Analysis of the Reasons for the Distortion of the Indicating Value of the Thickness Gauge

ultrasonic thickness in practice, especially in the monitoring of service equipment, if there is distortion of the display value, deviation from the actual thickness of the phenomenon, resulting in the existence of pipeline (equipment) hidden dangers, is based on the wrong data to replace the fittings, resulting in a large number of material wasteAccording to the tracking use of ultrasonic thickness measurement over the past few years, the distortion of the display and the reason syllable analysis is as follows:1, no display of the display or the cause of flickering of the display of the value of the analysis: this phenomenon in the field equipment and pipeline detection often appear, after a large number of phenomena and data analysis, the reasons are summarized as follows:(1) the surface of the workpiece rough through large, resulting in poor coupling effect between the probe and contact surface, low reflection echo, or even can not receive echo signalIn service equipment, pipes are mostly surface rust, coupling effect is very poor(2) the radius of curvature of the workpiece is too small, especially when the path tube is thicknessed, because the surface of the commonly used probe is flat, contact with the surface for point contact or line contact, the sound strong transmission rate is low (poor coupling)(3) the detection surface and the bottom surface is not parallel, the sound wave encountered the bottom surface scattering, the probe can not accept the final wave signal(4) castings, Auschwitz steel due to uneven tissue or grain coarseness, ultrasonic waves in which through the production of serious scattering attenuation, scattered ultrasonic waves along the complex path of transmission, may make the echo extinction, resulting in no display(5) probe contact surface has some wearCommon thickness probe surface syllensin, long-term use will make its surface roughness increased, resulting in reduced sensitivity, resulting in no display or flicker(6) there are a large number of corrosion pits on the back of the object under testDue to rust spots on the other side of the object under test, corrosion pits cause sound waves to decay, resulting in irregular changes in readings and, in extreme cases, no readings2, the display value is too large or too small reason analysis in the actual detection work, often encountered thickness gauge display value and design value (or expected value), significantly large or small, the reason analysis is as follows:(1) the object under test (such as pipe) within the sediment, when the sediment and the workpiece sound impedance is not much difference, the thickness gauge display value is thick wall thickness(2) When there are defects in the material (e.ginclusion, mezzanine, etc.), the display value is about 70% of the nominal thickness (at this time, the ultrasonic detector should be used for further defect detection)(3) temperature The sound velocity in general solid materials decreases with its temperature, and test data show that the sound velocity decreases by 1% for every 100 degrees C increase in hot materials This is often the case with high-temperature active equipment (4) laminated materials, composite (non-homogeneous) materials It is not possible to measure uncoupled laminated materials because ultrasonic waves cannot penetrate uncoupled space and cannot travel at a uniform speed in composite (non-homogeneous) materials For equipment made of multi-layered material wrap (like urea high-pressure equipment), special attention should be taken when measuring thickness, and the thickness gauge indicates only the thickness of the material in contact with the probe (5) the effects of the coupling agent Coupling agent is used to remove the air between the probe and the object under test, so that ultrasonic waves can effectively penetrate the workpiece for inspection purposes If you select a type or use method improperly, it will cause an error or a flickering coupling flag to fail to measure In actual use, due to the use of coupling agent too much, resulting in the probe leaving the workpiece, the instrument indicates the thickness value of the coupling agent layer (6) sound speed selection error Before measuring the workpiece, the sound velocity is preset according to the type of material or the sound speed is measured against the standard block When an instrument is corrected with one material (commonly used as steel) another material is measured, the wrong result is produced (7) effects of stress Most of the stress exists in the service equipment and the pipe, and the stress condition of the solid material has a certain effect on the sound velocity, and when the stress direction is consistent with the propagation direction, if the stress is the pressure stress, the stress effect increases the elasticity of the workpiece and the sound speed is accelerated; When the direction of stress and wave propagation is not the same, the vibration trajectory of the mass point in the process of fluctuation is disturbed by stress, and the propagation direction of the wave deviates According to the data, the general stress increases and the sound velocity increases slowly (8) the effects of metal surface oxides or paint coverings The dense oxides or paint anti-corrosion layers produced on the metal surface, although tightly combined with the substrate material, the nameless interface, but the speed of sound in the two substances is different, resulting in errors, and with the thickness of the mulch, the error size is also different