Check a suspect or critical depth reading by lifting the locator 50mm (2”) above the ground and repeating 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:
- Check that the route of the line is straight for at least 2 meters (6 ft) to either side of the measurement point.
- 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.
- Check that there are no adjacent lines carrying a significant signal within 1 to 2 meters (3 to 6 ft) of the target line. This is the most common source of error of depth measurements as a strong signal coupled to an adjacent line can often introduce ± 50% error.
- Make several depth measurements at points slightly displaced from the line’s apparent position. The shallowest indication will be the most accurate and will also indicate the line’s position most accurately.
Checking depth accuracy
This paragraph describes several quick and easy ways of verifying that the depth reading on the locator when used for markers or conductors and sondes is within acceptable limits.
Refer to this if you are getting inaccurate depth readings from a cable or pipe for which you know the approximate depth. 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. Both methods require the use of a transmitter:
Method 1
Place 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 transmitter 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.
- Switch the locator on.
- Select the same frequency as is selected on the transmitter but make sure that this frequency is in the sonde mode.
- Move the locator from left to right and when the bargraph reading is at its Peak note the depth as indicated on the locator. Measure the distance from the base of the locator to the center of the transmitter using a tape measure.
- Compare this reading with the depth reading on the locator.
The locator can be considered accurate if the difference between the depth reading on the locator and the distance measured with the tape is less than 10%.
Method 2
- Apply a signal to a cable or pipe of known depth.
- Locate the cable or pipe; the locator will display depth on the LCD automatically.
- Compare the depth reading on the locator with the actual depth.
NOTE: The accuracy of depth measurement is subject to a number of factors and is meant as a guide only. Exercise caution when performing any excavation.