Plate gauge with apertures ASTM E
Plate gauges with apertures (penetrameters) are usually required in the US regulations.
It is plates of unified thickness with drilled perpedicular apertures(with one diameter by API), or more various diameters (ASTM, ASME), while the required diameter depends on the wall thickness.
The testing sensittivity includes both the diferenciation of the gauge outlines (contrast), as well as the designated aperture in the gauge (distinctive ability).
In the ASTM system there are the plates arranged in the way that their thickness presents the required testing sensitivity 1%, 2% or 4% of the tested object's wall thickness. The diameter of the aperture is multiplication of the plate thickness, and it is marked as 1T, 2T and 4T. The qualty of the picture 2 - 2T is the most frequent requirement in the ASME/ASTM specifications and it means that the thickness of the penetrameter (gauge) must be 2% of the wall thickness and there must be a visible 2T aperture on the radiogramm.