Questions regarding high-temperature thickness measurement

Based on some reference materials, the factors that affect the readings of ultrasonic thickness gauges are as follows: (1) Excessive surface roughness of the workpiece leads to poor coupling between the probe and the contact surface, resulting in low reflected echoes, or even no echo signals being received. For equipment in service or pipes that suffer from surface rusting and have very poor coupling effects, 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 surface so that a good coupling effect can be achieved 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). An option is a probe designed for small pipe diameters (6mm), which enables more accurate measurement of curved surfaces such as pipes. 　　(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 returning waves to be extinguished, resulting 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 consequently inaccurate readings. Optional 　　Sand with 500# sandpaper 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 variations 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 differs little from that of the material of the object, the thickness measurement device 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 situation is often encountered with high-temperature equipment in service. 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 through 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 of material 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 selected 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. However, when the measurement temperature is high, the coupling agent 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 using a standard block. When the instrument is calibrated using one material (with steel being the commonly used 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 influence on the speed of sound; when the direction of the stress is consistent with the direction of propagation, 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 trajectory of particle vibration during the wave propagation is perturbed by the stress, causing the wave to deviate from its original propagation direction. 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.

This post was last edited by yuchenchf on 2013-7-10 at 16:57. I have a technology that allows for measurement errors as low as 0.5 mm; temperature has no impact on the measurement results. The maximum temperature for thickness measurement is 500 degrees. No coupling agent is required, and measurements can be taken directly without removing the insulation layer. If you’re interested, feel free to contact me

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Most of the current high-temperature thickness gauges suffer from inaccurate test data, with a significant difference between laboratory conditions and those in the field. Moreover, since it is a point contact, high requirements are placed on the surface.

High temperatures affect the coupling performance of the coupling agent. Additionally, the sound speed decreases at high temperatures; as a result, the measured values tend to be on the high side. This is why it is necessary to stop the machine, remove the material, lower the temperature, and turn off the power during regular inspections.

Correction formula for sound speed during high-temperature thickness measurement: Sound speed V = 5918 – 0.85T (where T is the surface temperature of the object to be measured, in °C). The sound speed decreases by 85 for every 100°C increase in temperature.

Reasons for inaccurate data: 1. Generally, high-temperature probes can only operate for a few seconds before they need to be cooled; otherwise, they will get damaged. As measurements are taken over short periods of time, the data is naturally inaccurate； 2. Generally, the probe is held by hand; since the surface chip is flat, the measurement becomes unstable and the values fluctuate. Solution: 1. Replace it with a probe that can perform measurements for an extended period of time (it can continue measuring at temperatures below 350 degrees for 45 minutes) ； 2. Install a probe stabilizer. If you need more information, you can add me on WeChat: 13522021452

Ultrasonic thickness measurement that is not afraid of damage: it can perform continuous measurements for 45 minutes at temperatures below 350 degrees, without the need for cooling, enabling online measurement. Up to 550 degrees. Please note: Add WeChat: 13522021452