Precautions of ultrasonic thickness gauge measurement (1)

The principle of ultrasonic thickness gauge measurement is similar to the principle of light wave measurement. The ultrasonic pulse emitted by the probe reaches the back-test object and propagates through the object. When it reaches the material interface, it is reflected back to the probe. The thickness of the material to be tested is determined by accurately measuring the time and the ultrasonic wave propagates through the material.

Cleaning surface
Before the measurement, all the dust, dirt and rust on the surface of the object to be tested should be removed, and the covering such as paint should be removed. Improving the roughness requirements excessively rough surfaces can cause measurement errors, even the instrument can not be measured, the surface of the material to be tested should be as smooth as possible before measurement, and it can be smoothed by grinding, throwing, rubbing. High viscosity coupling agents can also be used.

Measuring cylindrical surface
To measure cylindrical materials, such as pipes, oil drums, etc., it is important to choose the angle between the probe crosstalk and the axis of the material being tested.the probe is coupled to the material to be tested. The probe plate is parallel or perpendicular to the axis of the material to be tested. The ultrasonic thickness gauge underwater is slowly rocked vertically along the axis of the measured material. The readings on the screen will change regularly. The minimum value in the reading as the exact thickness of the material. The criterion for selecting the direction of the crosstalk between the probe crosstalk and the axis of the material to be tested depends on the curvature of the material, the pipe with a larger diameter, and the pipe cross-corrugated separator is perpendicular to the axis of the pipe, and the pipe with a smaller diameter is selected and the axis of the pipe. Both parallel and vertical measurements were taken and the minimum of the readings was taken as the measurement thickness.

Composite shape
When measuring composite shape materials (such as pipe elbows), the following method described in probe can be used, except that the second measurement is performed, and the probe cross-talk separator and the axis perpendicular and axis are respectively read. Two values are in parallel, the smaller of which is the thickness of the material at the measurement point.

Non-parallel surface
In order to obtain a satisfactory ultrasonic response, the other surface of the material to be tested must be parallel or coaxial with the surface to be measured, otherwise it will cause measurement errors or no reading at all.

Temperature influence of the material
The thickness of the material and the ultrasonic propagation speed are affected by the temperature. If the measurement accuracy of NDT ultrasonic thickness gauge is high, the test block comparison method can be used, that is, the test block of the same material is used to measure at the same temperature condition, and the temperature compensation coefficient is obtained. Use this factor to correct the measured value of the workpiece.

Large attenuation material
For some materials such as fiber, porous, and coarse particles, they cause a large amount of scattering and energy attenuation of the ultrasonic waves, resulting in abnormal readings or even no readings (usually abnormal readings are less than the actual thickness). In this case, the material is indicated. Not suitable for testing with this thickness gauge.

Reference test block
Accurate measurement of different materials under the different conditions, the closer the material of the calibration test block is to the material to be tested, the more accurate measurement. The ideal reference block will be a set of test pieces of different thicknesses of the material to be tested. The test block can provide instrument compensation correction factors (such as material microstructure, heat treatment conditions, particle direction, surface roughness, etc.). In order to meet the requirements of maximum accuracy measurement, a set of reference test blocks will be very important.

In most cases, accuracy of satisfactory measurement can be obtained by using a reference test block. This test block should have the same material and similar thickness as the material being tested. Taking a uniform material to be measured with a micrometer can be used as a test block. For thin materials, the test block can be used to determine the exact lower limit when its thickness is close to the lower limit of probe measurement. Do not measure materials below the lower thickness. If a thickness range is estimable, the thickness of the test block should be selected as the upper limit. When the material to be tested is thick, especially for alloys with complex internal structures, one of the test blocks should be selected close to the material to be tested to facilitate calibration.