Cable locating Depth and current readings
The RD8200 locator is a tool that provides automatic depth readings for buried cables, pipes, and sondes when the locator is correctly oriented above the target line or sonde. Current readings are displayed simultaneously if the locator is orientated correctly, but depth range and accuracy vary depending on the target utility, its depth, and external environmental factors like electromagnetic noise, ground conditions, and interference.
Depth measurement is to the center of the pipe, cable, or sonde, and the best readings are typically detected from active signals output by a transmitter rather than passive sources. The RD8200 locator is capable of determining cable depth when locating some passive power signals, but passive signals on lines are less suited for measuring depth due to interference.
To take a depth reading, do not apply the signal by induction and place the transmitter in induction at least 15m (50ft) from the point of any depth measurements. Depth measurements may not be accurate if there is audible interference or if part of the transmitter signal has coupled to a nearby line.
To accurately pinpoint the target line with the locator, check the locator is directly over the line, the antennas are at right angles to it, and the locator is vertical. Adjust the sensitivity level to bring the bar graph indication to approximately 50%. If the ground appears to radiate a strong field, check depth by holding the bottom of the antenna 50mm (2”) above the ground and subtract this reading from the indicated depth.
To verify depth measurements, lift the locator 50mm above the ground and repeat the measurement. If the measured depth increases by the same amount, it is a good indication that the depth reading is correct. When locating for a conductor or sonde, depth measurements should be precise to ±3% if conditions are suitable. However, you may not always know if the conditions are suitable, so the following techniques should be used to check critical readings:
1. Check that the route of the line is straight for at least 2 meters (6 ft) to either side of the measurement point.
2. Check that the signal response is reasonably constant over the 15 meter distance to the transmitter and make depth measurements to either side of the original point.
3. Check that there are no adjacent lines carrying a significant signal within 1 to 2 meters (3 to 6 ft) of the target line.
4. Make several depth measurements at points slightly displaced from the line’s apparent position.
To check depth accuracy, verify that the depth reading on the locator when used for markers or conductors and sondes is within acceptable limits. Inaccurate depth readings could be the result of the locator picking up a stronger signal such as another cable or pipe running in close parallel to the target pipe or cable.
There are two ways of checking the calibration of the locator in the field. Method 1 involves placing the transmitter on top of a non-metallic object, 18” / 500mm in height or more, on the ground and away from any buried lines. Switch the locator on in induction mode, hold the locator with the blade horizontal and pointing towards the front of the transmitter and approximately five meters from the front of the transmitter.
Method 2 involves applying a signal to a cable or pipe of known depth. The locator will display depth on the LCD automatically, and the depth reading on the locator will be the most accurate.
Current readings help confirm the identity of a line and may provide information about the condition of cable insulation or pipe coating. The rate of decrease in strength depends on the type of line and soil conditions.