Locating cables and pipes: Part 3Follow
The LCD compass provides a visual indication of the direction of the target cable, pipe or sonde. The compass is available for all frequencies apart from Power, Radio and Passive.
Line tracing can be accelerated by switching the locator to Null mode (PLM and TLM models only).
Move the locator left and right while walking along the path of the line to observe the null directly over the line and a peak response to each side of the line. As you move the locator over the line, the left and right arrows (and an accompanying tone) will indicate if the target line is to the left or right of the locator.
Periodically switch to peak mode, locate the target line, and verify its exact position.
Locating a target line in Peak mode accurately defines the position of a target line after it has been traced and its position is approximately known. Start with medium output power from the transmitter, medium frequency on the transmitter and locator, and peak mode on the locator.
Set the locator sensitivity to approximately 50% by pressing the orkey.
NOTE: it may be necessary to adjust the sensitivity level throughout the pinpointing to keep the bar graph on scale.
Pinpointing a target line
|Pinpointing with Peak and Null indicators|
- With the antenna perpendicular to the line, make traverses across the line. Define the point of maximum response.
- Without moving the locator, turn it round as if it is on a pivot. Stop at the point of maximum response.
- Hold the locator vertical with the antenna just above the ground and move the locator from side to side across the line. Stop at the point of maximum response.
- With the end of the antenna close to the ground, repeat steps 2 and 3.
- Mark the position and direction of the line.
Repeat the steps of the procedure to increase pinpoint accuracy.
Switch to null response mode and move the locator to find the null position. If the position of the peak and the null pinpoints correspond, it can be assumed that the pinpoint is accurate. The pinpoint is not precise if the marks do not correspond, but both marks will show an error to the same side. True line position will be close to peak position.
The line lies half the distance to the other side of the Peak position as the distance between the Peak and the Null positions.
Sweep and Search
There are a number of techniques available for locating unknown lines in an area. Using these techniques is particularly important before conducting any excavation work to help ensure that buried lines are not damaged.
A passive sweep is used to locate Power Radio or CPS signals that may radiate from buried conductors.
To perform a passive sweep:
- Press the key to select the passive frequency you wish to locate. You can select from the following passive frequencies.
- CPS (Cathodic Protection System)
2. Adjust the sensitivity to maximum; reduce the sensitivity to keep the bar graph on scale when there is a response.
3. Traverse the area in a grid search, at a steady walk, and hold the locator comfortably with the antenna in line with the direction of movement and at right angles to any lines that may be crossed.
Stop when the locator response rises to indicate the presence of a line. Pinpoint the line and mark its position. Trace the line out of the area being searched. Resume grid search in the area.
In some areas there may be a confusing amount of 50 / 60Hz power signals. Lift the locator 50mm / 2” from the ground and continue the sweep.
Switch the locator to Radio Mode if the locator supports this feature. Increase sensitivity to maximum and repeat the above grid search procedure over the area. Pinpoint, mark, and trace out any lines that are located.
In most, but not all areas, radio mode will locate lines that do not radiate power signals and a grid search should be made in both power and radio modes
An inductive search procedure is a more certain technique for locating unknown lines. This type of search requires a transmitter and locator and two people. This type of search is referred to as a ‘two person sweep’. Before starting the sweep, define the area of search and the probable direction of lines crossing the area. Ensure the transmitter is switched on in induction mode.
The first person operates the transmitter and the second person operates the locator. The transmitter induces a signal onto lines as it passes over them and the lines are then detected with the locator at a suitable distance away from the transmitter (around 15 meters / 50 feet – although this will depend upon the level of induction power used.)
Hold the transmitter with its length aligned with the assumed direction of any lines.
The second person holds the locator at the start of the area to be searched and with the locator antennas at right angles to the probable direction of the buried lines. Set the locator sensitivity level as high as possible without the locator picking up any airborne signals directly from the transmitter.
When the transmitter and locator are in line both operators start to move forward in parallel. The operator with the locator sweeps it backwards and forwards, keeping the locator vertical, as they proceed in parallel with the transmitter. This method allows for misalignment of the transmitter, locator and buried line.
The transmitter applies the strongest signal to the lines directly below it, which are then located with the locator. Move the transmitter from side to side to establish the highest signal which indicates that the transmitter is also directly above the line(s).
Mark the ground at the point of each Peak signal detected with the locator. Repeat the search along any other possible paths of lines. Once the positions of any lines have been marked, reverse positions, place the transmitter over and along each line in turn, and trace the line out of the search area.