AKA SIGNUM MFD 7272M GT Manual page 21

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When working on the ground with uneven salinity, such as arable land,
it is not always possible to accurately ground balance the unit in
automatic mode. This can be due to the uneven or lumpy nature of the
soil. This can sometimes lead to false responses from the identification
channel. In this situation the phase of the ground balance can vary by
more than 3 degrees. If you encounter this problem then perform the
ground balance procedure as follows.
Find a clean patch of ground with a width of 60 centimeters. Switch
to manual ground balance and making short horizontal search sweeps
with a width of 50cm at a height 3-5cm above the ground, you will see
a straight line on the ground the screen (if the line jumps, then there is
metal present).
In this case, if the balance is wrong, you will hear a buzz from the soil
and the soil line on the screen will not be horizontal.
Press and until the line is horizontal and the
buzz disappears.
Your device is fine-tuned to the ground.
For a small increase in the depth of the identification of large silver and
copper coins (kettles, basins and other large pieces of highly conductive
metals) it is possible, after performing automatic ground balancing, to
enter the manual balance mode and set the GB point from -1 to -2 from
the automatic GB reference point.
Care should be taken when setting the ground balance point either
negative or positive. When set negative it increases the possibility of
receiving false positive responses when bumping the coil on vegetation
or the ground. When set positive you risk the chance of losing sensitivity
to large silver or copper coins that will either become "invisible" or ID as
iron.
In the event that a "clean" patch of ground is not found you can excavate
a small amount of local soil (1-1.5 kg), removing all metal objects, and
place in a bag for convenience. Now lean the detector with the coil facing
up against a bush or tree with the coil at least 40-50cm off the ground.
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