Analysis of Measurement Errors of Ultrasonic Thickness Gauge

　　The ultrasonic thickness gauge measures thickness based on the ultrasonic pulse reflection principle. 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 to determine the thickness of the material to be measured 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. In actual testing work, we often encounter problems such as the ultrasonic thickness gauge indication being significantly larger or smaller than the design value (or expected value). This article analyzes some of the reasons for the above.

　　In actual testing work, it is often encountered that the ultrasonic thickness gauge indication is significantly larger or smaller than the design value (or expected value). The reasons are analyzed as follows:

　　1. 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 multi-layer materials (such as urea high-pressure equipment), special attention should be paid when measuring thickness, because the indication of the ultrasonic thickness gauge only indicates the thickness of the layer of material in contact with the probe.

　　2. 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.

　　3. 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.

　　4. The 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. In actual use, due to excessive use of coupling agent, when the probe leaves the workpiece, the instrument indicates the thickness of the coupling agent layer.

　　5. If there is sediment in 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 ultrasonic thickness gauge will display the wall thickness plus the sediment thickness.

　　6. 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 combined with the base material without obvious interface, but the propagation speed of sound in the two materials is different, which causes errors. The error size varies with the thickness of the coating.

　　7. 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 should be used for further defect detection).

　　8. 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 not consistent 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.