Dakota Ultrasonics MINI--MAX - Manual (Page 143)

MINI-MAX Ultrasonic Bolt Tension Monitor
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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.
12.6 Gate
The MINI-MAX is equipped with a single gate typically used to eliminate noise
that is present prior to the reflection off the end of the bolt, and also to force a
detection at or on a specific location or waveform. Often times the first reflection
off the end of the bolt will produce a poor signal. However, if the user looked
further out in time (length), a reflection from a mode conversion very well may be
a better alternative signal to detect on. A mode conversion is simply a reflection
of energy that took a longer path, reflecting off the sidewalls of the bolt, resulting
in a return echo slightly longer than the overall length of the bolt. Use of a gate
allows the user to force the MINI-MAX to use this signal for measurements.
Another potential issue is general transducer or low frequency noise. In this type
of situation, the noise interferes with the MINI-MAX, and is high enough in
amplitude to cause the MINI-MAX to detect on the noise rather than the true
return reflection. The diagrams below illustrate both cases:
Note: In order to adjust the gates they must be activated. The gates will only be
active if the measurement mode is set to P-E GT (pulse – echo w/gate). In
standard P-E (pulse echo) mode, the gates are not active.