Principle of Ultrasonic Thickness Gauge and Factors Affecting Accuracy

　　The ultrasonic thickness gauge measures thickness based on the principle of ultrasonic pulse reflection. When the ultrasonic pulse emitted by the probe passes through the object to be measured and reaches the material interface, the pulse is reflected back to the probe. The thickness of the material to be measured is determined by accurately measuring the time it takes for the ultrasonic wave to propagate in the material. This principle can be used to measure all kinds of materials that allow ultrasonic waves to propagate at a constant speed. The thickness gauge designed according to this principle can accurately measure various plates and various processed parts, and can also monitor various pipelines and pressure vessels in production equipment to monitor their thinning degree after corrosion during use. It can be widely used in various fields such as petroleum, chemical industry, metallurgy, shipbuilding, aviation, and aerospace.

　　Tips

　　1. General measurement method:

　　(1) Use the probe to measure the thickness twice at one point. The split surfaces of the probe should be 90° to each other in the two measurements, and the smaller value is taken as the thickness of the workpiece being measured.

　　(2) 30mm multi-point measurement method: When the measured value is unstable, multiple measurements are performed within a circle with a diameter of approximately 30mm with one measuring point as the center, and the minimum value is taken as the thickness value of the workpiece being measured.

　　2. Precise measurement method: Increase the number of measurements around the specified measuring point, and the thickness change is represented by equal thickness lines.

　　3. Continuous measurement method: Use the single-point measurement method to continuously measure along the specified route with an interval of no more than 5mm.

　　4. Grid measurement method: Draw a grid in the designated area and record the thickness at each point. This method is widely used in high-pressure equipment and stainless steel lining corrosion monitoring.

　　5. Factors affecting the indication of ultrasonic thickness gauge:

　　(1) The surface roughness of the workpiece is too large, resulting in poor coupling between the probe and the contact surface, low reflected echo, or even no echo signal can be received. For in-service equipment and pipelines with surface rust and extremely poor coupling effect, the surface can be treated by sanding, grinding, filing, etc. to reduce the roughness. At the same time, the oxide and paint layer can also be removed to reveal the metallic luster, so that the probe and the object under test can achieve a good coupling effect through the coupling agent.

　　(2) The radius of curvature of the workpiece is too small, especially when measuring the thickness of small-diameter pipes. Because the surface of the commonly used probe is flat, the contact with the curved surface is point contact or line contact, and the sound intensity transmittance is low (poor coupling). A special probe for small pipe diameter (6mm) can be used to more accurately measure curved materials such as pipes.

　　(3) The detection surface is not parallel to the bottom surface. The sound waves are scattered when encountering the bottom surface, and the probe cannot receive the bottom wave signal.

　　(4) Castings and austenitic steels have uneven structures or coarse grains, which cause severe scattering and attenuation of ultrasonic waves when they pass through them. The scattered ultrasonic waves propagate along complex paths, which may cause echo annihilation and result in no display. A low-frequency coarse-grained special probe (2.5MHz) can be selected.

　　(5) The probe contact surface is worn to a certain extent. The surface of the commonly used thickness gauge probe is acrylic resin. Long-term use will increase its surface roughness, resulting in decreased sensitivity and incorrect display. You can use 500# sandpaper to polish it to make it smooth and ensure parallelism. If it is still unstable, consider replacing the probe.

　　(6) There are a large number of corrosion pits on the back of the object being measured. Because there are rust spots and corrosion pits on the other side of the object being measured, the sound waves are attenuated, resulting in irregular changes in the readings, and even no readings in extreme cases.

　　(7) When there is sediment inside the object being measured (such as a pipe), and the acoustic impedance of the sediment is not much different from that of the workpiece, the thickness gauge displays the wall thickness plus the sediment thickness.

　　(8) When there are defects inside the material (such as inclusions, interlayers, etc.), the displayed value is about 70% of the nominal thickness. At this time, an ultrasonic flaw detector can be used to further detect the defects.

　　(9) Temperature influence. Generally, the speed of sound in solid materials decreases as the temperature increases. Test data show that the speed of sound decreases by 1% for every 100°C increase in hot materials. This situation is often encountered for high-temperature in-service equipment. A special high-temperature probe (300-600°C) should be used instead of an ordinary probe.

    (10) Laminated materials, composite (inhomogeneous) materials. It is impossible to measure uncoupled laminated materials because ultrasonic waves cannot penetrate uncoupled spaces and cannot propagate at a uniform speed in composite (inhomogeneous) materials. For equipment made of multiple layers of materials (such as urea high-pressure equipment), special attention should be paid when measuring thickness. The thickness gauge only indicates the thickness of the layer of material in contact with the probe.

　　(12) Influence of coupling agent. Coupling agent is used to remove the air between the probe and the object to be measured so that the ultrasonic wave can effectively penetrate the workpiece to achieve the purpose of detection . If the type or method of use is improper, it will cause errors or the coupling mark will flicker and measurement will be impossible. The appropriate type should be selected according to the usage. When used on a smooth material surface, a low-viscosity coupling agent can be used; when used on a rough surface, a vertical surface, and a top surface, a high-viscosity coupling agent should be used. High-temperature coupling agent should be used for high-temperature workpieces. Secondly, the coupling agent should be used in an appropriate amount and applied evenly. Generally, the coupling agent should be applied to the surface of the material to be measured, but when the measurement temperature is high, the coupling agent should be applied to the probe.

　　(13) Wrong sound velocity selection. Before measuring the workpiece, preset its sound velocity according to the material type or measure the sound velocity based on the standard block. When the instrument is calibrated with one material (the test block is usually steel) and then measured with another material, wrong results will be produced. It is required to correctly identify the material and select the appropriate sound velocity before measurement.

　　(14) Influence of stress. Most of the equipment and pipelines in service have stress. The stress state of solid materials has a certain influence on the speed of sound. When the stress direction is consistent with the propagation direction, if the stress is compressive stress, the stress effect increases the elasticity of the workpiece and accelerates the speed of sound; conversely, if the stress is tensile stress, the speed of sound slows down. When the stress is inconsistent with the propagation direction of the wave, the vibration trajectory of the particle during the wave wave is disturbed by the stress, and the propagation direction of the wave deviates. According to the data, generally, as the stress increases, the speed of sound increases slowly.

　　(15) The influence of oxide or paint coating on the metal surface. The dense oxide or paint anti-corrosion layer on the metal surface is tightly bonded to the base material without a visible interface, but the speed of sound propagation in the two materials is different, which causes errors. The error size varies with the thickness of the coating.