RIDGID SeekTech SR-20 Manuel De L'opérateur page 30

SeekTech SR-20
Figure 52 – Over a Distorted Field
Generally, distortion is likely to be worse at higher fre-
quencies, compared to lower frequencies. This is due
to the tendency of higher frequency signals to "jump" to
adjacent conductors. Large iron and steel objects such as
vault and manhole covers, trench plates, structural sup-
ports, rebar and vehicles can significantly distort even the
lowest frequencies. In general, passive locating is more
subject to distortion than active locating, especially in re-
gards to depth measurements. Power transformers,
buried and overhead power lines are a common source of
strong distortion. It may be impossible to get an accurate
locate close to a large power transformer.
Notes on Accuracy
Measured Depth, Proximity, and Signal Strength mea-
surements rely on a strong signal being received by the
SR-20. The SR-20 is used above ground to sense elec-
tromagnetic fields emitted from underground lines (elec-
trical conductors like metal cables and pipes) or Sondes
(actively transmitting beacons). When the fields are sim-
ple and undistorted, then the information from sensed
fields is representative of the buried object.
If those fields are distorted and there are multiple inter-
acting fields, it will cause the SR-20 to locate inaccu-
rately. Locating is not an exact science. It does require the
operator to use judgment and look for all the information
available beyond what the instrument readings may be.
The SR-20 will give the user more information, but it is up
to the operator to interpret that information correctly. No lo-
cator manufacturer will claim that an operator should fol-
low the information from their instrument exclusively. A
wise operator treats the information gained as a partial so-
lution to the problem of locating and combines it with
knowledge of the environment, utilities practices, visual ob-
servation, and familiarity with the instrument to arrive at an
informed conclusion. Locating accuracy should not be as-
sumed under the following conditions:
• When other cables or utilities are present. "Bleed
over" may produce distorted fields and illuminate ca-
bles or pipes unintentionally. Use lower frequencies
when possible and if possible eliminate connections
between the two cables (such as common bonding to
ground).
28
Tracing Line
Figure 53 – Bleed-over
• When tees, turns, or splits are present in the
• When Signal Strength is low. A strong signal is
• Far-end grounding will change Signal Strength sig-
• When soil conditions vary. Extremes in moisture, ei-
• In the presence of large metal objects. Simply walk-
Ridge Tool Company
line. When following a clear signal that suddenly
becomes ambiguous, try seeking in a circle of about
20' around the last known point to see whether the
signal picks up again. This may reveal a branch,
joint, or some other change in the line. Be alert to
"split opportunities" or sudden changes of direction in
the utility being traced. Turns or tees can cause a
sudden increase in the Distortion Indicator response.
necessary for accurate locating. A weak signal can be
improved by changing the grounding of the circuit, fre-
quency, or transmitter connection. Worn or dam-
aged insulation, bare-concentric cables, and iron
pipes exposed to ground will compromise Signal
Strength through leakage to ground.
nificantly. Where far-end grounding cannot be es-
tablished a higher frequency will provide a stronger
signal. Improving ground conditions for the locating
circuit is a primary remedy to a poor signal.
ther very dry or water-saturated, may affect measure-
ments. For example, ground that is saturated with
salty water will shield the signal severely and be very
difficult to locate in, especially at high frequencies. In
contrast, adding water to very dry soil around a ground
stake can make a major improvement in signal.
ing past a parked car during a trace, for example,
can cause an unexpected increase or decrease in
Signal Strength, which will revert to normal when past
the distorting object. This effect is stronger at high
frequencies, which "couple" more readily onto other ob-
jects.