Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

4.2.8.1: Linearity of time base
General
This check may be carried out using a standard calibration block eg A2,
and a compressional wave probe. The linearity should be checked over a
range at least equal to that which is to be used in subsequent testing.
Method
a) Place the probe on the 25mm thickness of the A2 block and adjust the
controls to display ten BWEs.
b) Adjust the controls so that the first and last BWEs coincide with the scale
marks at 1 and 10.
c) Increase the gain to bring successive backwall echoes to 80% FSH. The
leading edge of each echo should line up with the appropriate reticules
line.
d) Record any deviations at approximately half screen height. Deviations
should be expressed as a percentage of the range between the first and
last echoes displayed (ie 225mm).