CS615 WATER CONTENT REFLECTOMETER
2
3. SPECIFICATIONS
3.1 DIMENSIONS
Rods:
30.0 cm long
3.2 mm diameter
3.2 cm spacing
Head:
11.0 cm x 6.3 cm x 2.0 cm
3.2 WEIGHT
Probe: 280 g
Cable: 35 g m
-1
3.3 ELECTRICAL
Power
70 milliamps @ 12VDC when enabled
less than 10 microamps quiescent
Power Supply Voltage
9VDC minimum, 18VDC maximum
Enable Voltage
minimum voltage to enable probe is 1.3VDC
4. PERFORMANCE SPECIFICATIONS
4.1 ACCURACY
See the Calibration section for a discussion of
accuracy. The accuracy is
±
2% when using
calibration for specific soil. The accuracy when
using the general calibrations depends on soil
texture and mineral composition.
4.2 RESOLUTION
The resolution of the volumetric water content
measurement depends on which datalogger
instruction is used. When the CR10X, CR10 or
CR500 Instruction 27, Period Measurement, is
used, the resolution is on the order of 10
-6
m
3
m
-3
.
Period Measurement is not available on the CR7
or 21X.
When Instruction 3, Pulse Count, is used, the
resolution with an execution interval of 1.0
second is 10
-4
m
3
m
-3
when pulse period is 1.3
milliseconds. The resolution improves as the
water content decreases and as the execution
interval increases. A shorter execution interval
of 0.1 seconds yields a resolution of 10
-2
m
3
m
-3
at the same water content.
4.3 OPERATING RANGE
4.3.1 Soil Electrical Conductivity
The quality of soil moisture measurements
which apply electromagnetic fields to wave
guides is affected by soil electrical conductivity.
The propagation of electromagnetic fields in the
configuration of the CS615 is predominantly
affected by changing dielectric constant due to
changing water content, but it is also affected by
electrical conductivity. Free ions in soil solution
provide electrical conduction paths which result
in attenuation of the signal applied to the
waveguides. This attenuation both reduces the
amplitude of the high-frequency signal on the
probe rods and affects the shape of the
oscillating signal. The attenuation reduces
oscillation frequency at a given water content
because it takes a longer time to reach the
oscillator trip threshold.
Soil electrical conductivity can be described by
(Rhoades et al., 1976)
σ
σ
θ
σ
bulk
solution
=
+
v
solid
Τ
with
σ
the electrical conductivities of the bulk
soil, the soil solution, and 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. The above
equation is presented here the show the
relationship between soil solution electrical
conductivity and soil bulk electrical conductivity.
Soil solution electrical conductivity,
σ
solution
can
be determined in the laboratory using extraction
methods. Soil bulk electrical conductivity can
be measured using time domain reflectometry
(TDR) methods. Most expressions of soil
electrical conductivity are given in terms of
solution conductivity. Discussion of the effects
of soil electrical conductivity on CS615
performance will be on a soil solution basis
unless stated otherwise.
When soil solution electrical conductivity values
exceed 1 dS m
-1
, the slope of the calibration
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 the Calibration section.) At
