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The 3400 Series gauges are calibrated at the factory to relate
count ratios to standards of known density and moisture. For
density, materials that have proven to be dependable standards
are: aluminum, magnesium, granite, limestone, and layered
magnesium/polyethylene.
These standards cover the range of densities and moistures for
which the gauges are used. They also have had the stability
needed to calibrate the thousands of gauges built over the years.
To determine the calibration constants at the factory, gauge
readings are taken on standards as described above. The data is
processed to determine the calibration values for the particular
gauge.
The common factors that affect the calibration over the life of
the instrument are aging of electronics, decay of the source, shift
in detector plateau, and field use.
If factory standards are not available, a designated spot on the
concrete slab floor may be used for comparison. A historical
base of readings can be developed by testing all gauges at the
designated spot at the 4-minute count time when they come
from the factory. These readings can be averaged, and then the
serviced gauge can be compared to this average.
Also, another gauge can be used as a standard. Readings can be
taken with a gauge that has a known good calibration. Then
readings on the identical spot can be taken with the gauge in
question. Be sure there are no gauges within 10 meters (30 ft.)
of the gauge taking the reading. The 4-minute count is
recommended for this procedure.
Some users construct standards of their own by packing soil in a
box. In our experience, this method is very difficult and does not
provide a dependable standard. Also, such standards often do
not maintain a constant density or moisture over time.
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