Technique Sheet Conflict: Viewing Distance and Angle for a Critical Weld
A written VT procedure for structural welds specifies: viewing distance not greater than 600 mm, viewing angle not less than 30° to the surface, and surface illuminance of at least 1000 lx. A Level II inspector examining a 25 mm thick complete-joint-penetration groove weld finds that a stiffener plate blocks direct viewing of one weld face. The only access is at a 15° angle from a distance of 900 mm, using a mirror. The inspector proposes to proceed under the mirror, arguing that the mirror restores the effective viewing geometry. The weld is in a primary structural member and the procedure is invoked by contract.
1. Assess the inspector's justification that the mirror restores the effective viewing geometry.
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The mirror does not restore the geometry. Two separate quantities are involved:
- Line of sight to the mirror: the inspector's eye-to-mirror distance and angle.
- Angle of incidence on the surface: the angle between the viewing ray and the weld surface at the point of interest.
A mirror at 900 mm from the eye, with the surface seen at 15° incidence, still presents the surface at 15° - the mirror changes the path, not the incidence angle. The procedure requires ≥30°. Additionally, mirror viewing introduces:
- Image degradation (reflectivity, flatness, dirt, double reflection in front-surface vs. rear-surface mirrors),
- Reduced effective illumination at the surface (the mirror intercepts light), and
- A reversed/handedness image that can confuse orientation and location recording.
So the proposed setup violates both the angle and the distance requirements and adds optical penalties. The justification is not valid.
2. The procedure is contractually invoked. What are the acceptable paths forward, and what must be documented in each?
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Two legitimate paths:
Path 1 - Conform to the procedure. Improve access: remove or temporarily relocate the stiffener (with engineering approval), use a different viewing position, or employ a qualified remote visual method (borescope/videoscope) that can be qualified to meet or exceed the procedure's detection capability. Document the access change and the re-examination.
Path 2 - Deviate by controlled revision. A deviation requires:
- A written procedure revision or approved deviation authorized by the responsible engineering authority (not the inspector alone),
- Justification that the alternate geometry still provides equivalent detection capability (e.g., a demonstration on a reference specimen with the mirror at 15°/900 mm showing the required discontinuities are detectable),
- Records of the deviation, the demonstration, and the acceptance by the authorized party.
In both cases the inspector must not unilaterally proceed. Document the blockage, the proposed deviation, the demonstration results, and the authorization.
3. How would you verify that a mirror-based view still meets the intent of the illuminance requirement?
Read the worked answer
Measure illuminance at the weld surface as seen through the mirror path - i.e., place the meter at the surface location, with the light source and mirror in their inspection positions, and read the value. Do not rely on the lamp's rated output or on a reading taken without the mirror in place. Check:
- Mirror reflectivity and cleanliness (a dirty or degraded mirror can cut the delivered illuminance substantially),
- Whether the light source can actually be aimed through the mirror path to the surface,
- Uniformity across the region of interest, not just a single spot reading.
If the measured value at the surface falls below the procedure's 1000 lx, the setup fails the requirement regardless of the mirror's optical quality, and the deviation path in task 2 must be followed.