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8) Press the OK key to set the Threshold and return to the menu screen, or
ESC to cancel entering the Threshold.
9) Finally, press the MEAS key to return to the measurement screen and
begin taking readings.
12.7 Interpreting the Waveform
One of the most important things to consider, when using the MAX II, is how to
interpret the waveform in order to choose the proper phase (polarity) and gain
setting. It's also important to understand what's taking place during a basic
measurement. The user has the option of selecting the phase only once during the
L-REF (reference length measurement). Once the phase is chosen and the L-REF
measurement taken and stored for a particular bolt, all the elongation and loads
taken for that particular bolt will assume the sample phase. At first, this might not
seem all that critical. In most cases, with well prepare bolts, this might very well be
the case. While both phases may look fine at first sight, during the L-REF 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 MAX II to lose the first cycle altogether. If the fist cycle is lost, the
MAX II will "peak Jump" to another cycle later in time with greater amplitude. Often
times the transducers have a very slow ramp up time in their construction. In other
words, the amplitude of the first cycle is lower in amplitude than the second cycle.
For this reason, selecting the proper phase is very important.
The Auto Set feature built into the MAX II will do a lot of this work for you. However,
it's always a good idea to understand how to interpret the waveform. The following
diagrams offer a brief explanation:
Correct Phase (Polarity) – Fig. 1
Refer to the illustrations above. The first thing to note is that the screen shots above
are split into positive and negative regions. The centerline is assumed zero, and the
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Incorrect Phase (Polarity) – Fig. 2
127
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