Skip Distance (Angle-Beam Ultrasonics)

Before the probe touches the plate, the operator has to know how far to clean the surface and how far back to pull. Skip distance answers both: it is the surface distance between the beam entry point and the point where the beam returns to that same face after bouncing off the back wall. Since the weld cap can seldom be ground off, scanning happens away from the bead, on the second leg of the beam, and the width of the scanning band comes straight out of this figure.

S = 2 × t × tan(θ), with θ measured from the normal to the surface. Half of it, t × tan(θ), is the half skip, where the beam meets the back wall. For 25 mm of thickness and a 60° probe the tangent is 1.7321: half skip at 43.30 mm, full skip at 86.60 mm. The field check is the 45° probe, which draws a right-angle V whose two legs equal the thickness — the skip has to be exactly two thicknesses, and 25 mm returns 50.00 mm. Switching to 70° stretches the skip to 137.37 mm, close to three times the 45° figure.

The angle in the formula is the refracted angle inside the material, not the number stamped on the wedge. Nominal 45, 60 and 70 hold for shear waves in carbon steel; in austenitic stainless, Inconel or aluminium the true angle shifts and has to be measured on a calibration block, and it drifts further with wedge wear and with temperature. The distance runs from the beam exit point, not from the front of the probe. The geometry assumes a flat, parallel back wall, and the page takes any angle below 90°: 89° returns 2864.50 mm, right in arithmetic and useless in practice.