TOFD Procedure Requirements: ASME, ISO 10863 & Common Non-Compliances

What Makes TOFD Different from Conventional UT?

Time of Flight Diffraction (TOFD) uses two angled transducers in a pitch-catch configuration — a transmitter and a receiver straddling the weld. Instead of measuring amplitude of reflected signals (as conventional pulse-echo UT does), TOFD measures the time of flight of diffracted signals from defect tips. This gives highly accurate sizing of embedded defects but comes with a fundamental limitation: the dead zone near the surface.

TOFD procedures must explicitly acknowledge and address the dead zone, provide complementary examination methods for near-surface regions, and include TOFD-specific calibration procedures that have no equivalent in conventional UT. These unique requirements are the primary source of TOFD procedure non-compliances.

Governing Standards

• ASME Section V, Article 4, Appendix IV: TOFD as a supplementary technique under UT
• ASME Code Case 2235: When TOFD is used as an alternative to RT for pressure vessel welds
• ISO 10863:2020: Ultrasonic testing — Use of time-of-flight diffraction technique (TOFD)
• EN ISO 10863: European harmonised standard, used for EN 13445, EN 13480 pressure equipment
• ASTM E2373: Standard practice for use of the TOFD technique
• DNV-OS-F101 / DNV-ST-F101: Subsea pipeline TOFD requirements (offshore applications)

Mandatory TOFD Procedure Elements

1. System Configuration

Your procedure must specify the complete TOFD system setup:

• Instrument make, model, and pulser/receiver settings (pulse width, gain, filter)
• Transducer pair: frequency, element size, refracted angle, near-field length
• Probe centre separation (PCS) — must be calculated and stated for each examination zone
• Wedge angle, material, and coupling method
• Data acquisition system and software version
• Encoder type and scan increment

2. Dead Zone Analysis — The Most Critical TOFD Requirement

The dead zone is the near-surface region where the lateral wave masks defect signals. Every TOFD procedure must:

• Calculate the dead zone depth for each PCS and frequency combination used
• State the dead zone clearly: e.g., "Dead zone: 0–3.5 mm from scanning surface"
• Specify complementary coverage: State which technique covers the dead zone region (e.g., "Near-surface region 0–5 mm shall be examined using PAUT linear scan per Procedure [ref]")
• If no complementary technique is used, justify the acceptability with reference to the applicable code

3. Probe Centre Separation (PCS) Calculation

PCS directly affects the sensitivity and coverage of each examination zone. The procedure must document:

• PCS calculation for each material thickness range examined
• Basis for PCS selection (mid-wall focus, or other focal strategy)
• Number of TOFD channels / zones for thick-wall components (typically one channel per 30–40 mm thickness)
• Zone boundaries for multi-zone TOFD (overlap between zones)

4. Calibration Requirements

5. Reference Block Requirements

TOFD calibration blocks must contain specific reflectors:

• Side-drilled holes (SDH) at multiple depths to verify lateral wave, tip diffraction, and backwall signals
• Notch at the scanning surface to verify dead zone depth
• Material matching the examination component (same P-number group, similar velocity)
• Block drawing or controlled document reference — dimensions, SDH depths and diameters, notch dimensions must be fully stated

6. Data Recording and D-Scan Documentation

TOFD produces D-scans (time vs. position displays). Your procedure must state:

• Recording format (proprietary software format, version)
• Minimum data density (scan increment ≤ half beam width in scan direction)
• D-scan display parameters (time window, gain settings for recording)
• Indication sizing method: lateral wave arrival time used as reference, backwall arrival for through-thickness sizing
• Reporting threshold: all indications with signal amplitude exceeding [X]% of lateral wave amplitude, or all indications visible in the D-scan
• Data storage media, file naming convention, retention period

7. Personnel Qualification

ISO 9712 has a specific sector qualification for TOFD. Under ASNT SNT-TC-1A, TOFD is considered an advanced technique requiring demonstrated competence beyond general UT Level II. Your procedure must state:

• Minimum qualification level (UT Level II with TOFD qualification, or TOFD-specific PCN/ISO 9712 certification)
• Reference to qualification standard and company Written Practice
• Required practical experience with TOFD (hours or number of examinations)

Most Common TOFD Procedure Non-Compliances

1. Dead zone not quantified — Procedure mentions dead zone exists but does not calculate or state the depth
2. No complementary technique for dead zone — Dead zone acknowledged but no coverage method specified
3. PCS not calculated — PCS stated as a value but calculation or basis not provided
4. Calibration block not described — "TOFD calibration block" stated without SDH depths, notch dimensions, or material specification
5. Lateral wave timing verification not required — Procedure does not require lateral wave check before/after examination
6. D-scan display settings not specified — Time window, gain, and display range omitted
7. TOFD-specific personnel qualification not stated — General UT Level II cited without TOFD-specific requirement