Departement of international trade:
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VDI scale for a given target. IE a 1 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 bal-
ance 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 balanc-
ing procedure. In the absence of ferrite, a piece of red brick or red fired
ceramics can be used. In the absence of these items, you can take a CD,
and in a search mode, hold them flat in front of the coil. If the CD gives
a vertical hodograph and VDI of 0 (+1 or -1), then the device is ready
for “air” tests.
If the VDI of the CD differs from these values, then in auto
GB click
and adjust the phase of the ground balance in the
desired direction. When the VDI of he CD is at 0 (+1 or – 1) your device
is ready for air testing.
BALANCING PROCEDURE
1. Find a metal-free area. You can find it by the threshold tone response
in RT mode. On an unbalanced device there will be a response to the
clean ground, however the sound will be soft and “diffused”, without a
strong localization. While the sound would be sharp and with a strong
increase in sound above a metal object. Select an area with no localized
sharp threshold tone increase while swinging the search coil 5-10 cm to
the left and to the right above the ground. If there is no localized sharp
threshold tone increase then the area is suitable for balancing.
