T.O. 33B-1-1
5-52
c. Suppose the signal obtained from the fastener hole in the skin is 80% of saturation. This is an increase
of 60% (4 dB) of the signal from the reference standard fastener hole (30 = 60% of 50). This is the
amount of transfer, the amount by which the rejection (alarm) level has to be raised.
d. Place the search unit back on the reference standard to obtain the signal from the saw cut. Increase the
gain until the signal is at some convenient level, for example, 80% of saturation. At this gain 20% of
full scale would be the rejection level, since any signal exceeding 25% of the saw cut signal is cause for
rejection. However, this rejection level must be increased to 32% of full scale by the amount of transfer
(60% or 4 dB). Therefore, any discontinuity that exceeds 32% of saturation is cause for rejection. As
in the previous example (paragraph 5.3.5.3.1), the initial scanning is performed at a higher gain setting.
5.3.5.3.3
Straight Beam Technique of Transfer Applied to Angel Beam Inspection.
The technique given in paragraph 5.3.5.3.1 for transfer for a straight beam inspection may also be used for angle beam
inspections. A straight beam search unit is used to determine the amount of transfer. This amount of transfer is then
applied to the angle beam inspection. When using this method, the following conditions must be met.
a. The frequency of the straight beam search unit shall be approximately double the frequency of the angle
beam search unit. For a 2.25 MHz angle beam search unit, use a 5 MHz straight beam search unit. For
a 5 MHz angle beam unit, use a 10 MHz straight beam search unit.
b. The back surface of the standard and the test part must be located in the far field of the straight beam
search unit.
c. The back surfaces of the reference standard and the test part must be parallel with the front surfaces.
5.3.5.4
Transfer Limits.
When using the transfer technique, if the signal from the test part is less than 25 percent (-12 dB) or more than 60
percent (+4 dB) of the signal from the reference standard (see Figure 5-58), the reference standard may be of the wrong
material, heat treat condition and/or surface condition. If the signal from the test part is not within the above limits,
another reference standard should be tried, or the prime depot should be contacted.
