Ridge Tool Company
●
Elyria, Ohio
●
U.S.A
www.seektech.com
13
SeekTech ST-305 Line Transmitter
Note:
The inductor core of the ST-305 emits the
signal when the unit is transmitting in inductive
mode. A hard hit or a drop could possibly damage
the core. To test that the core is intact using a
SeekTech SR-20 receiver:
1.
Power transmitter on.
2.
Set frequency output to 33kHz.
3.
Set power level to high power.
4.
Power receiver on.
5.
Set to receive 33kHz.
6.
Move receiver 12 inches from transmitter.
If the inductor core is undamaged, the receiver
should show a Signal Strength reading larger than
2000.
High Voltage Indicator
Whenever the line transmitter encounters a live
voltage on the line higher than 62 VAC, it will flash a
red LED at the top of the keypad. Should this occur,
carefully disconnect the transmitter using high-
voltage precautions.
Useful Information
Resistance and Impedance
A circuit has a certain amount of resistance to
current; this is measured in ohms (
Ω
). Higher
resistance reduces the amount of current that can
travel along 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 the signal by reducing the total resistance
the transmitted current 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 behavior of different frequencies
under different conditions can be important in doing
effective and accurate locates.
In both direct-connect and Inductive Mode, the ST-
305 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 (Hz), which
means cycles per second, or kilohertz (kHz),
thousands of cycles per second.
Low Frequencies
The ST-305 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
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 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 ST-305 will generate frequencies as high as 262
kilohertz (93 kHz in European version). 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
