T.O. 33B-1-11-571.8.1.6SurfaceWaveReferenceStandard.A variety of reflectors can be used for set up of surface wave inspections. Electrical discharge machined notches, sawcuts, chiseled notches, and drilled holes can be used. Suggested surface wave standards are the side-drilled holes or thenotch in the IIW block when the search unit is placed on the large front or back surface of the block. The reflectedsignal from one of the holes or the notch can be compared with the reflected signals from discontinuities in test parts.Signals should be compared at equivalent travel distances (distance from search unit to reference standard reflectorequal to distance from search unit to test part discontinuity).1.8.2CalibrationofEquipment.The most important calibration is the verification of each inspection setup through use of the applicable referencestandard. It is essential that this verification be accomplished for each and every inspection. However, there aregeneral calibration procedures that can be used to ensure that the equipment and its supporting components are withinthe parameters required to perform ultrasonic inspections. These procedures, which follow in the remainingparagraphs of this section, should be performed and documented at the time intervals prescribed in applicablespecifications or procedures, at any time an operator suspects that there is a problem with the equipment, or at leastannually.1.8.2.1VerticalLinearityofInstrument.1.8.2.1.1Limits.a. Upper Linearity Limit is the level of vertical deflection defining the upper limit of an observed constantrelationship between the amplitude of the indications on an A-scan screen and the correspondingmagnitude of the reflected ultrasonic wave from reflectors of known size. The minimum acceptablelimit of full screen height is 95 percent.b. Lower Linearity Limit is the level of vertical deflection defining the lower limit of an observed constantrelationship between the amplitude of the indications on an A-scan screen and the correspondingmagnitude of the reflected ultrasonic wave from reflectors of known size. The maximum acceptablelimit of full screen height is 10 percent.1.8.2.1.2ProcedureforDeterminingVerticalLimits.a. Use three ASTM blocks, all with 3-inch metal travel distances and one each with a 3/64, 5/64, and 8/64inch diameter flat-bottom hole (FBH).b. Move the search unit over the surface of the block with the 5/64 inch FBH until maximum response isobtained from the FBH. Make sure that the reject control and filters are in the "off" or minimumpositions. Adjust the instrument gain control until the FBH signal is 35% of saturation on the CRT.c. Leave the gain fixed as adjusted above in d. Maximize the FBH signal on the 3/64 and 8/64 FBHblocks. Record the FBH signal amplitudes.d. If the instrument is linear, the signals from the 3/64 and 8/64 FBHs will be 13% ± 3% and 90% ± 5% ofsaturation respectively. Thus, a 3/64 FBH signal between 10% and 16% of saturation is consideredlinear; an 8/64 FBH signal between 85% and 95% of saturation is considered linear.e. Instruments not linear within the above limits SHALL be repaired or replaced.
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