Wedge delay and shear velocity from an IIW block

An angle-beam probe measures from the beam index point — the place where the beam leaves the wedge — but the instrument starts timing when the crystal fires. The time spent crossing the perspex or Rexolite wedge, typically 1 to 3 µs and around 2730 m/s, must be removed before the display reads true sound path. That is the wedge delay or probe delay.

Calibration blocks make this easy by putting a machined radius in front of the probe. When the index point sits on the centre of the arc, the shear wave travels one radius out and one radius back, so the echo comes from a known 2R of steel. The V1 (IIW) block gives a 100 mm radius; the V2 (mini, DIN 54122) block gives 25 mm and 50 mm radii from opposite ends. Sliding the probe for maximum amplitude from the arc also locates the index point, which is then marked against the block’s centre line.

Timing two radii removes the delay the same way a two-point thickness calibration does: the delay is common to both echoes, so the difference in time over the difference in path gives the shear velocity, c = 2(R₂ − R₁)/(TOF₂ − TOF₁), and back-substitution gives the delay. With one radius only, the velocity must be assumed — 3240 m/s for steel — and any error in it goes straight into the delay.

A worn wedge shows up here first: the delay creeps up, the index point moves back, and the refracted angle rises. Re-establishing delay, index and angle at the start of every shift is the difference between a plotted depth you can defend and one you cannot.