Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

The standard shown in the above figure is commonly known in the US as an
IIW type reference block. IIW is an acronym for the International Institute of
Welding. It is referred to as an IIW "type" reference block because it was
patterned after the "true" IIW block but does not conform to IIW requirements
in IIS/IIW-23-59. "True" IIW blocks are only made out of steel (to be precise,
killed, open hearth or electric furnace, low-carbon steel in the normalized
condition with a grain size of McQuaid-Ehn #8) where IIW "type" blocks can
be commercially obtained in a selection of materials. The dimensions of "true"
IIW blocks are in metric units while IIW "type" blocks usually have English
units. IIW "type" blocks may also include additional calibration and references
features such as notches, circular groves, and scales that are not specified by
IIW. There are two full-sized and a mini versions of the IIW type blocks. The
Mini version is about one-half the size of the full-sized block and weighs only
about one-fourth as much. The IIW type US-1 block was derived the basic
"true" IIW block and is shown below in the figure on the left. The IIW type US-
2 block was developed for US Air Force application and is shown below in the
center. The Mini version is shown on the right.