T.O. 33B-1-11-601.8.2.4ResolutionofInspectionSystem.NOTEBefore evaluating entry surface resolution, perform the distance calibration to avoidmisinterpreting an indication of a multiple echo from a discontinuity as theindication 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 canbe 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 followingprocedures are concerned only with entry and back surface resolutions, which are defined as the inspectable distancesnearest to the respective surfaces of the test material. Resolution shall be checked when specified and shall meet theminimum requirements as given in Table 1-8. This evaluation requires a reference standard with referencediscontinuities at the respective distances from the appropriate surfaces of the standard.NOTEThe 1 MHz and 15 MHz requirements are applicable only when these frequenciesare used; they are not general requirements for all instruments.Table 1-8. Limits of Boundary Surface Resolution.Frequency (MHz)12.2551015Entry Surface Resolution in Aluminum (inch)0.50.3750.250.1250.125Back Surface Resolution in Aluminum (inch)0.50.30.20.10.11.8.2.4.2CheckingResolutionwithanIIWBlock.When no resolution is specified, the following procedures may be used to check resolution with the IIW block (seeFigure 1-13).Figure 1-13. Use of an IIW Block to Check Back Surface Resolution.
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