19
19
screen. Its purpose is to "block" the trailing edge echos associated with the initial pulse or any
spurious signals which might interfere with what is known as the true interface. This group of echos
contains ring-down echos from the transducer and "noise" accompanying stray reflections resulting
from coupling. The "noise" is minimal when both the front surface of the transducer and the surface of
the test object are smooth. This is the usual case with new, unworn transducers and with many finely-
machined test blocks or reference standards. However, actual test objects seldom have clean, smooth
surfaces. They often have rough machined or mill-finished surfaces (as-cast surfaces on castings),
sometimes corroded and sometimes painted surfaces. In these cases, the extent of coupling noise will
be greater than that returned from test blocks.
At default, the length of the blocking gate (IP-BLK gate) is about 0.200 inch equivalent in steel.
With medium to broadband transducers of 2.25 MHZ or greater, the distance is usually sufficient
to block IP echos and coupling noise from standard test blocks. Note, however, that it would not
be possible to material less than the width of the calibrated IP-BLK gate, less than 0.200 inch at
FACTORY defaults. If the total IP signal is greater, and the blocking gate must be increased,
any significant echos within the blocking gate, transducer back echos or otherwise, will be
blocked and will determine the minimum thickness that can be gaged with the correct digital
thickness readout. In such cases, it may be necessary to use a different transducer or a different
technique. With highly damped, broadband transducers of 5 or 10 MHZ, it should be possible to
shorten the IP-BLK gate to permit IP to first back echo digital readout of 0.030 inch steel or
equivalent.
15.
At this point, use the Up/Dn arrow keys to change the length of the
IP-BLK gate in order to observe the effects of the control.
16.
The IF-BLK (interface echo blocking) functions in a similar fashion as
the IP-BLK but when Interface Synchronization (IF) is selected. IF
Sync automatically starts the T-gate at the leading edge of the
interface echo (the echo between the delay line and the test object
surface. The delay line could be a plastic tip or a water path).
Whenever practical, if delay lines are used, interface synchronization
(IF Sync) should be selected. A complete description of IF Sync is
discussed in step 25e.
17.
Before examining the function of THICKNESS THRESHOLD
(THESH),press MENU to display the MAIN Menu. At THRESH, note
that the horizontal T-gate bar is at 40%. Up or Down arrow keys change the threshold level of
the T-gate, and the actual level of the T-gate is readout in % full-scale amplitude, variable from
10% minimum to 90% maximum. The FACTORY default amplitude of generally 40% (probe
selection dependant) is a good compromise and generally effective when the first back echo
amplitude is maintained between approximately 70% full-scale and to somewhat greater than
saturation amplitude (greater than 100% full-scale).
18.
Move the cursor to ALARM Menu item. Using the left or right arrow will move to the item. Press
ENTER to enter ALARM setup
19.
Both LO-THK and HI-THK function in the same manner. These selections represent low
thickness alarm and high thickness alarm, respectively. They can be used to alert the operator
(through the front panel visual LED) when pre-set thickness levels, low, high or both low and
high, have been exceeded. The level sets are scrolled when the Up or Down arrow keys are
depressed. With the cursor at the STATUS position, arrow keys select the alarm system (visual
LED) on and off. In gaining familiarization with these features, note that it is not possible to
"cross" these levels. That is, the high alarm setting cannot be set lower than the low alarm
setting and vice-versa.
