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

Gating. (Depth Gate)
An electronic depth gate is another essential element in C-scan systems. A
depth gate is an electronic circuit that allows only those echo signals that are
received within a limited range of delay times following the initial pulse or
interface echo to be admitted to the receiver-amplifier circuit. Usually, the
depth gate is set so that front reflections and back reflections are just barely
excluded from the display. Thus, only echoes from within the test piece are
recorded, except for echoes from thin layers adjacent to both surfaces of the
test piece. Depth gates are adjustable. By setting a depth gate for a narrow
range of delay times, echo signals from a thin slice of the test piece parallel to
the scanned surface can be recorded, with signals from other portions being
excluded from the display.
Some C-scan systems, particularly automatic units, incorporate additional
electronic gating circuits for marking, alarming, or charting. These gates can
record or indicate information such as flaw depth or loss of back reflection,
while the main display records an overall picture of flaw distribution.