Campbell CS616 User Manual - page 23
User Manual
15
constant for a soil. Bulk electrical conductivity increases with water
content so comparison of the electrical conductivity of different soils must
be at same water content. Discussion of the effects of soil electrical
conductivity on CS616/CS625 performance will be on a soil solution or
extract basis unless stated otherwise.
When soil solution electrical conductivity values exceed 2 dS m
–1
, the
response of the CS616/CS625 output begins to change. The slope decreases
with increasing electrical conductivity. The probe will still respond to water
content changes with good stability, but the calibration will have to be
modified; see Section
8.2
, Water Content Reflectometer User-Calibration
(p. 17)
. At electrical conductivity values greater than 5 dS m
–1
, the probe
output can become unstable.
8.1.3.2 Soil Organic Matter, Clay Content, and Soil Bulk Density
The amount of organic matter and clay in a soil can alter the response of
dielectric-dependent methods to changes in water content. This is apparent
when mechanistic models are used to describe this measurement
methodology.
The electromagnetic energy introduced by the probe acts to re-orientate or
polarize the water molecules. If other forces are acting on the polar water
molecules, the force exerted by the applied signal will be less likely to
polarize the molecules. This has the net effect of ‘hiding’ some of the
water from the probe. Additionally, some clays absorb water interstitially
and thus inhibit polarization by the applied field.
Organic matter and some clays are highly polar. These solid constituents
can affect CS616/CS625 response to water content change and require
specific calibration. This affect is opposite to that of the ‘hiding’ effect. It
would be convenient if the calibration of water content to CS616/CS625
output period could be adjusted according to some parameter of the soil
which reflects the character of the signal attenuation. However, such a
parameter has not been identified.
The response of the water content reflectometer to changing water content
has been shown to change for some soils when bulk density exceeds 1.5 g
cm
–3
. The response to changing water content is still well behaved, but the
slope will decrease with increasing bulk density.
8.1.4 Error Sources in Water Content Reflectometer Measurement
8.1.4.1 Probe-to-Probe Variability Error
All manufactured CS616s/CS625s are checked in standard media. The
limits for probe response in the standard media ensure accuracy of
2%
volumetric water content.
8.1.4.2 Insertion Error
The method used for probe insertion can affect the accuracy of the
measurement. The probe rods should be kept as close to parallel as
possible when inserted 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 that generates air voids around the rods will indicate lower water
content than actual. In some applications, installation can be improved by
using insertion guides or a pilot tool. Campbell Scientific offers the
CS650G insertion tool.