“…If this point is taken to its conclusion, calibration becomes a function only dependent on specimen geometry and the probe arrangement, meaning that geometrically similar test-pieces share identical calibrations. There are several methods to determine a reference potential value; measuring across a specified crack length such as the initial notch [6,40,42,55,117,140,153,155,170], measuring on the initially un-cracked test-piece [117,119,164], measuring on a separate reference specimen placed in the same condition as the test-piece [11,45,127,134] and measuring in a region remote from the crack so that the current field is independent of the crack and remains homogenous (as mentioned for the dual pair potential probes) [3,48,51,76,77,81,82,102,183,184]. Normalisations using a singular reference value allows the elimination of the effects of inherent material properties but excludes sustaining errors due to the variations of temperature and current during testing [151].…”