Visual Testing (VT) — Complete Field Guide

How the physics works

Visual testing relies on the inspector's eye, ambient or aided lighting, and physical or optical access to the inspection surface. The physics is straightforward — reflected light from a defect contrasts with the surrounding surface — but the discipline of executing VT to a procedure is anything but trivial. ASME V Article 9 [1] sets procedure rules, acceptable equipment categories (direct visual, remote visual, translucent visual), lighting minimums (≥100 footcandles for direct VT at the part), and inspector visual-acuity requirements. ASTM E165 cross-references VT with PT [4]; ASTM E1316 defines VT terminology [5]. Inspector visual acuity is verified annually by a Jaeger 2 near-vision card at 12 inches and a color-vision plate test per ASNT CP-189 [6]. An inspector who cannot read Jaeger 2 type with corrective lenses fails the visual-acuity test and cannot perform VT under any code.

When to choose this method

VT is mandatory on every weld under every common construction code as the first inspection — ASME B31.3 §344.2.1 [7], AWS D1.1 §6.9 [2], API 1104 §11.1 [8] all require 100% visual before any other NDT. VT is the standard in-service method for external condition assessment of pressure vessels (API 510 §6.4 [3]) and aboveground storage tanks (API 653 §6 [9]). Remote VT with a borescope or RVI camera handles internal vessel inspection where confined-space entry would require permits, atmospheric monitoring, and standby rescue.

VT cannot find subsurface defects. It cannot resolve cracks tighter than about 0.005-0.010" width without aiding magnification or supplementary methods. It is operator-dependent — different inspectors interpret the same indication differently, particularly on borderline undercut or porosity. Lighting below the procedure minimum (100 footcandles direct, application-specific for remote VT) invalidates the inspection. VT in cold-weather field conditions where the inspector cannot maintain steady visual focus through bulky gloves or face shields gets a high false-negative rate.

Materials & geometries

Method coverage depends on couplant, surface, and section thickness. Compatible forms include:

Procedure

Procedure qualification and inspector vision

A written VT procedure references the governing code (ASME V Article 9 [1], AWS D1.1 §6.9 [2], API 510 §6.4 [3]) and lists the inspection equipment (mirror, magnifier, light source), the lighting minimum, the access plan, the acceptance criteria, and the inspector certification. The procedure is reviewed and signed by a Level III qualified to SNT-TC-1A or CP-189 [6].

Inspector visual acuity is verified annually. ASNT CP-189 §9.2 [6] requires near-vision verification with a Jaeger J-2 chart at 12 inches (corrective lenses allowed) and color-vision verification with an Ishihara plate test. The vision test results are documented in the inspector's certification file. An inspector with vision below the threshold cannot perform VT — and most field crews now run a quick vision check on Monday morning before mobilizing to verify nothing has changed.

Lighting and surface preparation

Direct VT requires ≥100 footcandles at the inspection surface per ASME V T-952 [1]. A white-light meter (Spectroline DM-365XL) verifies the level at the start of every shift and any time the inspection location changes. Overhead and confined-space inspection often falls below the threshold even with portable lighting — supplemental LED work lights bring the level above the minimum. Translucent VT (looking through thin glass or plastic for fractures) and remote VT have their own lighting requirements specified in the procedure.

Surface prep removes loose scale, slag, weld spatter, and oil that mask indications. ASME B31.3 §344.2.1 [7] requires the weld surface to be cleaned to a condition that permits a meaningful visual examination — wire-brush cleaning is the field standard, with grinding for cap dressing where the as-welded condition would prevent VT (e.g., heavily-piled MIG cap that hides toe cracks). Welds with arc burn outside the weld zone get rejected under AWS D1.1 §5.30 [2] regardless of size — clean the surface and look at the heat-affected zone carefully.

Direct VT execution

The inspector positions to view the inspection surface within 24 inches at an angle of at least 30° from the surface plane per ASME V T-953 [1]. A pocket mirror, a magnifier (typically 5×-10×), a fillet weld gauge (Cambridge gauge, V-WAC gauge), and a flashlight cover most field VT work. The scan is methodical — start at the start of the weld and follow the joint, marking every indication on a sketch or directly on the part with paint pen.

Weld dimensional checks happen during VT. The fillet leg length, throat thickness, undercut depth, reinforcement height, and concavity are all measured against the procedure or WPS limits. A Cambridge gauge (Palmgren or G.A.L.) measures fillet leg, throat, undercut, and porosity-pit depth in a single tool. AWS D1.1 §5.24 [2] gives the dimensional limits for structural fillet welds; ASME B31.3 §341.3 covers pipe welds.

Remote VT and borescope inspection

Remote VT uses a borescope, fiberscope, or video camera to inspect surfaces not accessible by direct VT. ASME V Article 9 §T-952 [1] requires the remote system to demonstrate the ability to resolve a 1/32" (0.8 mm) test character at the maximum working distance — typically demonstrated against a black-on-white test card. Standard tools include Olympus IPLEX GX/GT articulating videoscopes (4-6 mm tip), Karl Storz Techno Pack, and Everest VideoProbe systems.

Internal vessel inspection during turnaround typically uses remote VT to clear the vessel for confined-space entry. A pre-entry borescope shot through the manway documents internal condition, identifies surface defects for follow-up direct VT, and flags any structural concerns that would change the entry plan. API 510 §5.5 [3] permits remote VT in lieu of internal entry where the asset owner approves and the inspection plan documents the substitution.

Indication recording and disposition

Every reportable indication is recorded by location (station, distance from datum, clock position) and dimension (length, width, depth). Photographs at the indication with a scale (typically a paint-marker length-reference or a fillet-gauge in frame) accompany the report for any indication above the acceptance threshold. Modern field reporting (Atlantis NDT Reporting, GoCanvas) attaches photos automatically to the digital indication record.

Disposition follows the code. For new-construction welds, indications above the limit go back to the welder for repair, with re-VT after repair grinding and re-welding. For in-service VT, indications go to the inspection engineer for fitness-for-service evaluation under API 579 [10] or similar. The VT report names every indication and its disposition — accept, repair, monitor, or refer.

Reporting and certification

The VT report names procedure, inspector name and certification level, vision-test date, lighting (footcandles measured), equipment used (mirror, magnifier, borescope model and serial number), inspection date, every indication, and disposition. ASME V §T-980 [1] gives the required fields. AWS D1.1 §6.9.4 [2] requires the AWS Certified Welding Inspector (CWI) signature on the report for structural work.

For API in-service work, the inspector must be API Certified — 510 for pressure vessels, 570 for piping, 653 for storage tanks. The certification number and expiry date appear on every report. An expired API certification voids the report, even if the inspector retook the exam the next day — the report date must fall within the certification validity period.

Equipment

Direct VT tools

Standard field VT kit: a 5× pocket magnifier (Bausch & Lomb 81-31-65), a high-CRI LED flashlight (Streamlight 88040), a fillet weld gauge (Palmgren V-WAC), a pocket mirror, a calibrated pit gauge, a steel rule, and a paint marker. White-light meter (Spectroline DM-365XL) verifies the 100-footcandle minimum.

Crack-comparator cards (Western Instruments WCG-1) give a visual reference for crack-width estimation when a crack is found. Surface-roughness comparator (GAR Plate) helps the inspector classify finish quality where surface preparation is a contract requirement.

Remote VT and borescopes

The Olympus IPLEX GX/GT videoscope with 4 mm or 6 mm articulating tip dominates the industrial remote VT market. The Everest VideoProbe Mentor Visual iQ and Karl Storz Techno Pack are the other workhorses. Working channels range from 3 mm to 12 mm OD; the smaller scopes get into heat-exchanger tubes and small-bore piping, the larger scopes handle vessel internal and bundle inspection.

Resolution verification — the system must resolve a 1/32" (0.8 mm) character at the maximum working distance per ASME V T-952 [1] — happens at the start of every shift against a black-on-white test card. The card and the resolution verification log ship in the borescope kit.

Drone and crawler systems

Drone VT (Flyability Elios 3, Skydio X10) inspects tank internals, flare stacks, and confined spaces without human entry. The drone carries a high-resolution camera, LED lighting, and obstacle-avoidance lidar. ASME V Article 9 [1] does not yet explicitly recognize drone VT but does not exclude it — most operators run drone footage as supplementary to a procedure-qualified borescope or direct VT.

Crawler systems (Inuktun MaggHD, Eddyfi Inuktun) carry cameras inside piping for in-service VT of internal corrosion and erosion patterns. The crawler's odometer reports indication position relative to a datum (typically the inlet flange).

Codes & standards that govern this method

Procedures and acceptance criteria are anchored in published codes:

Acceptance criteria

For new-construction structural welds under AWS D1.1 §5.24 [2], undercut deeper than 1/32" or longer than 10% of the weld length is rejectable, reinforcement above 1/8" on butt welds is rejectable, any crack indication is rejectable, and arc strikes outside the weld zone are rejectable regardless of size. For ASME B31.3 piping welds, §344.2 [7] gives the visual acceptance limits — no cracks, lack of fusion limited to weld-toe areas with specific length caps, porosity per Table 341.3.2, reinforcement caps by service category. For in-service VT under API 510 §6.4 [3], findings drive fitness-for-service evaluation or repair-and-rerate. For API 653 tank inspection [9], shell, roof, and bottom defects each have category-specific dimensional limits — bottom plate corrosion exceeding 50% of original thickness drives repair-or-replace decisions. Arc strikes, weld undercut, and external corrosion are the most common reject categories in field VT and account for the bulk of weld rework dispatches on Gulf Coast turnaround projects.

How this compares to other methods

Choosing between methods is rarely about capability alone — cost, throughput, and code coverage all weigh in:

Cost range

Typical VT pricing in the US market runs $1–$4 USD per linear foot of weld inspected (Gulf Coast crew, 2025), with most jobs landing around $2 USD per linear foot of weld inspected (Gulf Coast crew, 2025). Mobilisation, access, and certification level shift the band.