False Signals And Chatter - Fisher Labs F75 User Manual

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 visual ID and the discriminator are independent systems which analyze different sets
of signals. 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 electrical interference
or internal circuit noise. 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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