Requirements of industrial thickness gauges on the inspection surface of the workpiece
At this stage of the thickness gauge on the detection of mechanical instruments mostly use ultrasonic technology, ultrasonic transmission loss in the solid is very small, the detection depth is large, due to the ultrasonic wave in the heterogeneous interface will occur in the reflection, refraction and other phenomena, in particular, can not pass through the gas-solid interface. If there are pores in the metal, cracks, delamination and other defects (defects in the gas) or inclusions, ultrasonic propagation to the interface between the metal and the defects, it will be all or part of the reflection.
The reflected ultrasonic waves are received by the probe, processed by the internal circuitry of the instrument, and waveforms of different heights and with certain spacing are displayed on the fluorescent screen of the instrument. The surface roughness of the workpiece is too large, resulting in poor coupling between the probe and the contact surface, the reflected echo is low, or even can not receive the echo signal. For surface corrosion, coupling effect is very poor in-service equipment, pipelines, etc. can be sanded, grinded, thwarted and other methods of surface treatment, reduce the roughness, but also oxides and paint layer can be removed, revealing the metal luster, so that the probe and the object being examined through the coupling agent can achieve a very good coupling effect.
The radius of curvature of the workpiece is too small, especially when the small diameter pipe thickness measurement, because the commonly used probe surface for the plane, and the curved surface contact for the point contact or line contact, the sound intensity of low transmittance (coupling is not good). The special probe for small pipe diameter (<6mm) can be used to measure the curved surface materials such as pipes more accurately. The detecting surface is not parallel to the bottom surface, the acoustic wave encounters the bottom surface to produce scattering, the probe can not accept the bottom wave signal.
Castings, austenitic steel due to uneven organisation or coarse grain, ultrasound in which the ultrasonic wave through the serious scattering attenuation, the ultrasonic wave is scattered along the complex path propagation, it is possible to make the echo annihilation, resulting in non-display. A special probe (2.5MHz) for coarse crystals with a lower frequency can be used. Probe contact surface has some wear. Commonly used thickness probe surface for acrylic resin, long-term use will increase its surface roughness, resulting in decreased sensitivity, resulting in incorrect display.
500# sandpaper can be used to sand, make it smooth and ensure parallelism. If it is still unstable, consider replacing the probe. There are a lot of corrosion pits on the back side of the measured object. As the other side of the object under test has rust spots, corrosion pits, resulting in acoustic attenuation, leading to irregular changes in readings, and in extreme cases even no readings. (The object under test (e.g. pipe) has deposits inside, when the difference between the deposits and the acoustic impedance of the workpiece is not large, the thickness gauge shows the value of the wall thickness plus the thickness of the deposits.
