TOFD Calculator — Probe Centre Separation, Tip Depth and Dead Zone
Set the thickness, probe angle and velocity to get the probe centre separation (PCS) that puts the beam crossing at your target depth, conventionally two-thirds of the thickness. The calculator gives the expected lateral wave and backwall arrival times, converts a measured diffraction time into tip depth, and estimates the near-surface dead zone from the pulse length.
The interactive TOFD calculator runs in your browser; nothing you enter is sent anywhere.
Formulas used
| Probe centre separation | PCS = 2 × d(focus) × tan θ, with d(focus) = ⅔ T by convention |
|---|---|
| Half separation | S = PCS ÷ 2 |
| Lateral wave arrival | t(L) = 2S ÷ v |
| Backwall arrival | t(BW) = 2 × √(S² + T²) ÷ v |
| Tip depth from a measured time t | d = √((v × t ÷ 2)² − S²) |
| Dead zone estimate | d(dz) = √((v × (t(L) + t(p)) ÷ 2)² − S²), t(p) = pulse length |
Worked example
Worked example: 25 mm carbon steel, 60° longitudinal-wave probes, v = 5,920 m/s, beam crossing at 16.7 mm (two-thirds of the wall). PCS = 57.7 mm. The lateral wave arrives at 9.75 µs and the backwall at 12.90 µs. A tip signal at 11.00 µs is at 15.1 mm depth. With a 5 MHz probe and a two-cycle pulse (0.4 µs), the near-surface dead zone is about 8.4 mm, which is why TOFD is usually paired with another technique for the near surface.
Before you rely on the numbers
- Times here run from the probe exit points. Real instruments add wedge delay; calibrate on the lateral wave and backwall, not on the calculated times alone.
- TOFD uses longitudinal waves, so use the longitudinal velocity of the material (about 5,920 m/s for carbon steel).
- A wider PCS gives better coverage of the lower wall but a larger near-surface dead zone; a narrower PCS does the opposite. Thick sections often need more than one probe pair.
Frequently asked questions
What is PCS in TOFD?
Probe centre separation is the distance between the index points of the transmitter and receiver. It sets the depth at which the two beam centrelines cross. The usual starting point puts the crossing at two-thirds of the thickness: PCS = 2 × ⅔T × tan θ.
How is flaw depth calculated in TOFD?
The diffracted signal travels from the transmitter to the flaw tip and on to the receiver. With the probes symmetric about the tip, depth = √((v × t ÷ 2)² − S²), where t is the arrival time from the exit points, v the longitudinal velocity and S half the probe separation.
What is the TOFD dead zone?
Near the scanning surface the tip signal arrives so soon after the lateral wave that the two cannot be separated. The depth of that zone grows with probe separation and pulse length. The estimate here uses the lateral wave time plus one pulse length.
Is this calculator a substitute for a qualified TOFD procedure?
No. It does the geometry. A TOFD setup still needs a written procedure, a scan plan, calibration on a reference block and a qualified operator, under the code that applies (for example ASME Section V or ISO 10863).
Related
- Try the simplified A-scan demo
- Sound velocity reference table
- Ultrasonic thickness calculator
- UT, PAUT and TOFD training: how training, exams and certification fit
- UT angle beam calculator
Building UT, PAUT or TOFD skills for a team? Atlantis NDT training is led by an ASNT Level III, and the Practical NDT simulator gives hands-on practice between courses.
What this tool is actually modelling
Every estimate of this kind rests on assumptions, and the useful ones state them. The output here is a starting figure for a conversation with your own operations and finance teams — not a quotation, and not a substitute for a scoped assessment. Inputs you supply about your own operation dominate the result; industry defaults are only used where you have no figure of your own, and they are deliberately conservative.
Getting the inputs right
- Use your own historical figures wherever you have them. Operations teams are usually accurate about downtime and mobilisation cost because they have lived through the events; vendor benchmarks are not.
- Count the non-billable time honestly — report preparation, audit-evidence assembly, standby and rework are where inspection businesses actually lose margin, and they are routinely excluded from estimates.
- Separate one-off transition effort from recurring effect. Benefits that depend on a workflow change take one to two inspection cycles to appear, not one month.
- Test the result at the edges. If the conclusion reverses when a single input moves 20%, the conclusion is the input, not the model.
How to read the output
Treat the figure as a range, not a point. In practice the largest and most reliable component of value in inspection operations is time recovered from work that produces no revenue — report formatting, chasing certification and calibration records, and assembling evidence for audits. The least reliable components are those that assume immediate behaviour change across a whole organisation. Weight your interpretation accordingly, and if you are building an internal business case, present the conservative end.
What it does not tell you
It does not tell you whether your data is in a state to support the change, which is usually the real constraint. Before committing to any programme on the strength of a calculator, check whether you can reconstruct one issued inspection report end to end — technician qualification, instrument calibration and procedure revision as at the date of inspection. If you cannot, that gap will consume more of the timeline than anything this tool models.
Related: all NDT tools · inspection management software · asset integrity management software · ASNT Level III consulting. Ask for a scoped assessment instead of an estimate.