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Ultrasonic thickness gauge Measuring casting
Mar 14, 2018

Ultrasonic thickness gauge has specialty for measuring casting. The crystal particles for castings are coarse, the structures are not dense enough, plus that they are in gross state, which makes difficulty in measuring their thickness.
First of all, due to coarse crystal particles and not-so-dense structure, it will cause large attenuation in sound energy. The attenuation is due to material’s scatter and absorption of sound energy. The attenuation degree is closely relative with the size of crystal particle and ultrasonic frequency. Under the same frequency, the attenuation will increase with the crystal diameter, but it has a high limit, when it reaches to this limit, if the crystal diameter increases, the attenuation will tend to be a fixed value. For attenuation under different frequencies, it will increase as the frequency.
Secondly, due to coarse crystal particle and if coarse out-phase structure exists, it will cause abnormal reflection, i.e. the grass-shaped echo or tree-shaped echo, so that the measuring will have error reading, and cause wrong judgement.
Thirdly, as the crystal particle is coarse, the anisotropy in flexibility in metal’s crystallizing direction will be obvious, which results in difference in sound velocities in different directions, and the max. difference can even be up to 5.5%. What’s more, the compactness in different positions of the workpiece is different, which will also cause difference in sound velocity. All of these will produce inaccuracy in measurement. Therefore, one must be very careful in measuring castings.
During measuring castings, please pay attention to the following points:
1.    When measuring casting with unmachined surface, please use engine oil, consistent grease and water glass as coupling agent.
2.    Calibrate the sound velocity for the object with a standard test piece having the same material and measuring direction as that for the object to be measured.
3.    If necessary, take 2-point calibration.

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