Metrotech Corp. |
17
Metrotech Copyright © 2008
Doc. No.:
R12015
5.1.1.3
Frequency
The active frequency, or the passive band name (power or RF), is always displayed on the top left of the i5000
receiver display. When the frequency soft key is used to change frequency, the display is updated as well.
The active frequencies available are below. Please contact Metrotech for extended or special frequency sets.
Region
Frequency (Hz):
North America
Direct: 491, 982, 8.44k, 9.82k, 35.4k, 82k, 83k
Passive: 50, 60, 100, 120, RF
International
Direct: 491, 512, 577, 640, 982, 8.192k, 8.44k, 9.82k,
35.4k, 65.5k, 82k, 83k
Passive: 50, 60, 100, 120, RF
Electronic Utility Markers
66.35k, 77.0k, 83.0k, 101.4k, 121.6k, 145.7k, 169.8k
5.1.1.4
Positive Line ID
Positive Line ID is an essential task of a modern locate receiver. The i5000 incorporates three elements that aid
the user in positively identifying the target utility from others that might be carrying the signal because of blee
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dover or bleedoff. Bleedover is defined as a loss of signal due to capacitive or inductive coupling between lines,
and increases in significance with higher frequencies. Bleedoff results when two or more lines are galvanically
connected through a common bond point such that the signal in the target line is reduced as it branches out and
finds multiple paths to ground. Bleedoff occurs and has an equal effect at all frequencies.
Both bleedover and bleedoff can cause signal distortion, since the magnetic field induced from an applied AC cur
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rent (the directly coupled signal) exists on multiple lines and can constructively or destructively combine. This, of
course, depends on the relative positions and currents of the utilities with respect to the target line. When distor
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tion is present, the normal assumptions about the field behaving as a cylindrical shell around the conductor can
be (in various degrees) invalid. Therefore, centerline, current, and depth estimates made in such distorted fields
may also be biased.
In the i5000 receiver, Distortion Alert™ and Signal Select™ are used to define the degree of distortion present
and the direction of the signal in the targeted line, respectively. Both of these methods are enabled by Signal
Select modulation. Line Guidance™ composes the third leg of the triad, and facilitates a “look-ahead” of the line
direction. Curvature in the target line, or galvanically connected junctions like “Y” and “T” connections create
anomalies in the shape of the field that are detected by the Line Guidance function.
All of these elements are combined into a single graphical component, called the Guidance Compass, in the
upper right-hand corner of the receiver display. The yellow arrow shows the detected direction of the line at all
times, including the look-ahead guidance feature. The ways that Distortion Alert and Signal Select and Line Guid
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ance combine is discussed in this section.
5.1.1.5
Signal Select
The Signal Select locate method allows for the detection of galvanically connected reverse (bleedoff) currents, as
well as bleedover currents caused by capacitive and inductive coupling.
Positive Line
inverted Line
The direction of the signal in the targeted line is determined by analyzing the sign of the demodulated Signal
Select. When the receiver is positioned over a signal that carries an inverted field (i.e., one in which the phase is
–180° from the expected, the Guidance Compass points downward for a line that is bonded to the target line and
carries return current, as shown in Figure 5-6.
