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For precision thickness gaging, the type of waveform used for setups has a significant effect on
gaging results. Note that the RECT default is +HW, meaning that the current display is of
signals that are positive half-wave rectified.
Appropriate Selection of Echo Half cycle
To better illustrate the effects produced by the various waveform displays, press MENU to return
to the Main Menu. Cursor down to RANGE. Use the left arrow key to change the RANGE
readout to 1.00 inch. Press MENU key to return to
PUL/REC
menu. With the cursor positioned
on RECT, use right / left arrow keys to select RF. For this example, the display will be similar to
that shown below:
Note that the back echos are now displayed on a centered horizontal baseline and have both
positive and negative components. This mode of display, RF , reveals all the details of the
signal. If necessary to more faithfully reproduce the waveform in the figure above, it may be
necessary to key to GAIN and change the gain. Now examine the details of the back echo. The
first half-cycle is negative-going, but has less relative amplitude than the next half-cycle with
respect to each following half cycle (+ or -), which is positive-going. This being the case,
imagine what the back echo would look like if everything below (on the negative side of) the
base line were removed from the display. Vertically enlarged, that is what actually happens
when +HW is selected. Selection of -HW results in the converse -- only the negative-going
components of the back echo signal are displayed. On the A-trace, the negative-going parts of
the signal are "flipped" upright (rectified). FW represents full-wave rectification; that is, both the
positive half-cycles and the flipped over negative half-cycles are displayed simultaneously.
