c. Measure the distance between the leading edge of adjacent back reflections. Ideal horizontal linearity
will be indicated by an equal distance between the leading edges of adjacent back reflections. If all the
values are equal within ± 3% of the full scale width, the instrument is considered linear in the
d. Instruments not linear within the above limits shall be repaired or replaced.
Sensitivity of Inspection System.
Sensitivity is a measure of the ability of an inspection system (instrument and search unit) to detect discontinuities
producing relatively low-amplitude signals because of the size, geometry or location of the discontinuities. Noise can
limit detectability of discontinuities by masking their indications. Generally, sensitivity, resolution and signal-to-noise
ratio are interdependent and should be evaluated under similar test conditions.
Procedure for Determining General Sensitivity.
The 1 MHz and 15 MHz requirements are applicable only when these frequencies
are to be used; they are not specific requirements for all instruments.
Unless otherwise specified in a detailed procedure, use ASTM reference blocks with flat-bottom holes (FBHs). Table 1-
7 shows the FBH that should be detectable with the respective frequencies.
a. Select the ASTM block with the appropriate FBH at a depth of three inches.
b. Obtain a peak signal from the appropriate FBH.
Table 1-7. Minimum Sensitivity Requirements.
FBH Size (inch/64)
c. Adjust the Gain control of the instrument until the discontinuity indication is 60 percent of full screen
d. Note the baseline noise in the test region (adjacent to the FBH indication). The noise should be no
higher than 20 percent of full screen height.
e. When a reference standard specified by a detailed inspection procedure is used, the minimum signal-to-
noise ratio is also 3 to 1.
f. If the inspection system does not meet these sensitivity requirements, the search unit and/or cable
SHALL be replaced and the sensitivity checked again. If the inspection system still does not meet the
above requirements, the instrument SHALL be repaired or replaced.