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 normalize 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 only about one-fourt 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.
IIW type blocks are used to calibrate instruments for both angle beam and normal incident inspections. Some of their uses include setting metal-distance and sensitivity settings,determining the sound exit point and refracted angle of angle beam transducers, and evaluating depth resolution of normal Beam inspection setups.