Capabilities & Limitations - Fisher Labs F75 User Manual

CAPABILITIES AND LIMITATIONS
DEPTH
The F75 can detect U.S. coins to a depth of up to 15-16 inches (37-40cm) under good
conditions. Large objects (55 gallon drums, manhole covers, etc.) can be detected to a
depth of up to several feet (1-2 meters).
Electrical interference from power lines and from electrical appliances and electronic
equipment can reduce detection depth, or cause audible interference, making it necessary
for the user to reduce the sensitivity setting. Soils with large amounts of iron or salt
minerals may also reduce detection depth or necessitate a reduction in the sensitivity
setting.
TARGET IDENTIFICATION
The F75 identifies the probable type of metal object by measuring its effective electrical
conductivity, which is displayed as a number from 0 to 99 on the LCD screen. The
effective electrical conductivity of an object depends on its metallic composition, size,
shape, and orientation relative to the searchcoil. Since coins are minted to tightly
controlled specifications, they can be accurately identified. Identification of pull-tabs and
foil is less consistent because these kinds of targets come in wide variety. In general,
smaller objects, and objects made from lower conductivity alloys such as iron, bronze,
brass, lead, pewter, and zinc will read lower on the effective conductivity scale. Larger
objects and objects made from higher conductivity alloys such as silver, copper, and
aluminum, will tend to read higher. The notable exceptions are gold, which usually reads
low because it is rarely found in large pieces, and zinc pennies, which read moderately high
because of their size and shape. Although nails and other iron and steel objects will usually
give low readings, ring-shaped pieces of iron (for instance steel washers and harness rings)
will usually produce medium to high readings. Flat pieces of iron or steel, such as can lids,
will occasionally do the same.
Most targets can be identified accurately in air to a distance of about 10 inches. The
minerals in many soils will cause identification to be less accurate. In most soils, effective
target identification can be had to a depth of at least 8 inches.
REQUIREMENT FOR MOTION: PINPOINT FEATURE
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As with other modern metal detectors, the F75 s searchcoil must be kept in motion in order
to both detect and identify targets. The All Metal modes are more forgiving of sweep
speed variation than is the Discrimination mode.
The trigger-activated PinPoint feature continues to detect metal if searchcoil motion stops
over the target. The PinPoint feature is used primarily to pinpoint the exact location of a
target so that it can be retrieved with a minimum of digging, and does not provide target
identification.
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