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

• High-voltage or low-voltage driving current, which is selected for the
transducer with a transducer voltage switch.
• Gated alarm units, which enable the use of automatic alarms when flaws
are detected. This is accomplished by setting up controllable time spans
on the display that correspond to specific zones within the test piece.
Signals appearing within the gates may automatically operate visual or
audible alarms. These signals may also be passed on to display devices
or strip-chart recorders or to external control devices. Gated alarm units
usually have three controls: the gate-start or delay control, which adjusts
the location of the leading edge of the gate on the oscilloscope trace; the
gate-length control, which adjusts the length of the gate or the location of
the gate trailing edge; and the alarm-level or sensitivity control, which
establishes the minimum echo height necessary to activate an alarm
circuit. A positive/negative logic switch determines whether the alarm is
triggered above or below the threshold level.