Campbell CS650, User Manual

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User Manual
21

v
= -5.3*10
-2
+ 2.92*10
-2
K
a
– 5.5*10
-4
K
a
2
+ 4.3*10
-6
K
a
3
It has been shown in numerous research efforts that this equation works well in
most mineral soils, so a soil specific calibration of the CS650 probe is usually not
necessary. If a soil specific calibration is desired, the user can generate an
equation relating K
a
to

v
following the methods described in Section 8.4 , Water
Content Reflectometer User-Calibration
(p. 23)
.
8.3.3 Electrical Conductivity
8.3.3.1 Soil Electrical Conductivity
The quality of soil water measurements which apply electromagnetic fields to
wave guides is affected by soil electrical conductivity. The propagation of
electromagnetic fields in the configuration of the CS650 is predominantly affected
by changing dielectric permittivity 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 reduces the bandwidth. The attenuation reduces
oscillation frequency at a given water content because it takes a longer time to
reach the oscillator trip threshold.
It is important to distinguish between soil bulk electrical conductivity and soil
solution electrical conductivity. Soil solution electrical conductivity refers to the
conductivity of the solution phase of soil. Soil solution electrical conductivity,

solution
can be determined in the laboratory using extraction methods to separate
the solution from the solid and then measuring the electrical conductivity of the
extracted solution.
The relationship between solution and bulk electrical conductivity can be
described by (Rhoades et al., 1976)
   
bulk solution
 
v solid

with

bulk
being the electrical conductivity of the bulk soil;

solution
, the soil
solution;

solid
, the solid constituents;

v
, the volumetric water content; and

, 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 firmware corrects the oscillation frequency
for the effects of

solution
up to 3 dS m
-1
for the CS650 and up to
10 dS m
-1
for the CS655. This is equivalent to

bulk
values of approximately 0.8
dS m
-1
and 2.7 dS m
-1
respectively. If

bulk
exceeds these limits, the CS650 probe
will return 99999 for dielectric permittivity and volumetric water content. The
measured period average and voltage ratio values will continue to be reported
even if the bulk EC is outside the operational range of the probe.