Introduction

The state-of-the-art, DML2000 Magnetic Locator packs all of its sophisticated electronics in an incredibly light 1.8-lb package. Shock-mounted sensors maintain proper alignment-even when exposed to harsh physical abuse. A waterproof housing stands up to the harshest weather conditions. The unit has six sensitivity settings for better control. Normal locates are performed on setting three, while lower settings can be used for large, relatively shallow targets, higher settings work best for weak or deep targets. The instrument audio output idles at a nominal 20 Hz when no magnetic objects are present and increases whenever the DML2000 approaches a ferromagnetic object.

The DML2000 Magnetic Locator is the ideal instrument for pinpointing buried iron or steel pipe, survey markers, water valves, water meters and any other ferrous object covered by dirt, pavement, water, snow or ice. If you hear a warble output from the instrument: BEWARE! A strong warble output from the instrument means you are probably over an energized power line, whereas a weak warble output often indicates the presence of a telephone or communications cable. The capabilities of this instrument are limitless and an experienced operator will find many uses for the DML2000.

The separate Power/Volume and Sensitivity controls allow the operator to leave the Sensitivity control in a favorite or preset position, to deactivate the instrument and later resume operation with this setting unchanged.

There is no need to worry about the instrument battery level since a built-in Low Battery indicator begins flashing whenever the operator has 2 to 3 hours of normal operation remaining.  Typically the DML2000 will provide in excess of 40 hours of normal operation from standard alkaline batteries.

Theory of Operation

The primary sensing elements used by the DML2000 Magnetic Locator are fluxgate magnetometers. Fluxgate magnetometers are vector magnetic field sensors that measure the average magnetic field component along their sensitive axis, i.e. the magnetic field component along the longitudinal axis of the sensor tube.

 

 Figure 1, Sensor Location

For the magnetic locator to work properly, the magnetometer sensors are aligned opposing so that the magnetic field measured by one sensor is the negative of the magnetic field measured by the other. The instrument then electrically sums the output of the two sensors. By summing the two output signals, you cancel any field common to both sensors, such as the Earth’s Magnetic Field, and leave only the differential magnetic field. The differential magnetic field, the magnetic field detected by one sensor and not the other, is the magnetic field of interest and hopefully represents the magnetic field of your target and not the field of your pocketknife, watch or the magnetic field of the steel arch support in your shoe. The instrument’s audio output idles at approximately 20 Hertz when no magnetic objects are present, and increases in frequency as you approach a ferromagnetic target.

Magnetic Cleanliness

The importance of the operator’s magnetic cleanliness prior to beginning a search cannot be over-emphasized.  Some of the more common sources of local magnetic interference are watches, steel arch supports or toe protectors in shoes, key chains, and pocketknives.

Operation

To begin operation, you must first apply Power; then adjust the Volume Control to a comfortable setting; and set the sensitivity Control to position 3.

Figure 2, Instrument Controls

Sensitivity Setting

Most search operations work well with the sensitivity control set to level 3.  For small and relatively weak magnetic targets a higher sensitivity setting may be desirable, and conversely when the operator is searching for large, relatively strong magnetic targets, a lower sensitivity setting may be desirable.

Frequency Output

The instrument’s audio output idles at approximately 20 Hertz when no magnetic objects are present. The speaker output tone then increases in frequency whenever the DML2000 approaches a magnetic object as the magnetic locator is swept back and forth around the desired search area.

Pinpointing your Target

You may hold your locator at an angle to maximize your search area, however when you want to pinpoint your targets location, it is advisable to hold the locator vertically and use an “X” or crossing pattern.

Panel Meter   

The DML2000-M Magnetic Locator has all of the standard features of the DML2000, plus an easily readable Liquid Crystal Display (LCD) panel meter. The DML2000-M with its 3½ digit (0 to ± 1999) Digital Panel Meter provides the highest resolution display of any Magnetic Locator. The digital display provides an exact numeric readout with a separate polarity indicator that can be even more useful to the experienced professional.

 

Figure 3, Meter Option

The highly visible panel meter is also helpful if high background noise begins to overwhelm the speaker.  At that time the operator can frequently continue his search operation simply by referring to the LCD display.  The digital display on the LCD panel meter tracks the instrument output frequency changes.

Magnetic Signatures of common buried objects

Most common underground targets have a predictable magnetic pattern and consequently produce a predictable output frequency change in the magnetic locator.  In the figures that follow, the bold line above ground indicates the relative output frequency level of the DML2000 as it moves across the indicated target: the higher the line, the higher the instrument output frequency.

Figure 4, Survey Marker or Well Casing

The peaking of the DML2000 output frequency normally indicates that you are over the top center of a vertical dipole (survey marker or well casing).

Figure 5, Chain-Link or “Cyclone” Fences

When working around chain-link fences, set the sensitivity control to 3 or 4, hold the instrument vertically and walk along parallel to the fence approximately 8” to 1’ away. You will hear the magnetic field of the fence as you walk along including the field from the posts. However, if your target is near to or under the fence, there will be a dramatic increase in the instrument frequency as you approach the target and you will have no difficulty distinguishing your target from the fence.

Figure 5, Horizontal Pipe

The peaking of the DML2000 output frequency normally indicates that you are over the end of that pipe section, which can be either a weld or a “Bell” joint as shown above. When searching for horizontal gas and water lines, look for a polarity change on the digital panel meter [*].  A polarity change that occurs when the output frequency is low means you are nominally over the midpoint of the pipe section.  A polarity change when the output frequency is high typically indicates a pipe joint or weld. 

Figure 6, Service Connections and Valve boxes

Frequency peaking occurs over service connections and valve boxes, any place the pipe has been cut and a service connection, Valve box or other magnetic anomaly inserted.

The septic tank illustration needs to be derived from the manhole cover picture.

 

My Thoughts are this; Start with the survey pin signature followed by the survey pin near a chain ling fence. Then go to the horizontal pipe, followed by the service connections and Valve boxes. The last two illustrations would be the Septic tank followed by the Manhole cover. 

 

 

Figure 8, Septic Tank

Most concrete septic tanks have a cover with two handles. The handles are inverted, U-shaped pieces of rebar which are highly magnetic. In most cases the audio output of the DML2000 will reach its peak directly over the handles, which makes it easy for the operator to identify the correct place to dig. In other cases the DML2000 will detect not only the handles on the cover, but the magnetic field of the wire mesh or rebar in the concrete. This allows you to not only pinpoint the location of the cover but to also outline the tank and determine precisely its orientation.  

Figure 9, Manhole Cover

A typical manhole cover is highly magnetized and easily detected by the DML2000 magnetic locator. However in some cases, where the manhole cover has recently been removed and reinstalled but not it its original orientation, then another situation can exist. The magnetic field of a manhole is a combination of two magnetic fields, the magnetic field of the cover itself and the magnetic field of the steel support ring. When both fields are aligned they add and are easy to detect, this is the most common situation. However, when a manhole cover has been recently removed and then the cover reinstalled but rotated 180^o then the two magnetic fields tend to cancel each other and detection becomes more difficult.      

 

Historical Notes:

Some of the earliest work on Fluxgate magnetometer design took place at Bell Labs prior to and during World War II. Much of that work was later transferred to the Naval Ordinance Laboratory where they perfected the magnetic torpedo. The magnetometer sensors detected and measured the magnetic field of a target and detonate the torpedo when the signal polarity changed, just as it passed under the keel of the boat. The first practical use of two coaxial, fluxgate magnetometers arranged in the typical magnetic locator design, occurred during the Vietnam War where they were used to detect Vietcong Tunnels. Today, fluxgate magnetometers are used to monitor solar flare activity, control the attitude of satellites, and guide cruise missiles to their target. They are also used in earthquake prediction instruments, in underwater Search & Salvage operations and in solid state heading sensors on boats, cars, and airplanes.

General Specifications

 

Dimensions:                     42½” L x 3¾” W x 1¾” D

                                                            (108cm L x 9.4cm W x 4.4cm D)

 

Waterproof Length:                   36” (91.4cm)

 

Weight:                                      1.8 lb. (0.8 kg)

 

Operating Time:                         up to 40 hours

 

Power:                                       4 – “AA” Batteries.

 

Low Battery Indicator:               RED, Flashing LED.

 

LCD Panel Meter                        [0 to ± 1999] Digital Display:

(Applies to Model DML2000-M only)

 

Temperature Range:                  -20° to 120° F,

                                                            (-29° to 50°C)

 

Audio Output:                            Variable Frequency,

                                                                               (20 – 3,200) Hertz