Factors Affecting the Measurement Accuracy of Ultrasonic Thickness Gauges

There are many factors that can affect the accuracy of thickness gauge measurements, and any single factor can influence or even drastically alter the results. In general, these factors can be summarized as: operator performance, surface condition of the test material, coupling technique, and the acoustic properties of the material being measured.

01 Calibration
The accuracy of ultrasonic measurements depends on the accuracy of the thickness gauge calibration. In any case, whenever the test material or the probe is replaced, both velocity calibration and zero calibration should be performed. It is recommended to regularly check the gauge using samples with known thickness to ensure the instrument is functioning properly.

02 Surface Roughness of the Test Workpiece
Excessive surface roughness of the workpiece can result in poor coupling between the probe and the contact surface, leading to weak reflected echoes or even no detectable echo signal. For in-service equipment, pipelines, and other components with rusted surfaces and extremely poor coupling conditions, surface treatment can be applied to reduce roughness. This process can also remove oxides and paint layers, exposing the metallic surface so that the probe can achieve effective coupling with the test object through the couplant.

03 Curvature of the Test Workpiece
If the curvature radius of the workpiece is too small, especially when measuring the thickness of small-diameter pipes, the commonly used flat-faced probes may not couple well with the curved surface. To obtain reliable measurement results, the probe size should be reduced so that pipes and other curved-surface materials can be measured more accurately.

04 Non-parallel Detection and Bottom Surfaces

If the detection surface and the bottom surface are not parallel, the probe will be unable to receive the bottom echo signal, making measurement impossible.

05 Acoustic Properties of the Test Material
Castings and austenitic steel, due to their non-uniform structure or coarse grains, cause severe scattering and attenuation of ultrasonic waves as they propagate through the material. The scattered ultrasonic waves travel along complex paths, which may result in the bottom echo being completely lost and thus not displayed. In such cases, a low-frequency coarse-grain probe should be used.

06 Wear of Common Thickness Measurement Probes
Prolonged use of standard thickness measurement probes can increase the surface roughness, resulting in reduced sensitivity and inaccurate readings. In such cases, 500# sandpaper may be used to polish the surface to restore smoothness and maintain parallelism. If the measurement remains unstable after polishing, the probe should be replaced.

07 Coupling Technique
Couplant is used to eliminate air between the probe and the test object, allowing ultrasonic waves to effectively enter the workpiece for inspection. To achieve maximum accuracy, operators need to master reliable and stable coupling techniques. For stable measurement readings, the couplant should be applied in an appropriate amount and evenly spread, while consistent pressure must be applied to the probe. The Shenyang YUSHI INSTRUMENTS UM-5 series features an A-scan function, which helps operators practice the optimal probe pressure by observing the waveform.

08 Sound Velocity Variation
Ultrasonic thickness measurement is accurate only when the material's sound velocity matches the velocity calibrated in the instrument. In certain cast metal materials, the grain structure varies due to different cooling rates, causing anisotropic sound velocity. Additionally, many materials exhibit rapid changes in sound velocity when subjected to temperature variations. Therefore, for such materials, sound velocity calibration must be performed under the actual temperature conditions during measurement.