PCM
+
Operation Manual 23
Section 8 – Interpreting Results
This section provides an overview of interpreting
results generated by the PCM
+
Receiver.
8.1 Introduction
The principle of the PCM
+
is that the frequency used is
so low (4Hz) that the effects of induced and capacitance
coupling to other lines are reduced to almost zero. Also
natural decay of the signal due to these effects is also
reduced to almost zero.
The losses are due completely to resistive losses i.e.
coating defects or contact to other structures.
8.2 Avoiding errors
Always use the standard techniques to ensure
measurements are valid. Confirm that peak and null
signals coincide. If in doubt check for field distortion
by taking a depth reading and then raise the receiver a
known height, for example 0.5M, and confirm the depth
measurement increases by the corresponding amount.
As the PCM
+
uses such a low frequency the sources of
error are much reduced. However there are still certain
circumstances that will create errors.
8.3 Identifying Interference
Errors due to locate (ELF/LF) signal distortion can result
in errors in 4Hz current measurement. This is because
the 4Hz current measurement process relies on depth as
measured by the ELF or LF signal.
Poor Peak and Null locate, outside 15cm (6 inch)
maximum.
Unreasonable depth measurement.
Reading on the LCD bargraph not stable.
Possible causes of Interference
Parallel pipes.
Too close to transmitter. Tie-ins, T-junctions and
L bends.
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Measurements at Ts, bends, abrupt changes in depth etc.
should be avoided, as there will always be a degree of
field distortion at these points.
Taking readings too close to large parked vehicles,
moving vehicles, boots or shoes with steel toe-caps
or large metal structures.
Taking readings near to PCM
+
Transmitter and
anode cables or anode ground bed.
Measurements taken near the transmitter anode bed or
earth stake can be misleading. This is because all the
signal current passes through the earth stake or anode
bed. The signal ground currents close to the transmitter
are therefore significant and opposite to the pipe current.
The effect is that for the first 30 to 50 meters – and
depending on ground conditions – the measured current
may increase. In fact, the current on the pipe is probably
at a constant level.
If it is necessary to survey this section of pipe it will be
necessary to apply the transmitter at a different location
and work back to this point.
8.4 Locate current
In ELF – and with a locate reading current below
15mA – the PCM
+
readings will not be accurate.
In LF – and with a locate reading current below 2mA
– the PCM
+
reading will not be accurate.
8.5 PCM
+
Current.
PCM
+
current results depend on locate current. If the
PCM
+
current has fallen below 100mA on a long survey,
it is likely that the locate current is also low. It is possible
to obtain results by using a portable transmitter nearby to
provide a locate signal. Use a different frequency from the
PCM
+
Transmitter locate frequency.
8.6 Direction Arrows
The direction arrow indicates current flow to the
transmitter.
A direction arrow will not be displayed if the transmitter
is set to the ELF (4Hz and 128Hz) mode.
A direction arrow will not be displayed if the PCM
+
Receiver is unsure of the current direction. However
the current reading will still be accurate.
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