hydratight
142
If the fist cycle is lost, the
BOLTSCOPE PRO
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
BOLTSCOPE PRO
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
Incorrect Phase (Polarity) – Fig. 2
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 two regions in the upper portion of the display are positive, while
the two lower halves are negative. Refer to
Fig.1
. Notice that the height or
amplitude of
W1+
is greater than the height of
W2+
from the zero crossing
(centerline). These are the positive cycles, as they are in the upper regions.
Now consider that the height of
W1-
is lower in amplitude than
W2-
. These are
the negative cycles as they are in the lower regions. Keeping that in mind, the
BOLTSCOPE PRO
uses a zero crossing (Flank) style of detection. Therefore,
the detection will always occur to the right side of the positive or negative cycle
being detected. With this said, the detection in
Fig.1
(B)
is occurring on the zero
crossing of the positive first cycle
W1+
. In this example, the correct (positive)
polarity has been selected.
Why? I’m getting to that! Refer to
Fig.2
. Notice that when the bolt is tightened,
the signal amplitude has decreased due to the applied stress, and a peak jump
has occurred from
W1-
to
W2-
at
(B)
. If we look at the height of
W1-
it has a
much lower amplitude than
W2-
. Therefore, when stress was applied to the bolt,
the amplitude of
W1-
was decreased beyond the threshold
(A)
and the
BOLTSCOPE PRO
jumped to the next available signal with sufficient amplitude.
However, this could never happen in
Fig.1
. This is because
W1+
has more
amplitude than
W2+
, and a peak jump could never occur without losing the signal
entirely.
