Some codes require that shims be used under penetrameters to compensate for thickness of a backing ring or weld reinforcement. Shims are supplied in the same materials as penetrameters. They are available in 1/16" increments, from 1/16" thru 1" thick. These dimensions allow approximately 1/8" clearance on each side of the penetrameter with which they are to be used. Material and size identification indicating actual thickness is marked on each shim.