Capabilities & Limitations - Fisher Labs F 75 User Manual

F75
Comprehensive Operating Manual & Guide to Metal Detecting
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
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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