In refereed (PDI'S) based on fit-for-purpose in-service; the Oil and Gas and Drilling Contractors use 6 mm length and.5 mm depth and non-visual for width. Using MT based on API RP 2X, ET BSI 1711 and ACFM ASTM-A36 MT the probability of detection for MT and ET were 90% while ACFM was 76%.