TOFD Simulator: Practise Time-of-Flight Diffraction Set-Up and Scanning

Time-of-flight diffraction (TOFD) locates and sizes flaws from when diffracted signals arrive, not from how big an echo is, which makes it a technique about geometry and timing. The TOFD simulator in Atlantis Practical NDT, with 9 jobs in the catalogue, makes you set the probe-centre separation, couple both probes and scan, and the simulator computes the lateral-wave, back-wall and tip-diffraction arrival times from the probe geometry you chose. Get the geometry wrong and the timing shows it.

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What you set up and scan

  1. Open the work order and confirm the weld, the procedure, the objective and the limitations of the job.
  2. Set the probe-centre separation (PCS) for the wall thickness and the depth you need the beams to cover. Work the numbers out beforehand with the free TOFD calculator, which gives PCS, tip depth and dead zone.
  3. Couple both probes. TOFD relies on the transmitter and receiver staying coupled at a fixed separation, and a position where coupling is lost earns no data.
  4. Scan along the weld, then review data quality before you interpret anything.
  5. Read the arrivals: the lateral wave travelling just under the surface, the back-wall echo, and between them any tip-diffraction signals from the upper and lower ends of a flaw.
  6. Record what you found against its position, finish the post-check and submit the training report for instructor review.

What the TOFD model computes

The TOFD model is geometric. From the probe-centre separation, the beam angles and the wall thickness it computes when the lateral wave, the back-wall echo and each tip-diffraction signal arrive. That is the relationship a TOFD technician has to carry in their head: the depth of a flaw tip comes from where its signal sits in time between the lateral wave and the back wall, and the shallow region hidden under the lateral wave, the dead zone, changes with the separation you set. Moving the probes apart and watching the arrivals shift is a quick way to build that intuition, and it costs nothing to repeat.

Like every Practical NDT signal model, it is analytical rather than a full-wave simulation, chosen so that it runs at interactive frame rates on ordinary devices and gives the same answer on each one.

Rehearsing a technique you rarely use

Many technicians use TOFD only on certain contracts, and skills decay without use. A meta-analysis of skill retention found losses growing from negligible straight after training to large after more than a year without use, with greater decay for cognitive and accuracy-based tasks. A technician who has not run TOFD for a year can rehearse it before mobilising for the job, at the independent level of assistance, where only the work order is shown, and arrive with the set-up sequence already back in hand.

Where the TOFD jobs run

TOFD jobs belong with weld work: coupons and fabrication joints in the welding workshop and weld jobs on the process plant. The phased array and conventional UT jobs on the same welds make a natural sequence, because many weld procedures pair TOFD with a pulse-echo technique.

What the TOFD simulator cannot teach

Computing arrivals from geometry does not show every mode-converted signal, the effect of weld cap and root profile on a real scan, or the noise of a real acquisition system. Nor can a screen teach the handling of a two-probe scanner on a real weld: keeping both probes coupled over a crown, round an obstruction or on a hot surface. Flaw sizing for acceptance belongs to a qualified procedure on real blocks, and the training report states that code acceptance is not evaluated.

TOFD practice and SNT-TC-1A

TOFD is an ultrasonic technique, so under SNT-TC-1A it falls within the UT method. The hours table on this site has no separate TOFD row: the recommended UT figures are 40 and 40 training hours and 210 and 630 hours of method experience for Level I and Level II, and the written practice decides what TOFD adds. Simulator time can be logged as organised training where the written practice allows it; it is not work in the work environment, so it is not experience.

Check the theory with the free TOFD Level II practice questions.

Frequently asked questions

What does the TOFD simulator calculate?

The arrival times of the lateral wave, the back-wall echo and the tip-diffraction signals, from the probe-centre separation and the probe geometry you set.

How do I choose the probe-centre separation?

From the wall thickness and the depth the beams need to cross. The TOFD calculator works out PCS, tip depth and dead zone, and the simulator lets you see what the choice does to the arrivals.

Can I practise TOFD without a scanner?

Yes: in the simulator you set the separation, couple both probes and scan in the browser. Handling a real two-probe scanner on a real weld is learned on equipment, in person.

Next steps

Practical NDT is a training simulation, not a validated engineering tool. It runs in a standard browser, works offline once downloaded, extends to standalone headsets and is integrated with the Atlantis LMS and ERP, so learners reach it from the client portal with one sign-in. Practice in it does not replace the practical examination or the experience hours your written practice requires.

Speak to an ASNT NDT Level III

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