Manual No. OM5000
Jan X/04 Ver 1.40
48
into selecting the half-cycle polarity of the thickness period start echo. Instead, the T-GATE
STOP switch selects the same polarity for both the thickness period start and stop echos
(multiples). As shown in Figure 25, the first back surface echo and the multiples typically have
the same polarity (which are inverted when compared to the associated IF echo). Moreover,
this typically leads to measuring across a "true" echo period (no extra half-cycle involved).
6.4.3.4 Gaging IF Echo - First Back Echo
- Temporarily set the VIDEO switch in the RF
position and use SYNC SWEEP DELAY to position the IF echo near the middle of the A-Scan
display, as shown in Figure 26. Also, use the GAIN control in the manual position to establish
unsaturated echos. Then, set the SYNC T-GATE switch in the "1st" position.
Now, set the T-GATE START switch to the polarity of the higher amplitude of the two leading
half-cycles of the IF echo. Then, do the same for the T-GATE STOP switch with the first back
surface echo. See Figure 26. When this small-amplitude leading half-cycle is present, select
the next half-cycle (opposite in polarity) for starting the T-Gate. This usually leads to
incorporating an extra half-cycle into the thickness period; however, the extra time involved is
compensated for during calibration with the ZERO control.
If the third half-cycle is improperly selected for T-Gate start, then the thickness readout will
probably not be as stable because the T-Gate may sometimes false-trigger on the small-
amplitude leading half-cycle.
If the small leading half-cycle of the IF echo can be successfully suppressed at all times with
the RECEIVER REJECT or GAIN controls, the largest "third" half-cycle can be considered a
candidate for the T-Gate Stop. Typically, this small first half-cycle doesn't enter into the
situation for the first back surface echo or for multiples because it's damped to an
inconsequential level by then.
6.4.3.5 Gaging On Multiples
- Temporarily set the VIDEO switch in the RF position and use
SYNC SWEEP DELAY to position the IF echo near the middle of the A-Scan display. Then,
set the SYNC T-GATE switch in the "2nd" or "3rd" position.
Now, set the T-GATE START switch to the polarity of the highest amplitude of the two leading
half-cycles of the IF echo. (Refer to Section 6.4.3.4 regarding consideration of the possible
occurrence of a small amplitude leading half-cycle in the IF echo.) Then, do the same for the
T-GATE STOP switch and the pair of selected multiple echos representing the thickness
period. See Figure 27. The polarity setup on multiples typically leads to the T-Gate
synchronizing over a true echo period because the pair of echos have the same polarity.
As previously mentioned, thickness echo periods beyond the "3rd" can be selected by rotating
the IF-Gate control CW until the T-Gate "jumps" to the desired multiple period.
