CS650 and CS655 Water Content Reflectometers
21
The following describes the relationship between solution and bulk electrical
conductivity (Rhoades et al., 1976)
σ
σ
θ
σ
bulk
solution
=
+
v
solid
Τ
where
σ
bulk
is the electrical conductivity of the bulk soil;
σ
solution
, the soil
solution;
σ
solid
, the solid constituents;
θ
v
, the volumetric water content; and
Τ
, a
soil-specific transmission coefficient intended to account for the tortuosity of
the flow path as water content changes. See Rhoades et al., 1989 for a form of
this equation which accounts for mobile and immobile water. This publication
also discusses soil properties related to CS650 operation such as clay content
and compaction. The above equation is presented here to show the relationship
between soil solution electrical conductivity and soil bulk electrical
conductivity.
Most expressions of soil electrical conductivity are given in terms of solution
conductivity or electrical conductivity from extract since it is constant for a
soil. Bulk electrical conductivity increases with water content so comparison of
the electrical conductivity of different soils must be at the same water content.
The calibration equation in the CS650 operating system corrects the oscillation
frequency for the effects of
σ
solution
up to 3 dS m
–1
for the CS650 and up to
10 dS m
–1
for the CS655. This is equivalent to
σ
bulk
values of approximately
0.8 dS m
–1
and 2.7 dS m
–1
respectively. If
σ
bulk
exceeds these limits, the CS650
sensor returns 99999 for dielectric permittivity and volumetric water content.
The measured period average and voltage ratio values continue to report even
if the bulk EC is outside the operational range of the sensor.
8.3.3.2 Temperature Correction of Soil Electrical Conductivity
The EC value reported by the CS650 is bulk electrical conductivity. This value
is temperature dependent, changing by 2% per degree Celsius. To compensate
for the effect of temperature, convert EC readings to a standard temperature,
such as 25 °C using the following equation:
EC
25
= EC
T
/ (1 + 0.02•(T
soil
–25)
where EC
25
is the
σ
bulk
value at 25 °C and EC
T
is the
σ
bulk
value at soil
temperature T
soil
(°C).
8.3.4 Error Sources in Water Content Reflectometer Measurement
8.3.4.1 Sensor-to-Sensor Variability Error
All manufactured CS650s/CS655s are checked in standard media to develop a
sensor specific span and offset value for electrical conductivity and dielectric
permittivity measurements. These sensor specific values are written to the
sensor operating system and minimize sensor-to-sensor variability.
8.3.4.2 Insertion Error
The method used for sensor insertion can affect the accuracy of the
measurement. The sensor 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. Sensors inserted in a manner that generates air
