Resolution of Inspection System.
Before evaluating entry surface resolution, perform the distance calibration to avoid
misinterpreting an indication of a multiple echo from a discontinuity as the
indication of the first echo.
Resolution is the minimum spacing between discontinuities for which separate and distinct ultrasonic echo signals can
be obtained. Spatial resolution refers to the lateral separation of discontinuities. Depth resolution, as the name implies,
refers to depth separation between internal discontinuities or a discontinuity and a boundary surface. The following
procedures are concerned only with entry and back surface resolutions, which are defined as the inspectable distances
nearest to the respective surfaces of the test material. Resolution shall be checked when specified and shall meet the
minimum requirements as given in Table 1-8. This evaluation requires a reference standard with reference
discontinuities at the respective distances from the appropriate surfaces of the standard.
The 1 MHz and 15 MHz requirements are applicable only when these frequencies
are used; they are not general requirements for all instruments.
Table 1-8. Limits of Boundary Surface Resolution.
Entry Surface Resolution in Aluminum (inch)
Back Surface Resolution in Aluminum (inch)
Checking Resolution with an IIW Block.
When no resolution is specified, the following procedures may be used to check resolution with the IIW block (see
Figure 1-13. Use of an IIW Block to Check Back Surface Resolution.