Manual No. OM5000
Jan X/04 Ver 1.40
9
Figure 4
With respect to the transducer location, the echo on the left side of the A-scan is called the
interface
echo
, while the echo on the right side of the A-scan is called the first back surface echo. The time
duration between the occurrence of the interface echo and the first back surface echo is proportional
to the material thickness (see Section 4.1.2).
With all the other conditions being equal, the echo from a thickness twice as great will be displayed
twice as far to the right across the scope screen. As shown in Figure 3(b), the echo height from a
thicker section is normally reduced in amplitude due to the ultrasonic attenuation phenomenon
discussed in Section 4.1.1.
The ultrasonic velocity differences between two materials understandably affects the echo round-trip
time. For a given thickness of two different materials, the echo round-trip time is longer in the
material with a slower characteristic ultrasonic velocity. If one material is twice as thick and has twice
the velocity as that of another material, identical A-scans will result. Thus, it is not possible to
differentiate between material thickness and velocity variations by using the pulse-echo ultrasound
method - even with the A-scan scope readout. Fortunately, for applicable materials, changes in echo
round-trip time essentially can be ascribed to thickness changes rather than ascribed to unknown,
inherent velocity variations in a given material.
One fundamentally important
scope (instrument) control which
affects the A-scan appearance is
the sweep speed control. The
sweep speed control changes the
time base along the horizontal axis
of the A-scan trace. This feature,
as shown in Figure 4, permits the
space (not the time) between the
two echos to be conveniently
expanded (spread apart) or
compressed (squeezed together).
For ease of operation, it is
particularly valuable to spread out
the display between echos too
closely spaced together. For the
same reason, slowing the sweep
speed will "bring" an echo onto the
display which otherwise falls so far
out in round-trip time that it is "off"
the right-hand side of the scope.
It is important to understand that
sweep speed only affects the
horizontal aspect of the display
and does not change the time
(duration) of the actual ultrasonic
echo period (which is related to
material thickness and velocity).