28
en
detects the true end of the bolt (C). Note: the gauge will only detect on signals that are located inside the
dimensions of the gate (B). Therefore, the gauge cannot see (A) at all, with respect to the starting point of
(B). Also notice, the THR (threshold) level is the height of the distance (D) from the baseline. Zero threshold
is indicated by the bottom of the range (D), and a THR (threshold) value of 6 is indicated at the top of the
range at (D). Therefore, the vertical height of the gate is the threshold level. The threshold level can be
increased to decrease sensitivity, or decreased to increase sensitivity.
If the threshold level was increased in the no
gate diagram, so that the level was higher
than the amplitude of the noise (A), the
gauge would have detected on the correct
reflection (C). Alternatively, if the gain level
was decreased, the signal amplitude of the
noise (A) would have decreased below the
threshold level, and the gauge would have
also detected the correct reflection (C). This
example brings all the fine adjustments into
consideration, and demonstrates the
versatility of your gauge.
Now, let us look at the diagrams again but in
a slightly different way. Let us look at the
case of a potential mode conversion
problem. In the
No Gate
diagram let us
assume that (A) is the true reflection off the end of the bolt, and the gauge is detecting on the echo at (B).
However, the signal has a horrible first cycle and is relatively weak. But as we look further out in time or
length, the second mode conversion reflection (C) looks like a much cleaner signal to use. Now refer to the
With Gate
diagram. By activating and setting the start time of the GATE (B), we can force the gauge to use
the mode conversion reflection (C), instead of (B). Once again, the same gain and threshold tools can be
use for fine adjustment and signal optimisation.
13.1 ADJUSTING THE GATE
The quickest way to adjust GATE is directly from the hot menu. Alternatively, adjust the value using the
menus:
1.
Select MENU/AUTO/GATE
2.
Use LEFT and RIGHT to scroll the value
3.
Alternatively press ENTER to display the DIGITS EDIT BOX and adjust the value from there
4.
Press OK to accept or ESC to cancel
5.
Press MEAS to return to the measurement screen
14 INTERPRETING THE WAVEFORM
One of the most important things to consider when using your gauge, is how to interpret the waveform in
order to choose the proper polarity (phase) and gain setting. It is also important to understand what is taking
place during a basic measurement.
You have the option of selecting the polarity only once during measurement of the reference length. Once
the polarity is chosen and the reference measurement taken and stored for a particular bolt, all the
elongation and loads taken for that particular bolt will assume the sample polarity.
At first, this might not seem all that critical. In most cases, with well prepared bolts, this might very well be
the case. While both polarities may look fine at first sight, during the reference mode, deformations in the
joint can occur when a load is applied to the bolt.
When a load is applied on a bolt, the velocity of the fastener decreases. Any deformations in the joint
(bending) will disrupt the sound path and potentially cause an overall decrease in signal amplitude. When
this occurs, the decrease in amplitude can cause the gauge to lose the first cycle altogether. If the first cycle
is lost, the gauge will peak jump to another cycle later in time with greater amplitude. Many transducers have
No Gate
With Gate
A typical surface noise problem
