T.O. 33B-1-15-50Figure 5-55.ASTM Block and Test Part Back Surface Signals.c. The search unit is now placed on the ASTM block with 1-1/2 inch metal travel distance to the FBH.The signal from the FBH is maximized, and the gain is adjusted to bring the signal to the DAC curvelevel. Transfer is now applied by increasing the gain setting to double the amplitude. On an instrumentwith dB gain controls, this is easily accomplished by adding 6 dB to the gain or subtracting 6 dB ofattenuation. On an instrument without dB controls, the gain must be increased to double the amplitudeof a signal on the display. A correct way of doing this is as follows:(1) Place the search unit on the 3-inch travel distance block and adjust the position for maximum signalfrom the FBH. Note the amplitude of the signal. (It should be close to 30% of full screen height.)(2) Increase the gain until the amplitude of the signal is doubled (to 60% if it was 30% before. Thegain is now set for evaluation of discontinuities in the test part. Any discontinuity signal thatexceeds the DAC curve is cause for rejection.NOTEDoubling the gain by doubling the signal (from 50% of saturation to 100% ofsaturation) from the FBH in the 1-1/2 inch metal travel distance ASTM block wouldbe improper; the 100% of saturation signal is in a possible nonlinear area of thedisplay. Signals at levels above 90% of saturation shall not be used for applyingtransfer.d. The gain setting obtained after applying transfer is used for evaluation of discontinuities in the test part.It is advisable to perform the initial inspection using an even higher gain setting. This provides formore reliable detection of discontinuities. When discontinuities are found, the gain is reduced to thelevel established by the transfer technique. At this gain setting, any discontinuity signal that exceedsthe DAC curve is cause for rejection.
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
