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.
