ToFD PCS for a target crossing depth

The probe centre separation is set from the beam angle and the depth at which the two beam axes are to cross. Each probe is refracted at angle theta to the normal, so a beam that must reach depth d on the centreline covers a horizontal distance of d x tan(theta). The two probes are therefore separated by PCS = 2*d*tan(theta), measured between the beam exit (index) points and not between the wedge fronts or the probe casings.

The crossing point is where the two beams overlap most strongly, but detection is not confined to it. The useful zone extends over the whole region where both beams still carry appreciable amplitude, which for a typical 60 to 70 degree ToFD pair is a large fraction of the wall. Setting the crossing point too shallow starves the lower wall of energy; setting it too deep widens the PCS and with it the near-surface dead zone.

Because the depth calculation uses S = PCS/2 directly, the PCS that is actually achieved matters more than the one that was intended. Measure it between index points on the scanner frame, then confirm it against the predicted lateral wave and backwall arrival times on known-thickness material before scanning.