6 dB drop method for reflector length and height, and the reference sensitivity levels used to set up distance-amplitude and electronic DAC standardization
◉ ASTM E164-24 (CURRENT EDITION), §7, §9, §11 · NOT A REGULATORY SUBMISSION TOOL
⚠ Sizing aid, not an acceptance standard. Source: ASTM E164-24, the current edition, read from its own text. E164 is a practice for performing the examination and estimating reflector size and location — acceptance criteria (how large a reflector is permitted to be) come from the referencing code or contract, not from E164 itself. The 6 dB method (§11.2.1) is stated to be accurate for reflectors larger than the beam diameter; for reflectors smaller than the beam, the standard warns that significant errors may occur, and amplitude alone should not be treated as a reliable measure of flaw severity for adversely oriented planar flaws.
Identification
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1. Reflector Length, 6 dB Drop (§11.2.1)
Move the search unit along the reflector; length is the distance between the two points where the signal amplitude has dropped to half (6 dB) of its peak value at the extremities. Minimum reliably measured by this method: 1/4 in. (6.4 mm).
2. Reflector Height, 6 dB Drop (§11.2.1)
Height % of thickness = (\u0394SR \u00d7 100) / tSR. Only the area of the reflector that returns energy to the search unit is measured; minimum reliably measured by this method: 1/8 in. (3.2 mm) height.
3. Sensitivity Standardization Reference Levels
Distance-amplitude curve (DAC) method, §9.4.1.180% screen height, from the highest-amplitude reference reflector
Electronic DAC / time-corrected gain method, §9.4.2.150% screen height, from the highest-amplitude reference reflector
IIW IIW-type block calibration, §7.2.280% screen height signal from the highest of the standardization indications