BS EN 17290:2021 pdf free download – Non-destructive testing一Ultrasonic testing – Examination for loss of thickness due to erosion and/or corrosion using the TOFD technique.
7.5.2 Time-to-depth conversion
For a given PCS, the time-to-depth conversion shall be set using one or more time reference signals (lateral wave, back-wall echo, reflector at a known depth) with a known material velocity.
After conversion, the thickness or depth position measured on the reference block shall be within ± 0,2 mm of the thickness of the reference block or the depth position of the reference reflector.
Correct ions are necessary for components with complex geometry.
7.5.3 Sensitivity setting
The test sensitivity shall be set by one of the following alternatives:
a) on the reference block;
b) on the test object.
When using a):
The height of the echo from the reference reflector closest to the thickness of the test object shall be set to at least 80 % of FSH (full screen height).
Differences in signal amplitude between the reference block and the test object could require compensation, e.g. a transfer correction.
When using b):
If the hack-wall echo is applicable, the sensitivity shall be set such that the amplitude of the hack-wall signal is between 18 dB and 30 dB above FSI-l. When the use of a back-wall signal is not appropriate, the signal from the lateral wave shall be used and set to 40 % to 80 % of FSH.
For both cases the material grain noise shall not exceed 10 % of FSH.
Typically this sensitivity setting is suitable for detecting diffraction signals. In case only reflection signals are required a lower sensitivity setting may be suitable.
For both cases the signal-to-noise ratio shall he at least 6 dB. Otherwise different probes and / or settings shall be used according to Table 1.
7.5.4 Checking of the combined equipment
Checks to confirm the time base and sensitivity settings shall be performed at least every 4 hours and on completion of the testing.
Checks shall also be carried out whenever a system parameter is changed or changes in the settings are suspected.
If a reference block was used for the initial setup, the same reference block should be used for subsequent checks. A different block may be used, provided that it has been cross-referenced with the initial reference block.
If deviations from the initial settings are found during the checks, the corrections given in Table 2 shall be carried out.