ToFD probe choice is a trade-off between resolution and penetration. Higher frequency and fewer cycles shorten the pulse, which shrinks both dead zones and improves depth resolution, but attenuation and beam divergence then limit how much wall a single setup can cover. Larger elements produce a narrower, more directional beam that carries further, at the cost of covering less depth range from one probe position.

Beam angle follows thickness. Thin sections use steep angles so that a workable probe centre separation still puts the crossing point at two-thirds of the wall; heavy sections use 45 to 60 degrees so the sound path, and therefore the attenuation, stays manageable. Above roughly 50 mm a single setup can no longer cover the wall and the examination is split into depth zones with a probe pair for each.

Because ToFD relies on the diffracted longitudinal arrival being first, the probes are compression-wave probes on angled wedges. Beam spread is used deliberately: a small element with a wide divergence illuminates a large depth range from a single position, which is why ToFD probes are usually much smaller than the pulse-echo probes used on the same joint.

The values here are banded starting points taken from the ToFD standards. They are a starting point only. The setup has to be proved on a reference block representative of the material, thickness, surface condition and weld geometry, and adjusted if the required flaw sensitivity, depth accuracy and near-surface coverage are not demonstrated.

Compression wave

Worked example

Wall thickness	30 mm
Compression velocity	5900 m/s
Recommended probe frequency	5 MHz
Recommended beam angle	60 deg
Recommended element size	6 mm
Recommended number of setups	1
Wavelength at recommended frequency	1.18 mm
Crossing depth for a single setup	20 mm
PCS for a single full-wall setup	69.28 mm
Pulse duration assumed	0.4 µs
Near-surface dead zone	9.12 mm

T = 30 mm falls in the 15 to 50 mm band, giving 5 MHz, 60 degrees, 6 mm elements and a single setup. c = 5.9 mm/us so the wavelength is 5.9/5 = 1.18 mm. Crossing depth = 2 x 30 / 3 = 20.00 mm, half separation = 20 x tan(60 deg) = 34.6410 mm and PCS = 69.2820 mm, that is 69.28 mm. A two-cycle pulse at 5 MHz lasts 0.400 us, which is 2.36 mm of material, so the near-surface dead zone is sqrt(34.6410 x 2.36 + 1.18^2) = sqrt(81.7527 + 1.3924) = sqrt(83.1451) = 9.1184 mm, so 9.12 mm.