(b) Double-Wall Viewing. For materials and for welds
in components 31⁄2 in. (89 mm) or less in nominal
outside diameter, a technique may be used in which
the radiation passes through two walls and the weld
(material) in both walls is viewed for acceptance on
the same radiograph. For double-wall viewing, only a
source side penetrameter shall be used. Care should
be exercised to ensure that the required geometric
unsharpness is not exceeded. If the geometric unsharpness
requirement cannot be met, then single-wall
viewing shall be used.