RIDGID Metal Detector manual Notes on Accuracy, NaviTrack

NaviTrack® II

formers, buried and overhead power lines are a common source of strong distortion. It may be impossible to get an accurate locate close to a large power transformer.

Notes on Accuracy

Measured Depth, Proximity Signal and Signal Strength measurements rely on a strong signal being received by the NaviTrack II. Remember that the NaviTrack II is used above ground to sense electromagnetic fields emitted from underground lines (electrical conductors like metal wires and pipes) or Sondes (actively transmitting bea- cons). When the fields are simple and undistorted, then the information from sensed fields is representative of the buried object.

If those fields are distorted and there are multiple inter- acting fields, it will cause the NaviTrack II to locate inac- curately. Locating is not an exact science. It does require the operator to use judgment and look for all the infor- mation available beyond what the instrument readings may be. The NaviTrack II will give the user more infor- mation but it is up to the operator to interpret that infor- mation correctly. No locator manufacturer will claim that an operator should follow the information from their instrument exclusively. A wise operator treats the information gained as a partial solution to the problem of locating and com- bines it with knowledge of the environment, utilities prac- tices, visual observation and familiarity with the instrument to arrive at an informed conclusion.

Locating accuracy should not be assumed under certain conditions:

•When other lines or utilities are present. “Bleed over” causes distorted fields and will illuminate lines other than the target line. Use lower frequencies when possible and eliminate all connections between the two lines.

Figure 51 – Bleed-over

•When tees, turns or splits are present in the line. When following a clear signal that suddenly becomes ambiguous, try seeking in a circle of about 20' around the last known point to see whether the signal picks up again. This may reveal a branch, joint or some other change in the line. Be alert to “split opportunities” or sudden changes of direction in the utility being traced.

•When Signal Strength is low. A strong signal is nec- essary for accurate locating. A weak signal can be im- proved by changing the grounding of the circuit, frequency or transmitter connection. Worn or dam- aged insulation, bare-concentric cables and iron pipes exposed to ground will compromise Signal Strength through leakage to ground.

•Far-end grounding will change Signal Strength sig- nificantly. Where far-end grounding cannot be estab- lished, a higher frequency will provide a stronger signal. Improving ground conditions for the locating cir- cuit is a primary remedy to a poor signal.

•When soil conditions vary. Extremes in moisture, ei- ther very dry or water-saturated, may affect measure- ments. For example, ground that is saturated with salty water will shield the signal severely and be very difficult to locate in, especially at high frequencies. In contrast, adding water to very dry soil around a ground stake can make a major improvement in signal.

•In the presence of large metal objects. Simply walk- ing past a parked car during a trace, for example, can cause an unexpected increase or decrease in Signal Strength, which will revert to normal when past the distorting object. This effect is stronger at high frequencies, which “couple” more readily onto other ob- jects. Manhole covers and trench plates can strongly distort signals.

The NaviTrack II cannot change the underlying conditions of a difficult locate, but changing frequency, ground con- ditions, transmitter location or isolating the target line from a common ground can change the results by making a better ground connection, avoiding signal splits or re- ducing distortion. Other locating receivers will indicate that they may be over a line, but they have less ability to determine the quality of the locate.

The NaviTrack II provides more information. If all of the in- dicators are aligned and in agreement, mark-outs can be made with more confidence. If the field is distorted, it shows immediately. This allows the operator to do some- thing to isolate the target line, change the grounding, connection point, move the transmitter or change the frequency to get better reception with less distortion. For extra certainty, take steps to inspect the situation such as by requesting potholing.

28 Ridge Tool Company