Campbell Scientific CS616 and CS625 Water Content Reflectometers - Manual (Page 10)

CS616 and CS625 Water Content Reflectometers
5.4 Operational Details
The accuracy specification for the volumetric water content measurement
using the CS616/CS625 probes is based on laboratory measurements in a
variety of soils and over the water content range air dry to saturated. The soils
were typically sandy loam and coarser. Silt and clay were present in some of
the soils used to characterize accuracy.
Resolution is the minimum change in the dielectric permittivity that can reliably
be detected by the water content reflectometer. The CS616 or CS625 is typically
used to measure soil volumetric water content.
Precision describes the repeatability of a measurement. It is determined for the
CS616 and CS625 by taking repeated measurements in the same material. The
precision of the CS616/CS625 is better than 0.1 % volumetric water content.
Soil Properties
The water content reflectometer operation can be affected when the signal
applied to the probe rods is attenuated. The probe will provide a well-behaved
response to changing water content, even in attenuating soils or other media,
but the response may be different than described by the standard calibration.
Consequently, a unique calibration is required. Change in probe response can
occur when soil bulk electrical conductivity is greater than 0.5 dS m -1 . The
major contributor to soil electrical conductivity is the presence of free ions in
solution from dissolution of soil salts. Soil organic matter and some clays can
also attenuate the signal.
6. Installation
6.1 Orientation
The probe rods can be inserted vertically into the soil surface or buried at any
orientation to the surface. A probe inserted vertically into a soil surface will
give an indication of the water content in the upper 30 cm of soil. The probe
can be installed horizontal to the surface to detect the passing of wetting fronts
or other vertical water fluxes. A probe installed at an angle of 30 degrees with
the surface will give an indication of the water content of the upper 15 cm of
soil.
6.2 Potential Problems with Improper 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 sensitivity of
this measurement is greater in the regions closest to the rod surface than at
distances away from the surface. Probes inserted in a manner which generates
air voids around the rods will reduce the 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.
4