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.
Compression wave
Worked example
| Target crossing depth | 20 mm |
| Refracted beam angle | 60 deg |
| Compression velocity | 5900 m/s |
| Half separation | 34.64 mm |
| Probe centre separation | 69.28 mm |
| Beam path to crossing point | 40 mm |
| Two-way transit time in material | 13.56 µs |
tan(60 deg) = 1.7320508. S = 20 x 1.7320508 = 34.6410 mm, so PCS = 69.2820 mm, which is 69.28 mm to 2 dp. Half path = 20 / cos(60 deg) = 20 / 0.5 = 40.00 mm. Two-way time in material = 2 x 40 / 5.9 = 80 / 5.9 = 13.5593 us, so 13.56 us.
Use at your own risk — verify before you act
These calculators support, and never replace, the judgement of qualified NDT and engineering personnel. Results are provided as is, without warranty of any kind, express or implied, and must be independently verified against the governing code edition named in your contract before being used in any inspection, acceptance, rejection, radiation-safety or fitness-for-service decision. By using them you accept full responsibility for how the results are applied; NDT Inspect, its owners and contributors accept no liability for any loss, damage, injury or death arising from their use or from reliance on them. If a result matters to safety, check it by hand and have it reviewed by a competent person.