Table of Contents
Advertisement
WARNING
Connect leads before powering the transmitter
on to avoid electrical shock. Ensure the trans-
mitter is well grounded.
Useful Information
Resistance and Impedance
A circuit has a certain resistance that is measured in
Ω
ohms (
). Higher resistance reduces the amount of
current that can be put on an underground line at a
given voltage. (Current is equal to voltage divided by
resistance). Factors that affect resistance are
conductivity of the line itself, insulation material and
condition, breaks or faults in the line, and how well
the transmitter is grounded.
Grounding can be affected by soil conditions
(wetness or dryness, for example), length of
grounding stake, or how the line transmitter is
connected to the grounding rod. Improving the
ground connection is the quickest way to improve a
tracing circuit.
Note: It is difficult to set up a good ground connection
in extremely dry soil. This condition can be remedied
by moistening the soil around the grounding stake.
Impedance is resistance which varies with AC
frequency. The measurement units in both cases are
the same, ohms. Impedance increases with the
frequency transmitted. Total "resistance" can include
impedance, and can be effected by inductance and
capacitance in the circuit and nearby metallic objects.
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,
NaviTrack transmitter 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
www.navitrack.com
14
known as the frequency of the generated current and
of its consequent magnetic field.
Frequency is expressed in terms of hertz (Hz), which
means cycles per second, or kilohertz (kHz), which
means thousands of cycles per second.
Low Frequencies
The NaviTrack transmitter will generate frequencies
as low as 128 hertz. Low frequencies are especially
useful for several reasons.
farther at a detectable level along a continuous pipe
or wire 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, 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 lose signal 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 NaviTrack transmitter will generate frequencies
as high as 262 kilohertz. 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 losing as much signal.
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
the
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 (known as bleed-
over). If you have two wires side by side in a trench,
a higher frequency used to trace one of them may
Ridge Tool Company
First, they will travel
Elyria, Ohio
U.S.A
Advertisement
Table of Contents
