14 factors affecting the value of ultrasonic thickness gauges
(1) The surface roughness of the workpiece is too large, resulting in poor coupling between the probe and the contact surface, low reflection echo, or even unable to 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 to be examined through the coupling agent can achieve good coupling effect.
(2) The radius of curvature of the workpiece is too small, especially when measuring the thickness of small-diameter pipe, because the commonly used probe surface for the plane, and the curved surface of the contact for the point contact or line contact, the sound intensity of the transmission rate is low (coupling is not good). Can choose a small diameter pipe probe (6mm), can be more accurate measurement of pipelines and other curved materials.
(3) Detection surface and the bottom surface is not parallel, the acoustic wave encountered in the bottom surface scattering, the probe can not accept the bottom wave signal.
(4) Castings, austenitic steel due to uneven organisation or coarse grain, ultrasonic waves in which the passage of serious scattering attenuation, the scattered ultrasonic waves along the complex path propagation, it is possible to make the echo annihilation, resulting in non-display. Can choose a lower frequency of coarse crystal special probe (2.5MHz).
(5) The probe contact surface has a certain degree of 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. Use 500# sandpaper to make it smooth and ensure parallelism. If it is still unstable, consider replacing the probe.
(6) The back of the measured object has a large number of corrosion pits. Due to the other side of the object under test has rust spots, corrosion pits, resulting in acoustic attenuation, resulting in irregular changes in readings, and in extreme cases, even no readings.
(7) The object under test (such as pipes) has deposits inside, when the deposits and the workpiece acoustic impedance difference is not large, the thickness gauge display value for the wall thickness plus deposit thickness.
(8) When there are defects within the material (such as inclusions, interlayers, etc.), the display value of about 70% of the nominal thickness, then the ultrasonic flaw detector can be used to further defect detection.
(9) The effect of temperature. General solid materials in the speed of sound with its temperature increases and decreases, there are test data show that the hot material every increase of 100 ℃, the speed of sound decreased by 1%. For high-temperature in-service equipment often encountered this situation. Should be used for high-temperature special probes (300 ℃ ~ 600 ℃), do not use ordinary probes.
(10) Laminated, composite (inhomogeneous) materials. It is not possible to measure uncoupled laminated materials because ultrasonic waves cannot penetrate uncoupled space and do not propagate uniformly through composite (inhomogeneous) materials. In the case of equipment made of multiple layers of material (such as urea high-pressure equipment), special care should be taken when measuring the thickness, as the gauge only indicates the thickness of the layer of material in contact with the probe.
(11) Effect of couplant. Coupling agent is used to exclude the air between the probe and the object to be measured, so that ultrasonic waves can effectively penetrate the workpiece to achieve the purpose of detection. If the type is selected or used improperly, it will cause errors or flickering of the coupling mark, which cannot be measured.
Should choose the appropriate type according to the use of the situation, when used in the smooth material surface, you can use low viscosity coupling agent; when used in the rough surface, vertical surface and top surface, should use high viscosity coupling agent. High-temperature workpiece should use high-temperature coupling agent.
Secondly, the coupling agent should be used in the right amount, coated evenly, generally should be coated with the coupling agent on the surface of the measured material, but when the measurement temperature is high, the coupling agent should be coated on the probe.
(12) Wrong selection of sound velocity. Before measuring the workpiece, the speed of sound is preset according to the type of material or back-measured according to a standard block. When the instrument is calibrated for one material (steel is a common test block) and then another material is measured, incorrect results will be produced. It is necessary to identify the material correctly and select the appropriate speed of sound before measurement.
(13) Effect of stress. In-service equipment, pipelines, most of the presence of stress, solid material stress conditions on the speed of sound has a certain impact, when the direction of stress and propagation direction is consistent, if the stress is compressive stress, the stress effect of the workpiece elasticity increases, the speed of sound; Conversely, if the stress is tensile stress, the speed of sound slows down.
When the stress and wave propagation direction is not consistent, the fluctuation process of the mass vibration trajectory by the stress interference, wave propagation direction deviation. According to the data, the general stress increases, the speed of sound increases slowly.
(14) Influence of oxides or paint coverings on metal surfaces. The dense oxide or paint anticorrosive layer produced on the metal surface, although it combines closely with the base material and has no obvious interface, the propagation speed of sound velocity in the two substances is different, thus causing errors, and with the thickness of the coverings are different, the size of the error is also different.
