Campbell Scientific DOT600 Roadbed Water Content Meter - Manual (Page 21)

Appendix A. Design and Calibration
A.1 Description of Basic Technology
The DOT600 is based on the design of the DMM600 Duff Moisture Meter also
manufactured by Campbell Scientific. Both devices measure volumetric water
content of a porous-media sample by placing the sample in a cylindrical sample
chamber, compressing the sample to a known compaction and then measuring
the water content using a method that is sensitive to dielectric permittivity.
These devices are portable and are easily transported to the field work-site for
real-time measurements.
The method used to measure soil water content is an indirect measurement that
is sensitive to the dielectric permittivity of the material surrounding the probe
rods. Since water is the only soil constituent that (1) has a high value for
dielectric permittivity and (2) is the only component other than air that changes
in concentration, a device sensitive to dielectric permittivity can be used to
measure volumetric water content.
The waveguide that is the bottom of the 3 inch diameter sample chamber has
interlaced circuit traces that form the ‘plates’ of a capacitor. The amount of
water in the soil being measured determines the dielectric permittivity which is
directly related to capacitance. The oscillation frequency of the circuit is
dependent on the capacitance. The waveguide floats on precision springs.
When a preset travel distance has occurred from sample compression, a limit
switch is made and the measurement completed in about 0.5 milliseconds. A
scaled oscillation frequency is measured by the DMM 600 and used in an
empirically derived calibration equation to provide volumetric water content.
A-1