Campbell Scientific CS650 and CS655 Water Content Reflectometers - Manual (Page 15)

CS650 and CS655 Water Content Reflectometers
6. Installation
6.1 Orientation and Placement
The CS650 measures the bulk dielectric permittivity, average volumetric water
content, and bulk EC along the length of the rods, which is 30 cm for the
CS650 and 12 cm for the CS655. The probe rods may be inserted vertically
into the soil surface or buried at any orientation to the surface. The probe may
be installed horizontal to the surface to detect the passing of wetting fronts or
other vertical water fluxes.
The sensitive volume depends on the surrounding media. In soil, the sensitive
volume extends approximately 7.5 cm (3 in) from the rods along their length
and 4.5 cm (1.8 in) beyond the end of the rods. Consequently, if the probe is
buried horizontally closer than 7.5 cm from the soil surface, it will include air
above the surface in its measurements and underestimate soil water content.
The thermistor used to measure temperature is in contact with one of the
stainless steel rods at the base of the epoxy probe body. Because of the low
thermal conductivity of stainless steel, the thermistor does not measure the
average temperature along the rod, but instead provides a point measurement of
the temperature within the epoxy. For a valid soil temperature reading, the
probe body must be in thermal equilibrium with the soil. If the probe is
installed vertically with the epoxy probe body above the surface, then the probe
body must be shielded from solar radiation and in direct contact with the soil or
media of interest.
6.2 Proper Insertion
The method used for probe installation can affect the accuracy of the
measurement. The probe rods should be kept as close to parallel as possible
when installed to maintain the design wave guide geometry. The probe is more
sensitive to permittivity close to the rods so probes inserted in a manner which
generates air voids around the rods will have reduced measurement accuracy.
In most soils, the soil structure will recover from the disturbance during probe
insertion.
In some applications, installation can be improved by using the CS650G
insertion guide tool. The CS650G is inserted into the soil and then removed.
This makes proper installation of the water content reflectometer easier in
dense or rocky soils.
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