PCMx Operation Manual
© 2021 Radiodetection Ltd 23
If the locates are different by more than 15cm (6 inches),
assume that the magnetic field is distorted and take
PCMx readings at a different position.
6.3 TruDepth
The PCMx locator features TruDepth™, a feature that
helps to ensure the accuracy of locate or survey
measurements. When the locator is not aligned
correctly with the pipe direction, if the locator is too far
away to the side or the signal conditions are poor for
reliable results, the depth and current measurements
are automatically removed from the display.
As well as non display of depth and current readings, it
will not be possible to take 4Hz current measurements
if the locator has not met TruDepth conditions.
Use the compass to accurately align the locator with
the pipe. The compass should be in the 6 o’clock
position when over and in line with the pipe. If the
compass is not aligned by more than 7.5 degrees
both depth and current readings will be disabled.
6.4 Current Direction (CD)
The PCMx transmitter is capable of outputting a CD
(current direction) signal and this is available in the
ELCD and LFCD modes. This can be used to provide
direction of current flowing on the pipeline. This feature is
particularly useful to identify the target pipeline, which
has had the PCM signal applied to it and inform users
that they are surveying the correct pipeline.
When a 4Hz measurement is taken in either ELCD or
LFCD mode, CD direction arrows are displayed on the
PCMx display. When locating and taking measurements
on the target pipeline, by default, the CD arrow will point
downwards. On some applications the PCMx output
signal can find its way onto an adjacent pipeline and in
this condition the CD arrow will point upwards, indicating
incorrect target line.
6.5 Types of PCMx Surveys
The PCMx magnetometer foot is a high precision, high
performance sensor, which detects and measures very
low frequency magnetic fields. Advanced signal
processing technology provides push button current
measurement and direction of the 4Hz signal.
6.6 ACCA Surveys
An Alternating Current, Current Attenuation, (ACCA)
survey measures the attenuation of the transmitted 4Hz
signal to establish the pattern of current loss. The results
can be used to establish the condition of the pipeline
coating, locate faults, or find shorts caused by contact
with other metal objects.
In all diagrams the arrows indicate the direction of current
flow to the transmitter. The PCMx transmitter applies a
current to the pipeline and this current reduces in strength
as the distance from the transmitter increases. The rate of
reduction depends on the condition of the pipe coating,
ground resistivity and pipe electrical resistance.
The PCMx Locator compensates for depth changes
during current measurements, and current readings
remain constant even when the depth of the pipeline
changes.
When a fault is encountered the current quickly drops. A
fault will result from coating damage, contacts with other
services, etc.
The loss of PCMx current will be virtually proportional to
the amount of CP current being used at the fault.
See section 4.14 on how to take a 4Hz current
measurement using the PCMx.
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