AUT corrosion mapping and manual UT grids both provide ultrasonic thickness evidence, but they collect and represent that evidence differently. An encoded AUT or HydroFORM scan records position-linked readings across an area. A manual grid records readings at defined points selected and measured by a technician. The appropriate method depends on the feature, access, required resolution, procedure, equipment and the decision the data must support. The main difference between AUT corrosion mapping and a manual UT grid is data density and spatial referencing: encoded mapping records many position-linked ultrasonic measurements across an area, while a manual grid records discrete thickness readings at predetermined locations.
Coverage and resolution
Encoded mapping is suited to high-density, position-referenced area coverage over a prepared surface. The scan can show spatial changes in remaining wall thickness and help define the shape and extent of a corrosion area between individual manual measurement points. Its effective resolution is still limited by probe, encoder, scan spacing, surface condition, coupling and acquisition settings.
A manual grid provides discrete readings at a defined axial and circumferential or X/Y spacing. It can be deployed quickly for a focused feature or where the surface, access or geometry is not suitable for an encoded scanner. A coarse grid may miss a narrow pit between points, while a tight grid takes more time and requires disciplined location control.
Repeatability and measurement control
Encoded mapping can be repeatable when the scanner, coordinate origin, surface preparation and scan plan are controlled. The record can show the same area as a map and support comparison with a future scan when registration and acquisition conditions are compatible. Repeatability should not be assumed; surface condition, coupling, curvature, scan origin, probe configuration, calibration, and acquisition settings remain important.
Manual UT repeatability depends on probe placement, technician technique, grid marks, calibration, couplant and the ability to return to the same point. A well-documented grid with permanent or recoverable references can be very effective for trending. A freehand set of spot readings is not equivalent to a controlled grid and should not be represented as one.
Documentation and review
| Consideration | AUT / encoded mapping | Manual UT grid |
|---|---|---|
| Coverage | High-density encoded area coverage within the qualified scan limits | Discrete readings at defined points |
| Resolution | Controlled by scan spacing, probe and acquisition quality | Controlled by grid spacing and technician placement |
| Repeatability | Supports repeat mapping when setup and registration are consistent | Supports trending when points and references can be relocated |
| Documentation | Source scans, stitched map, indication boundaries and profiles | Grid layout, readings, calibration and field references |
Both methods need calibration records, instrument identification, units, nominal wall or reference basis, surface condition and limitations. Both methods should document the lowest valid thickness measured within the examination coverage and its location. The report should not imply that a discrete manual grid necessarily captured the absolute minimum wall between measurement points. The method chosen should be clear in the final report so a reader does not mistake a mapped area for a complete surface examination or a grid for continuous coverage.
Where each method fits
Use encoded mapping when area coverage, corrosion shape, indication sizing or a repeatable surface record is important and the scanner can be deployed properly. Use a manual grid when the examination is localized, access is constrained, the geometry is difficult for a scanner or a procedure calls for point-based thickness readings. The methods can complement each other. Encoded mapping can characterize a broader corrosion area, while targeted manual UT or profile measurements can provide additional localized measurements where required by the procedure, geometry, or review objective.
The C-Scan / Corrosion Mapping workflow organizes encoded scan sets, stitching, indication review and profiles. For API piping or other inspection programs, API/NDE reporting can retain thickness data, drawings, photos, findings and the final report. Selection and interpretation remain the responsibility of qualified personnel.
Questions inspectors ask
Is a C-Scan always better than a manual grid?
No. Neither method is universally superior. The appropriate examination depends on geometry, access, required spatial resolution, surface condition, procedure, inspection objective, and the engineering decision the data must support.
Can a manual grid prove there is no corrosion between points?
No. A grid characterizes the selected points and its spacing. It should not be reported as continuous coverage unless the method actually provides it.
Can the two methods be used together?
Yes. A mapped area and a targeted grid or profile can provide complementary evidence when their references and measurement bases are documented.