Factors Affecting Ultrasonic Thickness Gauge Readings
1. Surface roughness can significantly impact the accuracy of ultrasonic thickness measurements. When the surface is too rough, it leads to poor coupling between the probe and the material, resulting in weak or no echo signals. For surfaces with rust, paint, or oxide layers—common in in-service equipment, pipes, and other industrial components—it's essential to clean the area by sanding, grinding, or scraping to reduce roughness and expose a smooth metallic surface. Applying a suitable coupling agent will help improve contact and ensure reliable signal transmission.
2. The curvature of the workpiece can also affect measurement accuracy. If the radius of curvature is too small, especially when measuring thin-walled tubes, the flat face of a standard probe may only make point or line contact with the curved surface. This reduces the sound energy transmission and causes poor coupling. To address this, use a small-diameter probe (less than 6mm) designed for curved surfaces, such as those used for pipe thickness testing, to achieve more accurate readings.
3. Misalignment between the detection surface and the bottom surface of the material can lead to scattering of the ultrasonic waves. If the two surfaces are not parallel, the sound waves reflect unpredictably, making it difficult for the probe to detect the bottom wave. Ensuring that the test surface is properly aligned with the back wall is crucial for accurate measurements.
4. Materials like castings and austenitic stainless steels often have non-uniform grain structures or coarse grains, which cause significant ultrasonic wave scattering and attenuation. This can result in weak or absent echoes, leading to incorrect readings. In such cases, using a low-frequency, coarse-grain probe (typically 2.5MHz) can help improve signal penetration and reliability.
5. Wear on the probe’s contact surface can also affect measurement results. Most thickness gauges use acrylic resin probes, which can become worn over time, increasing surface roughness and reducing sensitivity. To restore performance, gently sand the probe with 500-grit sandpaper to smooth the surface and maintain parallelism. If the issue persists, consider replacing the probe to ensure consistent and accurate readings.
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