ToFD probe selection by wall thickness

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.

f, θ, element size and number of setups are read from thickness bands
λ = c / f
d_cross = (2/3) × T
PCS = 2 × d_cross × tan(θ)
τ = n_cycles / f
near-surface dead zone = √( S × c × τ + (c × τ / 2)² )