From measurements, I found the near field structures of Universal DGS curves tend to underestimate the FBH amplitude. I know the near field curves are obtained based on a compromise derived from measurements and calculations for various probes.
Are the curves were designed to be conservative and represent the minimum responses so that to incorporate the variations in the field applications ?
DGS curves do not include any reserve for measurement tolerances. The compensation for measurement uncertainties will be given by the measurement procedure. This has to set an acceptance level which is below the critical flaw size. For example, when the critical flaw size is a 3 mm FBH the acceptance level has to be set e.g. 6 or 12 dB below the 3 mm FBH DGS curve.