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The wet density of coal often lies below the calibrated range of
1100-2700 kg/m
3
(70-170 pcf) of the gauge. Many users adjust
the calibration using samples prepared to simulate field
conditions.
A minimum of three to four “standards” should be constructed,
each having dimensions of 50 cm x 50 cm (20 in. x 20 in.). To
represent infinite volume to the instrument, each box should be
10 cm or 4 in. deeper than the maximum depth setting to be
used. Each box is then weighed and compacted to a calculated
wet density value and count rates are accumulated for each.
Repeat this procedure and vary the compaction effort for each
“standard” to achieve different wet densities.
A correlation can then be established between wet density of
material at the site and values obtained on the standards.
Backscatter mode is not recommended for use on coal.
Moisture data obtained with the instrument is subject to error
due to the chemical composition of the material, which is
inherently high in hydrogen content. Therefore, most operators
use conventional methods for moisture readings on coal.
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