Resolution of Inspection System

T.O. 33B-1-1 1-60 1.8.2.4 Resolution of Inspection System. NOTE 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. 1.8.2.4.1 Definition. 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. NOTE 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. Frequency (MHz) 1 2.25 5 10 15 Entry Surface Resolution in Aluminum (inch) 0.5 0.375 0.25 0.125 0.125 Back Surface Resolution in Aluminum (inch) 0.5 0.3 0.2 0.1 0.1 1.8.2.4.2 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). Figure 1-13. Use of an IIW Block to Check Back Surface Resolution.