Five important factors affecting the measurement data of ultrasonic thickness gauge
(1) laminated materials, composite (heterogeneous) materials. It is not possible to measure uncoupled stacked materials because ultrasound cannot penetrate uncoupled spaces and does not propagate at a uniform velocity through composite (heterogeneous) materials. For equipment made of multi-layer materials (such as urea high-pressure equipment), special attention should be paid when measuring the thickness. The indicated value of the thickness gauge only indicates the thickness of the layer of material that is in contact with the probe.
(2) 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 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.
(3) 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 common test block is steel) and then measured with another material, it will produce wrong results. It is required to correctly identify the material and select the appropriate sound velocity before measurement.
(4) The influence of stress. Most of the in-service equipment and pipelines have stress, and the stress state of solid materials has a certain influence on the sound velocity. When the stress direction is consistent with the propagation direction, if the stress is compressive stress, the stress will increase the elasticity of the workpiece and accelerate the sound velocity; otherwise , if the stress is tensile stress, the speed of sound slows down. When the stress and the propagation direction of the wave are different, the particle vibration trajectory is disturbed by the stress during the wave process, and the wave propagation direction deviates. According to the data, the general stress increases and the speed of sound increases slowly.
(5) The effect of metal surface oxide or paint coating. Although the dense oxide or paint anti-corrosion layer produced on the metal surface is closely combined with the base material and has no obvious interface, the propagation speed of the sound velocity in the two substances is different, resulting in errors, and the error varies with the thickness of the covering. Also different.
