Number of samples
It is important to note that the number of samples cannot be changed after calibration, otherwise the calibration is invalidated. The following steps are used to calculate the number of samples necessary:
- Calculate the wavelength. For a 5 MHz probe in shear mode in steel, the wavelength is approximately 0.65 mm
- Calculate the UT path. If the start is at 10 mm and the range is set at 30 mm then the UT path is 46.7 mm at 50°
- Calculate the number of samples. The UT path is divided by 15% of the wavelength. Hence, the approximate number of samples needed is 479
- If the calculated number of samples is bigger than the maximum number of samples permitted by the instrument, the UT path must be shrunk
- Some instruments offer an auto-set for the number of samples. It is, in general, a good idea to use that setting
It should be noted that the start and range change for every angle when in True Depth mode.