Campbell Scientific DOT600 Roadbed Water Content Meter - Manual (Page 20)

DOT600 Roadbed Water Content Meter
16
A circuit board containing water content measurement electronics is mounted
below the sample chamber and is electrically connected to a waveguide
mounted in the bottom of the chamber. As water content of the sample
increases, the oscillation frequency of the circuit decreases, and this frequency
is related to water content through empirical calibration.
6.3.2 Sample Chamber Volume and Compression Force
The sample volume is calculated from known diameter of the sample chamber
and the distance between the bottom of the chamber and the compression
piston. The distance is measured with a magnetic displacement sensor. The
compression force is calculated by knowing the distance between the
compression piston and the spring compression plate. This distance will
determine how far the springs between these two plates have compressed. This
is determined by comparing two different magnetic displacement sensors.
Knowing the distance the springs have compressed and the spring rate we can
then calculate the compression force.
6.3.3 Water Content Calibration
The water content measurement uses a calibration derived by the DOT600
manufacturer using soils with a range of textures. The calibration coefficients
are selected using material type. The DOT600 response can be affected by soil
salinity in addition to clay content. Soils high in clay and electrical
conductivity (greater than about 1.5 deciSiemens/meter) may require a soil
specific calibration. The water content response is linear so a one-point
calibration will work.
7. Maintenance
Sample Chamber and Compression Cap
The sample chamber and compression cap should be cleaned after use. Failure
to do so could affect future water content measurements.
The lead screw should be lubricated as needed with a liquid graphite lubricant.
Enclosure
Before placing the sample chamber in the location for measurement, the
surface should be clear of any dust or soil.
Calibration
The unit should be recalibrated at least once for each two-year period.