Overview

The TFM Grid Verification Tool, introduced in OmniScan™ onboard software version 6.3, streamlines the experimental verification of the required and optimal grid resolution for your Total Focusing Method (TFM) inspections. This feature provides additional support for complying with ISO 23865:2021 and ASME BPVC 2023 Section V.

Background and Codes Requirements

The spatial resolution of a TFM image—defined by spacing between adjacent pixels or the number of pixels per millimetre—must be fine enough to keep amplitude variations from a reflector within an acceptable tolerance when the probe position shifts slightly. Maintaining consistent sensitivity helps ensure that small reflectors are not missed and that sizing remains accurate throughout the inspection.

The maximum amplitude variation for a given reflector at a specific grid resolution is expressed through the TFM Amplitude Fidelity indicator (ASME – BPVC 2023 Section V) or Amplitude Stability (ISO 23865:2021) indicator, defined as follows:

where A_min and A_max are the minimum and maximum measured amplitudes for the same reflector at a fixed grid resolution.

Amplitude Fidelity (or Stability) is primarily influenced by:

• TFM grid resolution
• TFM input signal (RF or envelope)
• Probe characteristics (center frequency, bandwidth)
• Material properties (wave speed)

Both ISO and ASME standards specify that amplitude variation must not exceed 2 dB.

Amplitude Fidelity and TFM Grid Resolution Verification

ASME BPVC 2023 Section V allows amplitude fidelity verification through calculation or experimental validation. ISO 23865:2021 encourages experimental verification and mandates that results be included in inspection reports.

OmniScan X3 and X4 previously introduced a calculated Amplitude Fidelity indicator, which remains available and provides a conservative estimate without requiring a full experimental procedure.

However, experimental validation is often beneficial—particularly for confirming the TFM grid resolution. Both standards recommend validating results using a test block containing vertically aligned side-drilled holes (SDH). The new TFM Grid Verification Tool in version 6.3 enables this validation using SDH or any other reflector type relevant to the inspection.

Accessing the TFM Grid Verification Tool

The tool is available in the Plan & Calibrate > Calibration Tools menu when the Law Configuration is set to FMC/TFM or PWI/TFM for a specific group. Note that the Calibration Tools menu is not available when the Law Configuration is set to FMC/PCI.

How It Works

The TFM Grid Verification Tool follows the recommended procedures outlined in ISO and ASME codes and builds on existing calibration tools such as TCG Calibration for TFM and Velocity Calibration for PA. It verifies the TFM grid for each individual reflector by allowing users to select a region of interest (ROI) and automatically shifting the reconstruction grid in small horizontal and vertical increments.

Before starting the verification process:

1. Configure your TFM group(s) with the desired parameters (specimen, probe, wedge, wave set, reconstruction zone).
2. Place the probe on a test block with appropriately spaced reference reflectors (e.g., a row of vertical SDH).

Workflow

1. Define the ROI around a single reflector
2. Run verification for that reflector.

Repeat these steps for each reflector as needed.

Available Controls

• Gain Adjustment: Sets the gain of the TFM image. Gain does not influence the calculation.
• ROI Definition: Define the ROI using Depth Start, Depth End, Index Start, and Index End. The ROI should be large enough to avoid edge effects during verification.

• Measurement Management:
  • Add Point: Start a measurement for the selected ROI.
  • Reset: Clear all measurements.
  • Done: Save and accept the verification results for the TFM group.

Measurement and Verification

When a measurement is added, the verification process begins:

• TFM grid shifts by λ/20 horizontally and vertically (20 increments per direction), where λ is the minimum wavelength in the active TFM group.
• For each grid position, a TFM image is reconstructed and the maximum amplitude (A) within the ROI is recorded.

• For each direction, the maximum amplitude variation across all increments is calculated:

Measurements Results

Once a measurement is complete, results appear in the Measurements table, including:

• Measurement ID (1–9)
• ROI center position (Index, Depth)
• Maximum amplitude variation in dB for horizontal and vertical directions

The Maximum Variation indicator displays the highest variation from all recorded measurements regardless of direction.

Complementarity with Estimated Amplitude Fidelity

While the new tool provides experimental validation, OmniScan software continues to offer an estimated Amplitude Fidelity value through the TFM Settings menu. This estimate—derived from an analytical model validated experimentally—helps users select an optimal TFM grid resolution that meets standard requirements.

The model accounts for:

• Horizontal and vertical grid resolution
• Probe center frequency
• Material wave velocity (slowest velocity based on wave set)

Because the model assumes variation along the grid diagonal (the worst-case scenario for TFM envelope), its estimates are generally more conservative than experimental results.

Users can rely on this estimate for most inspections, turning to the experimental verification tool when required by standards or when working with complex materials such as grainy alloys or composites.