ToFD scan increment and speed

ToFD data is collected as a series of A-scans triggered by an encoder as the scanner moves along the weld. The scan increment is the distance between consecutive A-scans. It fixes the along-weld resolution of the D-scan image and therefore the shortest flaw that appears as a coherent arc rather than as one isolated line.

Two constraints apply at the same time. The increment must not exceed the value allowed by the procedure, commonly 1 mm for walls up to 50 mm and 2 mm above that, and it must be an exact multiple of the encoder resolution, otherwise the instrument will drop or duplicate lines. An increment of 0.5 mm on an encoder producing 10 pulses per millimetre means one A-scan every 5 pulses, which is exact; 0.25 mm on the same encoder is not, because it falls between the second and third pulse.

Scan speed is then limited by how fast the instrument can deliver averaged A-scans. With a pulse repetition frequency f and n averages the effective line rate is f/n, and the maximum speed is that rate multiplied by the increment. Exceeding it causes missed lines, which appear in the D-scan as blank columns or as smearing of the flaw arcs.

If more averaging is needed for signal-to-noise, either the pulse repetition frequency has to rise or the scan has to slow down. Raising the PRF too far causes wrap-around ghosting from the previous pulse, so on thick sections and highly attenuative material slowing the scan is usually the correct answer.