False Signals & Chatter - Fisher Labs F70 User Manual

&
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.
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 F70 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
F70'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 search coil 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 F70 allows you to shift operating frequencies to avoid electrical interference. See the
Frequency Shifting section for information about this technique.
30