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In the PROBE Menu, select RECT then RF.
Adjust gain to produce and maintain an echo pattern which is less-than-saturated.
In the PROBE Menu, set DAMP at 375 ohms or adjust to produce the sharpest, or "cleanest"
echo pattern.
2.
Study this echo pattern carefully. Notice that there is a low amplitude
negative-going half-cycle, then larger amplitude positive and
negative half-cycles. During the optimization step above, observe
that the amplitude of the first small negative-going half-cycle
changes very little as a consequence of varying pulse width and
amplitude. In the PUL/REC Menu, alternately observe the effects of
+HW and -HW rectification. While -HW (shown on the right, below)
could be used, the echo half-cycle on which the T-Gate terminates
varies considerable with changes in echo amplitude. At +HW, the
first positive-going half-cycle is at full-scale amplitude (shown in the
left-hand example). Since both displays were produced at the same
level of gain, +HW rectification should be chosen.
3.
The next step is to complete the calibration. In the MAIN menu, adjust VEL to 0.2330. Then
move to and adjust ZERO to produce the thickness readout corresponding to a known thickness
(in this case, 0.500 inch). Note, when you check the measurement of the 0.200' inch step note,
it produces ~0.205 inch. This error is due to "V-Path Error", mentioned above. Then check the
thicknesses in between and note the variations.
4.
With the error having been determined to be ~0.005 inch in the range from 0.200 to 0.500 inch,
the "safer" way to calibrate is to establish the known thickness at the lower extreme of the
measurement range. Thus, ZEROing at 0.200 inch produces the "conservative" error of 0.006
inch at the 0.500 inch step. Note the ZERO difference.
AUTO PROBE ZERO
Although the TG400 was designed as a general purpose precision thickness gage, able to use
transducers of any manufacturer, one of the most significant advantages is the Probe Auto Zero
function. When used with the appropriate NDT Systems, Inc. NOVA Series transducers, incorporating
a patented Auto Probe mechanism, setup and calibration are made significantly easier. Also, errors
due to V-Path are corrected and linearized in software. Since each manufacturers V-Path factor can
be different it is not possible to provide a full table of possible variations.
In order to calibrate using an NDT Systems Probe (such as the TG506) follow the following simplified
procedure:
