An amplitude type gate is essential for which of the following examinations?

The correct choice is that an amplitude type gate is essential for automatic examinations. In automated ultrasonic testing systems, amplitude gates are used to enhance the detection and characterization of defects by establishing preset thresholds for signal amplitudes. These gates help in accurately distinguishing between valid echoes from discontinuities and background noise, which is critical in an automatic setup that relies on consistent and repeatable evaluation without operator intervention.

Automatic examinations benefit significantly from amplitude gates because they enable the system to process the received signals in real-time and make decisions based on pre-defined criteria, which can improve reliability and efficiency in inspection. By setting an amplitude threshold, the automated system can identify relevant signals that exceed this threshold, ensuring that even subtle defects are detected while minimizing false readings that could lead to misinterpretation.

In contrast, shear wave and longitudinal examinations may utilize amplitude gates under certain conditions, but they are not exclusively reliant on them as their primary mechanism for effective signal interpretation. Manual examinations, while they can benefit from amplitude gating, depend more on the operator's judgment and subjective interpretation of the signals rather than an automated process. Thus, the emphasis on automatic examinations explains why the amplitude type gate is considered essential in this context.