Useful Information; Resistance And Impedance; Using High And Low Frequencies - RIDGID 20168 Operator's Manual

Useful Information

Resistance and Impedance

A circuit has a certain amount of resistance to
current; this is measured in ohms. (
resistance reduces the amount of current that can be
put on an underground line. Factors that affect
resistance in the transmitter circuit are conductivity of
the line itself, breaks or faults in the line, insulation
problems with the line, and how well the transmitter is
grounded. (Poor grounding makes the return path of
the circuit more resistive).
Grounding can be affected by soil conditions, length
of grounding rod, or how the line transmitter is
connected to the grounding rod. Good grounding
improves your signal by reducing the total resistance
the transmitter encounters.
Impedance is a form of resistance which is caused by
a back-force in the electrical field caused by
alternating current (AC). Impedance can be thought
of as "AC resistance", and adds to the resistance in
the circuit in proportion to the frequency being used
(i.e., higher AC frequencies add more impedance
than lower ones).

Using High and Low Frequencies

Understanding the behaviour of different frequencies
under different conditions can be important in doing
effective and accurate locates.
In both direct-connect and Inductive Mode, the Brick
is essentially doing the same thing – imposing a
wave of traceable energy onto the target pipe or line.
This electrical energy rises and falls a certain number
of times per second, which in turn causes a magnetic
field to build and collapse around the conductor at a
regular rate. This rate is known as the frequency of
the generated current and of its consequent magnetic
field.
Frequency is expressed in terms of hertz, which
means cycles per second, or kilohertz, thousands of
cycles per second.
Low Frequencies
The Brick will generate frequencies as low as 1
kilohertz. Low frequencies are especially useful for
several reasons. First, they will travel farther at a
detectable level along a continuous pipe or wire
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NaviTrack Brick Line Transmitter
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conductor than a high frequency will. Secondly, lower
frequency fields lose less energy to the area around
the conductor. If you can get a clear signal on your
receiver using a low frequency it is generally
preferable, because you will be able to trace it further
and it will tend to confine itself to the original
conductor more than a high-frequency signal will. But
a low-frequency signal is more likely to be interrupted
by gaps in the line or poor insulation, or hidden by
other magnetic fields in the area. It is a "weaker"
signal in that respect. While it doesn't jump as readily
onto other lines, it will dissipate if traveling on a line
with poor insulation, bare-concentric cable, or bare
pipe exposed to earth, and will follow the path of least
resistance, which is not always the path intended by
the operator. This can make tracing the original
conductor difficult.
High Frequencies
The Brick will generate frequencies as high as 262
kilohertz. (European versions are limited to 93 kHz.)
There are certain conditions where only higher
frequencies will serve. High-frequency signals are
especially valuable when you are tracing a line that
has some sort of interruption—such as a gasket, or
decayed insulation – in the continuity of the
conductor. The reason is that a high-frequency signal
can "jump" some barriers and continue without
dissipating as much signal as a lower frequency
would.
A high-frequency signal can also be valuable in
getting a signal on a receiver when there is a poorly
grounded circuit, compared to the signal the same
receiver will detect at a lower frequency. While all
currents tend to follow the path of least resistance, a
high-frequency current will "buck" this tendency to
some degree, reaching across incidental barriers.
The disadvantage to higher frequencies is that they
also jump onto other conductors. If you have two
wires side by side in a trench, a higher frequency
used to trace one of them may illuminate both of
them. Additionally, nearby metallic objects, or even
highly metalized soil, may pick up a higher frequency
and distort the picture at the locator. If a gas line is
being "illuminated" with a high frequency current, it
may bleed over onto a water line or a power cable
running nearby, confusing the picture of where the
original line is.
As a general rule, detecting with lower frequencies is
more reliable for the reasons given above, IF you can
get a good signal.
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