ASNT VT Level III cheat sheet

Visual testing (VT) relies on human vision plus illumination; detection depends on contrast, luminance difference, and the observer's visual acuity.

Snellen acuity 20/20 means resolving detail at 20 ft equivalent to a standard observer; 20/40 resolves detail at 20 ft that a standard eye resolves at 40 ft.

Angular resolution of the unaided eye is about 1 arc minute; near-point accommodation limits practical inspection to roughly 250 mm unless magnification is used.

Contrast sensitivity falls at low light levels; illuminance on the test surface, not ambient room light, governs detectability of surface discontinuities.

Specular (mirror-like) surfaces reflect light directionally and can wash out indications; diffuse surfaces scatter light and give more uniform contrast.

Indication visibility depends on the angle between illumination, surface, and observer; oblique lighting enhances surface relief and shallow scratches.

Reflected luminance ratio between indication and background should generally be at least 2:1 for reliable visual detection under typical conditions.

Color perception is trichromatic (red, green, blue cones); color-blind observers may miss color-contrast indications, so color vision testing is required for VT personnel.

Human factors such as fatigue, glare, and adaptation time reduce detection reliability; inspection time and rest breaks are part of method theory.

Illuminance is measured in lux (lm/m²); 1 footcandle = 10.76 lux. Verify meter calibration before use.

Typical VT illuminance guidance: at least 500 lux (about 50 fc) on the surface for general inspection; higher levels (often 1000 lux or more) for fine or critical work.

Light meters (photometers) use a cosine-corrected photocell; cosine correction is needed because response must match surface orientation.

Magnifiers are specified by magnifying power and focal length; a 2x to 10x loupe is common, and higher power reduces field of view and depth of field.

Flexible borescopes and fiberscopes allow internal visual inspection; rigid borescopes give better image quality and are limited to straight-line access.

Videoscopes and cameras add resolution, lighting, and recording; verify camera resolution and field of view against the required detection size.

Mirrors, weld gauges, and pit-depth gauges are common accessories; they must be clean and undamaged to avoid false indications.

Reference standards or comparator specimens (e.g., surface finish comparators) help judge acceptability against a documented criterion.

Contrast aids such as dye penetrant or developer are not VT; VT uses light, optics, and surface preparation, not penetrant materials.

Direct visual testing: the observer's eye is close to the surface, typically within 600 mm (24 in) and at an angle of at least 30 degrees to the surface.

Remote visual testing uses borescopes, fiberscopes, or cameras when access or distance prevents direct viewing; resolution and lighting must be documented.

Translucent (backlight) testing uses transmitted light to reveal internal flaws in glass, plastics, and thin sections.

Surface preparation (cleaning, removing coatings, descaling) must be specified because coatings can hide or mimic indications.

A written VT procedure should define illumination level, viewing distance, angle, magnification, surface condition, and acceptance criteria.

Scanning technique should overlap adjacent fields of view to avoid missed areas; systematic patterns (raster or grid) improve coverage.

Angle of incidence and oblique lighting are chosen to maximize contrast for the expected discontinuity orientation.

Documentation should record equipment, calibration status, lighting, and environmental conditions, along with the inspection results.

Personnel performing VT must have demonstrated near vision acuity (often Jaeger 1 or equivalent) and color vision, per the applicable standard.

VT detects surface-breaking discontinuities: cracks, porosity, undercut, overlap, spatter, corrosion, and mechanical damage.

Weld acceptance criteria are usually defined by a code (e.g., AWS D1.1 or ASME Section VIII) rather than by the inspector's judgment alone.

Indications are evaluated against acceptance criteria; rejectable indications are reported, and non-rejectable ones are noted as appropriate.

Cracks appear as sharp, dark, linear indications; their orientation and length are measured and compared with the acceptance limit.

Corrosion and erosion are evaluated by depth and extent, often using pit-depth gauges or surface comparators.

False indications can come from lighting reflections, surface finish, or contamination; changing illumination angle helps confirm a true indication.

Coating thickness and surface condition can mask discontinuities; removal or localized preparation may be required before VT.

Interpretation requires distinguishing relevant indications (from actual discontinuities) from non-relevant ones (geometry or finish artifacts).

Reporting should include location, size, orientation, and a clear description so results can be reproduced and compared over time.

ASNT SNT-TC-1A is a recommended practice for employer-written qualification and certification programs; the employer is responsible for certification.

ANSI/ASNT CP-189 is a standard for qualification and certification of NDT personnel, more prescriptive than SNT-TC-1A.

Level III duties include procedure development, technique selection, interpretation of standards, and training/qualifying lower-level personnel.

Certification requires documented training hours, experience, and passing the general, specific, and practical examinations for the method.

Vision requirements for NDT personnel include near vision acuity and color perception, with periodic re-examination per the applicable document.

Level III general examinations cover method fundamentals, equipment, techniques, and standards; specific exams cover the employer's procedures.

Codes and standards (ASME, AWS, ASTM, ISO) define acceptance criteria; the Level III must identify the governing document for each job.

Recertification typically requires periodic re-examination or documented continued satisfactory performance, per the certifying program.

Ethics and impartiality: Level III personnel must avoid conflicts of interest and document results accurately and objectively.