Factors Affecting Ultrasonic Thickness Gauge measurement

(1) When the ultrasonic Thickness Gauge measures, the surface roughness of the workpiece is too large, resulting in poor coupling between the probe and the contact surface, low reflection echo, and even failure to receive the echo signal. For surface corrosion and in-service equipment and pipelines with poor coupling effects, the surface can be treated by sanding, grinding, filing, etc. to reduce the roughness. At the same time, the oxide and paint layer can be removed to expose the metallic luster, so that the probe A good coupling effect can be achieved with the tested object through the couplant.

(2) The radius of curvature of the workpiece is too small, especially when measuring the thickness of small 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 transmission rate is low (poor coupling). A small-diameter special probe can be selected to accurately measure curved surface materials such as pipes.

(3) The detection surface is not parallel to the bottom surface, the sound wave encounters the bottom surface and scatters, and the probe cannot receive the bottom wave signal.

(4) Castings and austenitic steel have serious scattering and attenuation when ultrasonic waves pass through them due to uneven structure or coarse grains. The scattered ultrasonic waves propagate along complex paths, which may cause echoes to be annihilated, resulting in no display .

(5) The contact surface of the probe is somewhat worn. The surface of commonly used ultrasonic thickness measuring probes is made of acrylic resin. Long-term use will increase the surface roughness, resulting in a decrease in sensitivity, resulting in incorrect display. You can use 500# sandpaper to grind the periphery of the probe to make it smooth and ensure parallelism. If still unstable, consider replacing the probe.

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

(7) There is sediment in the measured object (such as a pipeline). When the acoustic impedance of the sediment and the workpiece is not much different, the value displayed by the Thickness Gauge is the wall thickness plus the thickness of the sediment. For example, the scale in the pipeline is in close contact with the pipeline.

(8) When there are defects (such as inclusions, interlayers, etc.) inside the material, the displayed value is obviously inconsistent with the thickness value. At this time, the ultrasonic flaw Detector can be used for further defect detection.

(9) The influence of temperature. Generally, the sound velocity in a solid material decreases with the increase of its temperature. According to experimental data, the sound velocity decreases by 1% for every increase of 100°C in a hot material. This is often the case for high-temperature in-service equipment. Special probes for high temperature should be selected (selected according to specific temperature conditions), and ordinary probes should not be used.

(11) The influence of coupling agent. The couplant is used to exclude the air between the probe and the measured object, so that the ultrasonic wave can effectively penetrate the workpiece to achieve the purpose of detection. If the type is selected or used improperly, it will cause errors or the coupling mark will flicker, making it impossible to measure. Due to the selection of the appropriate type according to the application, when using on a smooth material surface, you can use a low-viscosity coupling agent; when using on a rough surface, vertical surface and top surface, you should use a high-viscosity (such as butter) coupling agent. High temperature workpieces should use high temperature couplant. Secondly, the couplant should be used in an appropriate amount and applied evenly. Generally, the couplant should be applied to the surface of the material to be tested, but when the measurement temperature is high, the couplant should be applied to the probe.

(12) Wrong choice of sound velocity. Before measuring the workpiece, preset its sound velocity according to the type of material or reversely measure the sound velocity according to the standard block. When the instrument is calibrated with one material (the commonly used test block is steel) and another material is measured, wrong results will be produced. It is required to correctly identify the material and select the appropriate sound velocity before measurement. General thickness measuring instruments use the reverse sound velocity measurement function, which is called the material sound velocity, and can measure the workpiece sound velocity before measuring.