Instruction manual

CS650 and CS655 Water Content Reflectometers
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 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 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.
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, EC readings may be converted 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 Probe-to-Probe Variability Error
All manufactured CS650s/CS655s are checked in standard media to develop a
probe specific span and offset value for electrical conductivity and dielectric
permittivity measurements. These probe specific values are written to the
probe’s firmware and minimize probe-to-probe variability.
8.3.4.2 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.
8.3.5 Temperature Dependence and Correction
The two temperature dependent sources of error in CS650 water content
measurements are the effect of temperature on the operation of the probe
electronics and the effect of temperature on the dielectric permittivity of the
soil.
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