Is it accurate to measure wall thickness using ultrasound when there is a medium inside the pipeline?

An ultrasonic thickness gauge measures thickness based on the principle of reflection of ultrasonic pulses. When the ultrasonic pulses emitted by the probe pass through the object being measured and reach the interface between different materials, those pulses are reflected back to the probe. By accurately measuring the time it takes for the ultrasonic waves to travel through the material, the thickness of the material can be determined. Any material that allows ultrasonic waves to propagate through it at a constant speed can be used to measure using this principle. Factors affecting the measured value: (You can find your answer below) (1) Excessive surface roughness of the workpiece results in poor coupling between the probe and the contact surface, leading to low reflected echoes, or even no echo signals being received. In the case of surface rust, on operating equipment, pipes, etc., where the coupling effect is very poor, the surface can be treated using methods such as sanding, grinding, or rubbing to reduce its roughness. This also helps to remove oxide layers and paint, revealing the metallic luster, thereby enabling a good coupling effect between the probe and the object being inspected through the use of a coupling agent. (2) The radius of curvature of the workpiece is too small; especially when measuring the thickness of small-diameter tubes, since the surface of the commonly used probe is flat, contact with a curved surface occurs as point or line contact, resulting in a low sound intensity transmission rate (poor coupling). A special probe with a small diameter (6 mm) can be used to measure curved surfaces such as pipes with greater accuracy. (3) The detection surface is not parallel to the bottom surface; sound waves are scattered upon hitting the bottom surface, preventing the probe from receiving the bottom-wave signals. (4) In castings and austenitic steels, due to uneven microstructure or large grains, severe scattering and attenuation occur as ultrasonic waves pass through them. The scattered ultrasonic waves travel along complex paths, which may cause the echoes to overlap and result in no signal being detected. A coarse-grain dedicated probe with a lower frequency (2.5MHz) can be used. (5) There is some wear on the probe contact surface. The surface of common thickness measurement probes is made of acrylic resin; prolonged use increases the surface roughness, which leads to a decrease in sensitivity and thus inaccurate readings. 500# sandpaper can be used for grinding to smooth it out and ensure parallelism. If it remains unstable, consider replacing the probe. (6) There are numerous corrosion pits on the back side of the object under test. Due to rust spots and corrosion pits on the other side of the object being tested, sound waves are attenuated, resulting in irregular changes in the readings; in extreme cases, no readings are obtained at all. (7) If there are deposits inside the object being measured (such as pipes), and the acoustic impedance of these deposits is not significantly different from that of the workpiece, the thickness gauge will display a value that is the sum of the wall thickness and the thickness of the deposits. (8) When there are defects inside the material (such as inclusions, interlayers, etc.), the displayed value is approximately 70% of the nominal thickness; in such cases, an ultrasonic flaw detector can be used to further detect the defects. (9) Effect of temperature. In general solid materials, the sound speed decreases as the temperature rises. Experimental data show that for materials in a heated state, the sound speed drops by 1% for every 100°C increase in temperature. This is a common situation encountered with high-temperature operating equipment. Special high-temperature probes (300–600°C) should be used; ordinary probes must not be employed. (10) Laminated materials, composite (heterogeneous) materials. It is impossible to measure uncoupled layered materials, as ultrasonic waves cannot penetrate uncoupled spaces nor travel at a constant speed in composite (heterogeneous) materials. For devices made of multiple layers of material (such as urea high-pressure equipment), special care must be taken when measuring thickness, as the reading given by the thickness gauge only indicates the thickness of the layer in contact with the probe. (12) Effect of the coupling agent. A coupling agent is used to remove the air between the probe and the object being inspected, allowing ultrasonic waves to penetrate the workpiece effectively for detection purposes. If the type or usage method is selected improperly, it will cause errors or the coupling indicator to flash, making measurement impossible. Since the appropriate type should be chosen based on the application, a low-viscosity coupling agent can be used when applied to smooth material surfaces ； When used on rough surfaces, vertical surfaces, and top surfaces, a coupling agent with high viscosity should be used. High-temperature couplants should be used for high-temperature workpieces. Secondly, the coupling agent should be used in appropriate amounts and applied evenly. Generally, it should be applied to the surface of the material being tested, but when the measurement temperature is high, it should be applied to the probe. (13) Incorrect sound speed selection. Before measuring the workpiece, preset its sound speed based on the type of material, or determine it by measuring it against a standard block. When the instrument is calibrated using one material (with steel being the common test block) and then used to measure another material, incorrect results will be obtained. It is necessary to correctly identify the material and select the appropriate sound speed before making measurements. (14) Effect of stress. Most in-service equipment and pipelines are under stress. The stress condition of solid materials has a certain impact on the speed of sound; when the direction of the stress is consistent with the direction of wave propagation, and if it is compressive stress, this stress causes an increase in the elasticity of the material, thereby increasing the speed of sound ； Conversely, if the stress is tensile, the sound speed slows down. When the stress is not aligned with the direction of wave propagation, the vibration path of the particles during the wave propagation is perturbed by the stress, causing the wave to deviate from its original direction of propagation. According to the data, as stress generally increases, the sound speed increases slowly. (15) Effect of oxide layers or paint coatings on metal surfaces. The dense oxide layer or paint coating formed on the metal surface is tightly bonded to the base material, with no distinct interface; however, the speed of sound varies between these two materials, which leads to errors. Moreover, the magnitude of these errors changes depending on the thickness of the coating