The Reject control, even if it is linear, should be used with discretion, because even
small signals may be important when interpreting waveform displays. Reject should
not be used unless directed by the inspection procedure.
The Reject control is used to attenuate low-level irrelevant signals and noise signals on the waveform display. This
permits easier interpretation of echo signals. Most newer instruments have linear Reject controls, which eliminate the
low-level signals without affecting the amplitude of the relevant echo signals. On older instruments Reject also reduces
the amplitude of the relevant signals. Figure 5-28. illustrates the effect of the linear Reject control.
Figure 5-28. Reject Control
The Frequency control allows the inspector to select the frequency corresponding to a search unit or to select the
broadband mode to cover all frequencies. The selection that gives the best echo signal is normally used.
Single/Dual Search unit.
This control configures the search unit -cable receptacles for single-element search unit, dual-element search unit or
two separate search unit (for through-transmission inspection). The Dual position of the control is used for both dual-
element- search unit and two- search unit inspections; in these cases some instruments specify one receptacle as
transmitter and the other as receiver. For single-element- search unit inspections, only one receptacle is used. Consult
the instrument manual or procedure for the appropriate use of the receptacles.
Electronic Distance Amplitude Correction (DAC).
Depending on the instrument, DAC may also be called STC (Sensitivity Time Control), TCG (Time Corrected Gain),
TVG (Time Varied Gain) or if electronic gates are used, DAG (Distance Amplitude Gate). DAC electronically
compensates for material attenuation that causes different size echoes from equal-size reflectors located at varying
metal-travel distances from the search unit. After DAC is applied over a particular thickness, all the echoes from
reflectors of equal size and at the same orientation within that thickness will be of the same amplitude. Consult the
operating manual for the instrument being used or the specific inspection procedure for instructions for adjusting the