False Signals And Chatter - Fisher Labs F 75 User Manual

F75
Comprehensive Operating Manual & Guide to Metal Detecting
Search Techniques (continued)
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.
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.

False Signals and Chatter

At times the detector may beep when there is nothing there, or it may seem like there is
nothing there. There are five major causes for this: electrical interference, nuisance buried
objects, ground minerals, hot rocks, and sensitivity set so high that internal circuit noise is
audible. The problem can usually be corrected by reducing the sensitivity setting, but
sometimes other measures can also be taken.
ELECTRICAL INTERFERENCE
Electrical Interference can be caused by power lines, appliances, computer equipment, cell phones,
fluorescent and vapor type lamps, household light dimmers, other nearby metal detectors, electric
fences, radio transmitters, and electrical storms. If you get abnormal noise while holding the
searchcoil motionless in the air, the cause is either electrical interference or internal circuit noise. If
it is electrical interference, by walking around with the metal detector, you can often follow the
signal and track it back to the offending device; simply turn the device off, or come back at another
time when it may be off. If the interference is from power lines, you might try another time of day.
Interference on power lines is usually caused by something connected to them which may be idle
in the evenings or on weekends. If the interference is from a communications or broadcast
transmitting antenna, reducing the sensitivity is usually your only recourse.
The F75 allows you to shift operating frequencies to avoid electrical interference. See the
Frequency Shifting section for information about this technique.
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