Ultrasonic thickness measurement is based on the principle of ultrasonic pulse reflection. When an ultrasonic pulse is emitted by the probe, it travels through the material and reflects off the interface between the material and the opposite surface. The thickness of the material is determined by precisely measuring the time it takes for the ultrasonic wave to travel through the material and return. This method can be used to measure a wide range of materials that allow ultrasonic waves to propagate at a consistent speed. Designed with this principle in mind, the equipment can accurately measure flat plates, machined parts, pipes, and pressure vessels. It is especially useful for monitoring corrosion in industrial equipment, making it widely applicable in fields such as petroleum, chemical processing, metallurgy, shipbuilding, aviation, and aerospace.
Skills for Accurate Measurement:
1. General Measurement Method:
(1) Perform two thickness measurements at the same point using the probe. Ensure the probe faces are at 90° to each other, and use the smaller value as the thickness of the component.
(2) For unstable readings, perform multiple measurements within a 30 mm diameter circle around the target point. Select the minimum value as the actual thickness.
2. Accurate Measurement Method: Increase the number of measurements around key points, and use thick lines to indicate changes in thickness.
3. Continuous Measurement Method: Conduct continuous measurements along a specified path using single-point measurements, with intervals not exceeding 5 mm.
4. Grid Measurement Method: Create a grid over the area of interest and record thickness at each grid point. This method is commonly used in corrosion monitoring for high-pressure equipment and stainless steel linings.
Factors Affecting Thickness Gauge Readings:
(1) Excessive surface roughness can lead to poor coupling between the probe and the workpiece, resulting in weak or no echo signals. For rough surfaces like rusted metal or in-service pipes, sanding, grinding, or removing paint and oxide layers can improve coupling. Use a coupling agent to enhance signal transmission.
(2) A small radius of curvature, especially in thin-walled tubes, may cause poor contact between the flat probe and curved surface, leading to low sound intensity. A small-diameter probe (e.g., 6mm) is recommended for accurate measurements on curved surfaces.
(3) If the detection surface is not parallel to the back surface, the sound waves may scatter, preventing the probe from receiving the bottom wave signal.
(4) Castings and austenitic steels often have uneven grain structures, causing significant ultrasonic scattering and attenuation. A low-frequency coarse crystal probe (2.5MHz) is suitable for such materials.
(5) Worn probe contacts reduce sensitivity and accuracy. The probe’s surface, typically made of acrylic resin, may become rough over time. Sanding with 500-grit paper can restore smoothness. If issues persist, consider replacing the probe.
(6) Corrosion pits on the backside of the object can cause irregular readings or even no reading due to signal attenuation.
(7) Sediment inside the measured object, such as in pipelines, can affect readings if its acoustic impedance is similar to the material. The gauge may display the combined thickness of the wall and the deposit.
(8) Internal defects like inclusions or interlayers can cause the displayed thickness to be about 70% of the nominal value. In such cases, further inspection using an ultrasonic flaw detector is recommended.
(9) Temperature affects the speed of sound in solids, which decreases as temperature increases. For every 100°C rise, the speed of sound drops by approximately 1%. High-temperature environments require specialized probes (e.g., 300–600°C), as standard probes may not function properly.
(10) Laminated or composite materials present challenges due to their heterogeneous structure. Proper calibration and selection of the right probe are essential for accurate results.
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