What is the formula for tofd scan increment and speed?

pulses per step = increment × encoder resolution; effective line rate = PRF / averages; max scan speed = (PRF / averages) × increment; A-scans per metre = 1000 / increment. Checks the ToFD scan increment against the allowed value and the encoder resolution, and gives the maximum scan speed for the pulse rate and averaging in use.

What should I check before relying on the result?

Scan speed must be limited in the software or by the operator; most instruments will not stop a scan that is going too fast, they simply lose lines. Check the encoder calibration over a measured distance before every job, not just its nominal pulses per millimetre. Very high PRF can cause wrap-around ghosting from the previous pulse, particularly on thick or attenuative material. Slowing the scan is usually safer than raising the PRF. A smaller increment improves along-weld resolution and flaw arc definition, at the cost of file size and scan time. Verify against the governing edition: ISO 10863:2011 §8 (scan increment and encoder requirements); ASME BPVC Section V, Article 4, Mandatory Appendix III. Increment limits are the usual banding and must be checked against the governing procedure..

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.

Compression wave

Worked example

Wall thickness	25 mm
Scan increment	0.5 mm
Encoder resolution	10
Pulse repetition frequency	500 Hz
Averages per A-scan	4
Maximum allowed increment	1 mm
Encoder pulses per A-scan	5
Effective line rate	125 Hz
Maximum scan speed	62.5 mm/s
A-scans per metre of weld	2000
Minimum time per metre	16 s

A 25 mm wall falls in the band allowing a 1 mm increment, so 0.5 mm is acceptable. At 10 pulses per mm, 0.5 mm is exactly 5 encoder pulses per A-scan. With 500 Hz PRF and 4 averages the effective line rate is 500/4 = 125 A-scans per second, so the maximum speed is 125 x 0.5 = 62.5 mm/s. One metre of weld is 1000/0.5 = 2000 A-scans and takes at least 1000/62.5 = 16.0 s.

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