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F75-MANUAL(122811).qx 12/14/12 9:04 AM Page 14
F75
Comprehensive Operating Manual & Guide to Metal Detecting
Ground Balancing
What is Ground Balancing?
All soils contain minerals. Signals from ground minerals are often tens or hundreds of times as strong as the signal from a buried metal object. The magnetism of iron minerals, found in nearly all soils, causes one type of interfering signal. Dissolved mineral salts, found in some soils, are electrically conductive, causing another type of interfering signal.
Ground Balancing is the process by which the metal detector cancels the unwanted ground signals while leaving signals from buried metal objects intact. This is accomplished by establishing the detector’s internal Ground Balance setting; this setting is calibrated to the soil and eliminates the signal produced by ground minerals.
Calibration to the actual soil condition will result in deeper target detection and quieter operation. This calibration, or Ground Balancing, can be accomplished automatically with the detector’s internal computer, by pushing the
Trigger Switch forward, or manually in the All Metal menu.
The ground balance setting carries through into all operating modes. In
Discrimination mode, the ground signal is generally inaudible unless the discrimination setting is 0.
AUTOMATIC GROUND BALANCING PROCEDURE (FASTGRAB™)
1. Find a spot of ground where there is no metal present.
2. Hold the detector with the searchcoil about one foot above the ground.
3. Push the TRIGGER SWITCH forward with your index finger.
4. Physically pump the searchcoil and detector up and down over the ground.
Lift it about 6 inches above the ground and lower it to within 1 inch of the ground, about once or twice a second.
5. A 2-digit value will appear on the display. This is the Ground Balance setting.
If the detector’s internal computer is unable to ground balance, an error message will appear:
• If the message OVERLOAD RAISE COIL – CAN’T GB appears, you will also hear a siren sound.
You are probably over metal.
• If the message CAN’T GB appears, you may not be pumping the coil, or you may be over a small piece of metal.
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MANUAL GROUND BALANCING
In most situations, it is preferable to
Note: FASTGRAB will not automatically balance over highly conductive soils, such as a wet salt water beach.
F75
Comprehensive Operating Manual & Guide to Metal Detecting
Search Techniques
Sweeping Searchcoil (this does not apply to
Pinpointing with the trigger switch) Keep the searchcoil in motion to detect targets. Sweep the searchcoil parallel to the ground; do not lift the searchcoil at the end of the sweep.
RIGHT
When you have located a target and continue sweeping back and forth for verification, use broad, deliberate sweeps across the target for the most accurate target ID. Do not use short sweeps as you might with other metal detectors.
Shallow Targets
Shallow targets tend to give multiple responses, with the last response being the
WRONG one that remains illuminated on the visual display. This last response is usually sampled at the edge of the searchcoil and will tend to be inaccurate. If you suspect a shallow target
(within 2 to 3 inches of the searchcoil), lift the searchcoil slightly, and slow down your sweep speed until you notice a single response consistently in the same place.
Large shallow targets can cause signal overload, indicated by the siren sound. In these instances, raise the searchcoil until the overload warning disappears, and sweep at this increased height.
Large Targets
If an overload warning is not confined to a small spot, you are probably overloading on a large object, for example, a large iron pipe, reinforcing steel in concrete, or buried sheet metal. It is usually not possible to locate objects, such as coins, in close proximity to large masses of metal.
Pinpointing With the Trigger Switch
When you turn the F75 on, the ground balance setting is preset to give a positive response on nearly all soils. This means that if you are pulling the pinpoint trigger, the audio tone will get louder as you lower the searchcoil to the ground. But you do not want to hear the ground; you just want to hear the target. So always Ground Balance first.
After you have discovered a buried metal target using the ALL METAL or DISCRIMINATION
Modes, use the trigger switch to pinpoint its exact location.
Position the searchcoil an inch or two (2.5-5cm) above the ground, and to the side of the target. Then pull the trigger. Now move the searchcoil slowly across the target, and the sound will indicate the target’s location. As you sweep from side to side, and hear no sound at the ends of the sweep, the target is located in the middle of that zone, where the sound is loudest and the audio pitch is highest. If the sound is loud over a wide area, the buried object is large.
Use the PinPoint feature to trace an outline of such large objects.
Estimating Target SIZE, DEPTH, and SHAPE
When the trigger is pulled to activate Pinpoint, the LCD displays estimated depth. The estimate is based on the presumption that it is a coin-sized target.
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F75-MANUAL(122811).qx 12/14/12 9:04 AM Page 13
F75
Comprehensive Operating Manual & Guide to Metal Detecting
Search Techniques (continued)
But what if it is not a coin-sized target? The most common example is that of an aluminum can.
Flattened aluminum cans are usually identified as quarters. Their large size will produce a strong signal, tricking the microcomputer into thinking that it is a shallow coin.
The following explains techniques for differentiating buried aluminum cans from coins. Sweep back and forth to get a feel for the target, keeping the searchcoil close to the ground. Now, continue to sweep back and forth as you slowly raise the searchcoil higher and higher. If the response diminishes quickly and never gets very broad, the target is probably a coin. If the response diminishes slowly as you lift the searchcoil, and you get a broad response, the target is probably an aluminum can. If you practice this by laying a coin and a flattened aluminum can on the ground, you will quickly understand how to differentiate the two and you will probably never have to dig another aluminum can. And, you will know whether it was deep or shallow.
This technique works well in the All Metal modes, and to a lesser extent in Discrimination mode.
Objects which are ring-shaped, or flat and round like coins, tend to give a narrower, crisper response than objects of similar size with irregular shapes. The easiest way to demonstrate this is with an aluminum screwcap from a soda bottle. In its normal shape, it occupies a volume, and gives a somewhat broader response than that of a coin. But if you flatten it, the response will be crisper and more like that of a coin. Again, these differences are most readily noticed in the All Metal modes.
Long skinny iron or steel objects such as nails usually produce a double response when scanned lengthwise, and a weaker single response when scanned crossways. This is most noticeable in the Motion All Metal mode. However, a coin lying on its edge can produce a similar response, so rely on both the target ID as well as target feel to distinguish between different kinds of objects.
Objects within 2 to 3 inches of the searchcoil will often produce multiple responses as you sweep across them, because the response field close to the searchcoil is irregular.
Estimating Target I.D.
With a single sweep over a target, you will usually see a 2-digit target ID displayed on the LCD.
Repeated sweeps back and forth over the target may cause the 2-digit target ID values to change with each sweep of the coil; this may seem inconsistent with your discrimination setting. These variations and inconsistencies provide important clues regarding the identity of the buried object.
Most metal detectors have difficulty properly identifying steel bottle caps, and the F75 is no exception. Steel bottle caps will often read in the coin range, at the high end of the scale.
The ID number you receive from a buried coin will usually be consistent regardless of sweep speed or angle. The readings from a steel bottle cap will tend to bounce around a lot more, especially with variations in sweep speed or angle. By paying attention and taking this into account, you can minimize the number of bottle caps you dig.
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The information provided by the Visual Target ID may be different than the detector’s Audio
Feedback because the visual and audio systems are independent of one another. The internal signals and algorithms controlling the Visual and Audio output are different. Therefore what you hear, or do not hear, provides additional information regarding target ID. For instance, if discrimination is set at 12, and most sweeps result in no visual target ID, the target is most likely iron even though the majority of the ID numbers will be greater than 12.
The F75 has a tendency to up-average nonferrous targets in the proximity of iron, indicating ID numbers higher than would be obtained in an air test. This tendency is connected with the
F75’s enhanced see-through ability – that is, the ability to find valuable targets in an area where there is a lot of iron trash.
Comprehensive Operating Manual & Guide to Metal Detecting
The Menu System
The entire menu is printed on the LCD display. The display highlights the mode and settings which are in use.
F75
There are three search modes, Static All Metals, Motion All Metals, and Discrimination. To change between an All Metals mode and Discrimination mode, the top line of the menu system must be highlighted. Press the MENU button until the top line of the menu is highlighted.
When either All Metal or Discrimination is highlighted, rotate the SETTINGS knob to move between the two categories.
Each search mode has several adjustable function settings:
ALL METAL: Threshold (not adjustable in stat mode), Audio Pitch, Sensitivity, and
Manual Ground Balance
DISCRIMINATION: Sensitivity, Discrimination Level, Notch, Number of Tones, and
Process Number.
To select a function, push the MENU button and continue pushing in order to move to the function you want. The word SETTING will pop up in the middle of the display, and the present setting of that function will be displayed as a number.
To change a setting, rotate the knob.
To increase a value, rotate to the right (clockwise)
To decrease a value, rotate to the left (counterclockwise)
If you select a function and do not make a change to that function after 7 seconds, the detector will exit the menu system automatically, deactivate the SETTINGS knob, and resume normal operation.
If you press the MENU button while the machine is in normal operation, the user interface will return to the last menu feature setting. This feature allows you to have quick access to a function that you want to adjust frequently.
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