Campbell Scientific DOT600 Roadbed Water Content Meter - Manual (Page 24)

Appendix A. Design and Calibration
Typical displacement sensor calibration
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-40.000 -20.000 0.000 20.000 40.000 60.000 80.000
Displacement sensor output (millivolts)
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isp
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FIGURE A-2. Typical calibration data for the displacement sensors.
A.5 Sensitivity of Weight Position on Scale Surface
Small weights (diameter = 3 cm, weight = 200 g) were placed at various
locations of a symmetrically defined grid superimposed on the scale surface.
There were no significant differences among any of the positions. It is
concluded that placing the sample chamber anywhere on the scale surface such
that it is not extending over any edge of the scale will result in no error
contribution to the measurement.
A.6 Soil Water Content Calibration
Multiple soil water content calibrations are derived to cover a range of soil
textures. Water content measurement methods that are sensitive to dielectric
permittivity are inherently sensitive to soil texture and specifically clay
content.
The calibrations were obtained by independently measuring sample volumetric
water content. Soils mixed to a range of water contents were measured in the
DOT600 in normal fashion. The cylindrically-shaped sample was removed
from the sample chamber intact. This allows accurate measurement of the
sample volume. Oven drying at 105C for 24 hours was used to determine
sample gravimetric water content which is converted to volumetric water
content by multiplying by the sample dry bulk density. Three repetitions were
used for each soil at each water content.
A-4