Ultrasonic thickness gauge principle and use tips and influence factors
(1) The effect of temperature. Generally the speed of sound in solid materials decreases as their temperature increases, and experimental data show that the speed of sound decreases by 1 per cent for every 100°C increase in the thermal state of the material. For high-temperature in-service equipment often encountered this situation. High-temperature special probes should be used, do not use ordinary probes.
(2) laminated materials, composite (non-homogeneous) materials. To measure the uncoupled laminated material is impossible, because ultrasonic waves can not penetrate the uncoupled space, and can not be uniformly propagated in the composite material. For equipment made of multiple layers of material wrapped (, thickness measurement should be particularly careful, the thickness gauge indicates only the thickness of the layer of material in contact with the probe.
(3) When there are defects within the material, the displayed value is about 70% of the nominal thickness, at this time, the ultrasonic flaw detector can be used for further defect detection.
(4) Sound speed selection error. Before measuring the workpiece, according to the type of material preset its speed of sound or according to the standard block back-measurement of the speed of sound. When calibrating the instrument with one type of material and then going to measure another type of material, it will produce wrong results. It is required that the material must be correctly identified and the appropriate speed of sound selected before measurement.
(5) Influence of oxides or paint coverings on metal surfaces. Metal surface produced by the dense oxide or paint corrosion layer, although the base material and the combination of close, no name obvious interface, but the speed of sound in the two kinds of material
(6) the impact 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 the same, 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 the same, fluctuations in the process of mass vibration trajectory by stress interference, wave propagation direction deviation. According to the data, the general stress increases, the speed of sound increases slowly.
(7) The effect of the coupling agent. 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 choice of type or the use of improper methods, will cause errors or coupling mark flicker, can not be measured. Due to the use of selecting the appropriate type, 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 be selected high-temperature coupling agent. Secondly, the coupling agent should be used in the right amount, coated evenly, the coupling agent should generally be coated on the surface of the material to be measured, but when the measurement temperature is higher, the coupling agent should be coated on the probe
