Pulser / Receiver Controls

Figure 5-24. CRT Display Before Adjusting Sweep Delay

Figure 5-27. Decibel-to-Amplitude-Ratio Conversion Chart

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 5-25 5.2.1.3.9 Zero or Zero Offset. Some newer instruments have a Zero or Zero offset control that is a fine-delay control used to compensate for transducer face-plate wear. In shear-wave inspection, this control can be used to compensate for the distance the sound beam travels in a plastic wedge. 5.2.1.3.10 Velocity. On some instruments an inspector can enter the velocity of sound in the test material (if known), then enter the thickness of the test part. Then, the horizontal scale of the display will be automatically calibrated to provide the depth of any discontinuity that may be detected in that particular test part. 5.2.1.4 Pulser / Receiver Controls. 5.2.1.4.1 Pulse Repetition Rate (Rep Rate). On some instruments, the Pulse Repetition Rate is set automatically with the sweep controls. Other instruments allow changing the repetition rate with a separate control or menu selection. The repetition rate is the actual number of ultrasonic pulses per second generated by the instrument; that number can be set from hundreds to thousands. The Pulse Repetition Rate must be high enough so that several pulses are transmitted in the time it takes for the search unit r to move a distance equal to the size of the smallest defect that must be found. The higher the rate, the faster the scanning speed can be while still maintaining the required sensitivity. The maximum Rep Rate is the rate beyond which unattenuated echo signals occur on the display from an earlier pulse; this is called "wrap around" and is recognized by the occurrence of unexplained signals on the display which disappear if the Rep Rate is decreased while the search unit is held motionless on the test part. Some instruments include an automatic override to set the Rep Rate at a reduced value if the inspector tries to set it manually above a value that is compatible with the sweep settings. 5.2.1.4.2 Pulse Controls. On some instruments the following controls are automatically set to default values when a new setup is initiated or when other interactive controls are adjusted. Adjustments of the following controls (if permitted) should be made to more clearly define the discontinuity indications. NOTE Minimum pulse length, (maximum damping) is obtained with the load resistance as small as possible for the circuitry. Load resistance selections may range from 16 ohms for maximum damping to 500 ohms for maximum pulse length (minimum damping). a. Pulse Length (Damping). The Pulse Length or Damping control is used to adjust the time duration of the high-frequency pulse applied to the search unit. The length of the initial pulse should be kept to a minimum, and increased only to gain signal strength when required; excessive pulse length can obscure signals from discontinuities close to the inspection surface (poor near-surface resolution). A short pulse length provides the best near-surface resolution. b. Pulse Voltage. This control determines the amplitude of the generated initial pulse. Some instruments have incremental voltage adjustments; for example, from 40 to 400 volts in 5-volt increments. Other instruments have adjustments for only low, medium or high voltages. c. Pulse Width. Some instruments generate a square pulse as opposed to a spike pulse. The Pulse Width control sets the width of the square pulse, usually in nanoseconds. The effect of the Pulse Width is similar to the Damping control, although the electronic nature of each is different.

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