Capabilities & Limitations - Fisher Research Labs F70 User Manual

Capabilities and Limitations
DEPTH
The F70 can detect U.S. coins to a depth of up to 13-14 inches (35cm) 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 F70 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 search coil. 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, and steel bottle caps will occasionally do the same.
Most targets can be identified accurately in air to a distance of about 10 inches (25cm) . The
minerals in many soils will cause identification to be less accurate. In most soils, effective
target identification can be achieved to a depth of at least 6 inches (15cm).
REQUIREMENT FOR MOTION: PINPOINT
FEATURE
As with other modern metal detectors, the
RIGHT
F70's search coil must be kept in motion in
order to both detect and identify targets.
The Autotune mode is more forgiving of
sweep speed variation than is the
Discrimination mode.
The Pinpoint feature continues to detect
metal if search coil motion stops over the
target. The Pinpoint feature is used primarily
WRONG
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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