Usage tips of ultrasonic thickness gauge

2.30mm multi-point measurement method: When the measuring value is unstable, taking a measurement in the circle with a diameter of about 30mm centering on one measurement point, and take the small value as the thickness value of the workpiece to be tested.

3.Accurate measurement
The number of measurements is increased around the specified measurement point, and the thickness variation is represented by an equal thickness line.

4.Continuous measurement
Continuous measurement along a specified route is using a single point measurement with an interval of no more than 5 mm.

5.Grid measurement
Mark the grid in the specified area and record the thickness by point. This method is widely used in corrosion monitoring of high pressure equipment and stainless steel lining.

Factors affecting the measurement indication:

(1) The surface roughness of the workpiece is too large,which is resulting in the poor coupling between the probe and the contact surface, low reflection echo, and even failure to receive the echo signal. For surface rust, in-service equipment, pipes, etc., which have extremely poor coupling effects, which can be treated by sand, grinding, and frustration to reduce the roughness, and the oxide and paint layers can be removed to expose the metallic luster. A good coupling effect can be achieved by the coupling agent with the test object.

(2) The radius of curvature of the workpiece is too small, especially when the small diameter tube is thick, because the surface of the common probe is flat, the contact with the curved surface is point contact or line contact, and the sound intensity transmission is low (coupling is not good). A small diameter probe (6mm) can be used to accurately measure curved materials such as pipes.

(3) The detection surface is not parallel to the bottom surface, the sound wave encounters the bottom surface to generate scattering, and the probe cannot accept the bottom wave signal.

(4) Castings and austenitic steels are unevenly distributed or coarse grains. When ultrasonic waves pass through them, they cause severe scattering attenuation. The scattered ultrasonic waves propagate along complicated paths, which may cause the echoes to annihilate, resulting in no display. . A low-frequency coarse crystal is dedicated probe (2.5MHz) is available.

(5) There is some wear on the probe contact surface. The surface of the commonly used thickness measuring probe is made . The long-term use will increase the surface roughness, resulting in a decrease in sensitivity, resulting in incorrect display. It can be sanded with 500# sandpaper to make it smooth and ensure parallelism. If it is still unstable, consider replacing the probe.

(6) There are a large number of corrosion pits on the back of the object to be tested. Due to rust spots and corrosion pits on the other side of the object, the sound waves are attenuated, resulting in irregular readings and, in extreme cases, no reading.

(7) There is sediment in the measuring object (such as pipeline). When the sediment and the acoustic impedance of the workpiece are not much different, the shows the wall thickness plus the thickness of the deposit.

(8) When there are defects inside the material (such as inclusions, interlayers, etc.), the display value is about 70% of the nominal thickness. At this time, the defect detection can be further performed by the ultrasonic flaw detector.

(9) The effect of temperature. Generally, the speed of sound in solid materials decreases with the increasing temperature. Test data shows that for every 100 °C increase in hot material, the speed of sound drops by 1%. For high temperature in-service equipment
This is often the case. High-temperature dedicated probes (300-600 ° C) should be used. Do not use ordinary probes.

(10) Laminated materials, composite (heterogeneous) materials. It is impossible to measure uncoupled laminates because ultrasonic waves cannot penetrate uncoupled spaces and cannot propagate at a constant rate in composite (non-homogeneous) materials. For equipment made of multi-layer materials (like high-pressure equipment), special care should be taken when is measuring thickness. The steel ultrasonic thickness gauge indicates only the thickness of the material that is in contact with the probe.

(11) The influence of the coupling agent. The couplant is used to remove the air between the probe and the object to be measured, so that the ultrasonic wave can effectively penetrate the workpiece for inspection purposes. If the type is selected or the method of use is improper, it will cause an error or the coupling mark will flash and cannot be measured. Since a suitable type is selected depending on the use, a low viscosity coupling agent can be used when used on a smooth material surface; a highly viscous coupling agent should be used when used on a rough surface, a vertical surface, and a top surface. High temperature couplings should be used for high temperature workpieces. Secondly, the coupling agent should be used in an appropriate amount and evenly applied. Generally, the coupling agent should be applied to the surface of the material to be tested, but when the measurement temperature is high, the coupling agent should be applied to the probe.

(12) The sound speed selection is incorrect. Before measuring the workpiece, preset the speed of sound according to the type of material or reverse the sound speed according to the standard block. When the instrument is calibrated with one material (commonly used for steel) and another material is measured, erroneous results will result. It is required to correctly identify the material before measuring and select the appropriate speed of sound.

(13) The effect of stress. Most of the ultrasonic thickness measurement gauge and pipelines have stress, and the stress state of the solid material has a certain influence on the speed of sound. When the stress direction is consistent with the direction of propagation, if the stress is pressure.

(14) In the case of stress, the stress increases the elasticity of the workpiece and the speed of sound increases. Conversely, if the stress is the tensile stress, the speed of sound decreases. When the stress and the wave propagation direction are different, the vibration trajectory of the particle 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.

(15) The effect of metal surface oxide or paint overlay. The dense oxide or paint anti-corrosion layer produced on the metal surface, although tightly combined with the matrix material, has no obvious interface, but the speed of sound propagation in the two materials is different, resulting in errors, and the thickness of the cover varies with the thickness of the cover. It is also different.