16. Special Calibration
Special calibration provides the functionality to adjust the
factory calibration in the field. Special calibration constant, B,
can be derived by the gauge, or may be entered by using the
equation given in this section for calculation of B.
The gauge is calibrated at the factory to “average soil”, which
is defined as material that is half way between pure granite
and pure limestone. For most soil and aggregate materials,
average soil approach results in accurate density
measurements. However, there are cases when the material
composition being tested is much different than the range of
materials covered by the factory calibration. Special
calibration provides the user the opportunity to calibrate the
gauge to local materials and materials not covered by the
factory calibration.
In the special calibration mode, a new B constant is calculated
by using an actual field sample and obtaining the “true
density” of the sample. The B value is the only constant in the
gauge that is influenced by the material composition. So
changing this constant is the most effective way to account for
material composition changes in the field.
To perform a special calibration for a local material that may
not be covered by the factory calibration, take a minimum of
four and a maximum of 10 density counts in the field on the
material the gauge is to be used on. For granular materials, use
direct transmission readings and for asphalt, use BS readings.
Average the counts. Obtain samples from the locations where
gauge reading are taken. Analyze the samples in the lab and
determine the density of the material (“True Density”) in kg/m3.
Use the following equation to recalculate the B value that will
be entered in the gauge.
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