This equipment can be used in several modes but simplistically gates of variable width can be set at time-of-flight positions to include the echoes of interest. The procedure starts by establishing the correct positions for the gates with the scanner coupled to a flat area adjacent to the welds to be inspected. From this it is possible to establish the distance to the back face, the total stack thickness and the distance to the faying surface detectable by its non-continuity when there is no weld present. Once this has been done the scanner can be positioned over the weld area to be analysed. Traditional A-scan and C-scan images are automatically displayed after each scan is completed. Each colour pixel in the C-scan image corresponds to a single A-scan image and represents 0–100% signal amplitude within the selected gate. As annotated in the typical A-scan and C-scan results shown in Fig. 3, the echoes in the A-scan occur when sound waves are reflected back from an interface, but these will not occur when the same interface has been fused, such as in a spot welded area.