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extreme left edge of the A-trace and terminates on the leading edge of the back echo. This line,
or bar, represents the thickness gate (T-gate) and shows which echo stops the gate. In this
example, since the 0.500 inch step is the one being measured and the horizontal FS range is
2.00 inches, the T-gate terminates on the first echo above 40% FS amplitude, the back echo
from the 0.500 inch step. Along the left-hand vertical axis, a signal will also be seen. Since the
FACTORY setup defaults to initial pulse synchronization (IP SYNC), this signal is the initial
pulse (IP) with its leading edge coincident with the left-hand vertical axis.
One other feature that should be noted is the presence of a dark bar extending horizontally
along the thickness gate, in this case on the MAIN A-Trace window from the left-hand axis. This
bar represents the IP blocking gate (IP-BLK gate) which serves to "block" out the string of high
amplitude echos typically accompanying the IP. In its FACTORY setup default position, the IP-
BLK gate is at 0.070" to block out the IP echos and thus prevent the T-gate from triggering in or
on the IP. Control of the IP-BLK gate is explained further in this procedure. (If an undamped or
lightly damped transducer other than those recommended for this exercise is being used, it may
be necessary to make an immediate adjustment of the IP-BLK gate in order to proceed. If
necessary, go to Step 15, below, and perform the necessary adjustment.)
9.
For the transducers and stepped wedge recommended, the thickness readout (T-readout)
should be within a few thousandths of an inch from 0.500 inch plus or minus. Position of the
cursor in the Main Menu is at GAIN.
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With the cursor at GAIN, Up & Down arrow keys increase/decrease effective gain accordingly.
11.
To become familiar with the RANGE function, position the cursor at
RNG. The up arrow key will squeeze the echos on the A-trace to the
left, that is, except for the initial pulse. If multiple back echos are
present, they will move toward the left and approach each other
more closely. In our 0.500 inch steel block example, depress the UP
arrow key until the first back echo, originally at the second vertical
division of the electronic graticule moves to approximately fourth full
vertical division. The second back echo will be aligned (or nearly so)
at the 8th full division. The readout value under RNG should be
close to 1.2 inch.
However, note that T-readout did not change unless the gain was
substantially increased. With TG400’s digital electronics, it is not
necessary to use the graticule or other precise settings of range in
order to calibrate for, and to readout precise thicknesses.
13.
The DELAY function also has pronounced effects on the location of signals on the A-trace. Set
the full-scale range using the RNG menu selection at about the 1" inch and position the cursor
at DELAY. Depress the UP arrow key and observe the effect. Signals move toward the left,
including that of the initial pulse, which moves off-screen to the left. Note that the distance
between multiple back echos does not change as it did when range
was increased. Continued depression of the RIGHT arrow key
moves echos toward the left until they are completely delayed off-
screen. If calibration has been performed for the material under test,
the readout under DELAY indicates how far the display has been
delayed.
14.
With the RANGE at 1.00 inch cursor to Select GATES from the
MAIN Menu and select IP-BLK. The Up & Down arrow keys control
the length of the blocking gate, referred to as "IP-BLK gate", and
represented by the Thick dotted bar extending horizontally along the
thickness gate line on the A-trace from the left-hand side of the
