Adjusting Sensitivity - Fisher Research Labs F75 Comprehensive Operating Manual & Manual

F75
Comprehensive Operating Manual & Guide to Metal Detecting
Search Techniques (continued)
In spots where there has been intense fire, such as a campfire site or where a stump was
burned during land clearing, the soil minerals may be altered by oxidation so that their ground
balance setting is lower than that of the surrounding soil. In such cases, search slowly and
change the G.B. setting as frequently as necessary.
In some areas, electrically conductive industrial minerals such as fuel coke, slag, clinkers (left over
from burning mineral fuels), or charcoal have been dumped or used as landfill. Individual lumps of
these materials can usually be quieted by reducing sensitivity and searching with a discrimination
level of at least 25. However, where the ground consists primarily of such materials, you may not
be able to search quietly. In that case, do not dig unless a signal is crisp and repeatable.
Electrically conductive natural minerals such as graphite, graphitic slate, or sulfide ore minerals
are rarely encountered except when gold prospecting. When gold prospecting, you need to be
able to hear everything, and you can expect to dig conductive minerals that turn out not to be
gold. In a given locality you may learn to recognize what type of rocks these minerals are
found in, and to ignore them if people in the area say that gold is not found in rocks of that type.
HOT ROCKS
A hot rock is a rock which causes the metal detector to sound off because the rock contains
iron minerals. They come in two basic types.
Negative hot rocks (also called cold rocks) are usually magnetite or contain magnetite, and
give a negative response because their ground balance value is a higher number than the soil
they are found in. They tend to be dark in color, usually black, and usually heavy. In some
cases they will have rust stains. They are usually attracted to a magnet, and for this reason
gold prospectors always carry a magnet—the ultimate ferrous/nonferrous discriminator. In
Motion All Metal mode, negative hot rocks produce a boing sound rather than the zip sound of
a metallic target; recognize the difference and you will learn to ignore them.
Positive hot rocks are iron-bearing rocks which have been oxidized by natural weathering
processes so that their Ground Balance number is a number lower than the soil they are
found in. They are often small, right on the surface, sound just like a gold nugget, and are
common in many gold prospecting areas. They are usually, but not always, drawn to a
magnet. They are most often reddish in color but are often black, brown, or yellow. On relic
hunting sites, red clay bricks and rocks which have lined a fireplace or a campfire will often
be hot rocks. The discriminator will usually eliminate them without difficulty if widely
scattered, but if there is a large concentration of them, the discriminator may not quiet them
all. In that case, you can revert to the rule of thumb -- "don't dig non-repeatable signals".
Using the sensitivity control
When the F75 is first turned on, the Sensitivity is at a medium setting appropriate for most
coinshooting. For relic hunting or gold prospecting, higher Sensitivity settings are usually preferred.
In the event of detection of electrical interference from electrical power lines, electrical or
electronic appliances, or another metal detector, it is usually necessary to reduce the
Sensitivity setting to achieve quiet operation. Alternatively, use the Frequency Shift feature
described earlier in this manual.
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