Ask The dp-PRO

Damage Prevention Professional invites you to send us your questions about excavation safety and damage prevention, and we will have them answered by a professional working in an appropriate area of the industry. Send your questions to: Ask The DP Pro, Damage Prevention Professional, 10740 Lyndale Avenue South, Suite 15W, Bloomington, MN 55420 or email to Michele @Email IR.com.

Why doesn’t the locator provide the depth of a buried pipe? It would sure make it easier when we’re digging!

 

Bob Nighswonger, President, Utility Training Academy (UTA)

 

This is a very common question. My simple answer would be that for several reasons, including
signal strength and shape, the electronic depth estimation provided by locate equipment is not guaranteed. For the locator to provide a depth estimation may give an excavator a false sense of security when crossing over or under a buried line. The electronic depth reading is not a reading that you would want to bet any-one’s life on. The depth readings provided by the handheld line locating equipment is an estimation of distance from the bottom of the signal receiver to the center of a locatable line. The only sure way to know the depth of any buried line is to safely expose the line and see it with your own eyes.

Electronic Depth Estimation and Signal Fields
An electronic depth measurement is a distance calculation from the bottom of the locator’s signal receiver to the center of the signal field being detected. This reading is not a depth of cover over a buried pipe. Since the receiver is estimating distance to the center of a perfectly round signal, if you’re locating a large diameter pipe the depth calculation is to the center of the pipe. It has been claimed in many equipment manuals that in suitable conditions the accuracy of the depth reading provided by the signal receiver should +/- 5% for lines up to 10 to 15 feet in ideal conditions. The likelihood of error increases with the depth of the line because signals created on deeper lines are weaker and less reliable when detected at surface level.

Suitable conditions for depth measurement are when the signal transmitter is directly connected to a facility that is buried in a straight line without any adjacent facilities in the ground. The locatable signal would be a strong signal that is perfectly round in shape as it radiates or broadcast out from the underground line. It would be similar to a water ripple created by a golf ball in the middle of a pond of calm water. In areas of signal wave distortion, the signal is not perfectly round which causes the mathematical calculation of distance to line to be incorrect. In an area of signal distortion caused by target signal coupling with a nearby line, the depth reading can be in error up to 50% off the actual depth. That means a line buried 10 feet deep can produce a depth reading of either 5 feet or 15 feet.

A Few Tips for Electronic Depth Estimating
Choose point along the target line where it runs in a straight line for at least 10 feet in both directions from that point. Avoid taking a depth measurement within 15 feet of the transmitter due to interfering fields being broadcast for the temporary ground stake and wire connection leads.

“The depth readings provided by the handheld line locating equipment is an estimation of distance from the bottom of the signal receiver to the center of a locatable signal broadcasting from an underground line.”

The most accurate depth estimates are normally obtained from a buried line is when measurement is taken from a signal created by a signal transmitter that is directly connected to the targeted line. The depth assumptions are that the receiver is directly over the top of the line. Another assumption is that the receiver’s handle is aligned with the direction of the line or orientation of the signal field.

Use the guidance indicators and signal strength readout to pinpoint the exact location directly over the line. This will be the top dead center of the round signal field. Then establish the exact direction of the line. Some
receivers align the handle of the receiver to the direction of the buried line. Set the bottom of the receiver on the ground while maintaining alignment and obtain an electronic depth reading by either the push of a button or full time display. Note the depth at ground level and then raise the receiver 12-18 inches up from the ground. Check the depth reading again and do the math. The reading should equal the sum of the depth at ground level plus the distance you raised the receiver.

There are many areas along a buried line where depth measurements are not favorable. A few of these areas include any point along the line that is within 8-10 feet of an abrupt turn or change of direction in the target line. This includes the area where the line could be at a downward or upward pitch from the surface of the ground. Other areas to avoid taking a depth reading would be near a tee in a pipe or splice in a cable or tracer wire. In these areas, the signal splits in multiple directions and will collide and distort, or any point along the path of a target line where signal may bleed over to another nearby line or metallic object like a fence or heavy equipment, including joint trench scenarios where your targeted line is buried with several other lines. These areas may cause the target signal to become substantially distorted due to signal coupling.

With countless variables which can cause an electronic depth reading to be in error, it will never be as reliable as safely exposing a buried line prior to digging across, above or beneath a buried line. If the depth of the line is important, the only way to guarantee it… is to see it. Dig Safe!

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