thickness
Hole:  sensitivity % = 100 x IQI thickness / thickness
EPS (ASTM E1025) = 100 x sqrt(T_iqi x h / 2) / x
h = hole diameter = multiplier x T_iqi