8
GROUND BALANCE
Balancing the device to the ground is the single most important part of the device settings for
accurate search results. Improper ground balancing can result in decreased responses to targets and
may result in instability as well as an increase in false ground signals.
To explain the basic physics behind the ground balance procedure. Every object has its own shape,
conductivity and magnetic component, which together are called the VDI (Visual Digital Index).
Remember that the VDI is the angular response that is numerically equal to the angle of deflection of
the signal vector time curve (hodograph display) on the vertical axis. The soil, or ground, depending
on the degree of mineralization (presence of iron oxides) imposes its signal on the target signal thus
changing the VDI upscale. This is especially true on small or deep targets, as well as large highly
conductive targets (copper, silver, aluminum). This can actually push these high VDI targets up and
over into the high negative VDI range known as “VDI wrap around”. To minimize this effect, it is
crucial to perform a proper and accurate ground balance procedure.
The basic objectives of balancing:
1. Make the soil 'invisible' to the instrument, i.e. exclude false signals from the ground where there is
no target.
2. Install the correct (standardized) reference system number for the VDI. When receiving a signal
from both the ground and a detected object the detector must subtract mathematically the signal
from the ground in order to display a correct VDI indication of the detected target
Keep in mind that when performing “air tests” inside after the detector has been previously balanced
in soil may result in incorrect VDI readings. This is due to the correction factor for the VDI previously
set in the last ground balance. This may result in the loss or “invisibility” of large aluminum, silver and
copper targets. If the ground setting is too positive the VDI will be offset positive and vice versa if the
ground balance is negative.
This may also lead to targets air testing as iron, as well as a false increase in sensitivity when air
testing. Remember that for every 1 degree of ground balance offset from the 0 or neutral point we
get a corresponding increase or decrease in the VDI scale for a given target. IE a 1 degree+ ground
balance will increase the VDI by +1.
For example, if you are air testing a large copper coin that would normally give a VDI of +82 to +84
but is now air testing with a VDI of -86. The result is that the device ground balance is set +10 to +12
degrees too positive for performing an accurate air test.
Balancing with a piece of ferrite is required before performing an "air test" of the device. Put the
device on a non-metal surface, turn on automatic ground balance mode, count to three, then bring
the ferrite to the center surface of the coil from a distance of 25-30 cm (10”to12”) to complete the
balancing procedure. Puzzled? Let's make it easier: Take a CD and pass it flatwise in front of the
search coil.
If the CD gives a vertical hodograph and VDI of 0 (+1 or -1), then the device is ready for “air" tests.
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