Campbell CS616 User Manual - 8. Operation
CS616 and CS625 Water Content Reflectometers
10
measured for the average calculation. An over range value will be stored if
the Timeout period is exceeded. A value of 1 is recommended if 10 is used
for cycle’s parameter.
Port: The Port parameter is the control port or analogue channel that will
be used to switch power to the CS625 Water Content Reflectometer.
Mult, Offset: The Mult and Offset parameters are each a constant, variable,
array, or expression by which to scale the results of the raw measurement.
A multiplier value of 1 is recommended.
8. Operation
8.1 Water Content Reflectometer Method for Measuring
Volumetric Water Content
The water content reflectometer method for measuring soil water content is
an indirect measurement that is sensitive to the dielectric permittivity of the
material surrounding the probe rods. Since water is the only soil
constituent that has a high value for dielectric permittivity and is the only
component other than air that changes in concentration, a device sensitive
to dielectric permittivity can be used to estimate volumetric water content
The fundamental principle for CS616/CS625 operation is that an
electromagnetic pulse will propagate along the probe rods at a velocity that
is dependent on the dielectric permittivity of the material surrounding the
line. As water content increases, the propagation velocity decreases
because polarization of water molecules takes time. The travel time of the
applied signal along two times the rod length is essentially measured.
The applied signal travels the length of the probe rods and is reflected from
the rod ends traveling back to the probe head. A part of the circuit detects
the reflection and triggers the next pulse.
The frequency of pulsing with the probe rods in free air is about 70 MHz.
This frequency is scaled down in the water content reflectometer circuit
output stages to a frequency easily measured by a datalogger. The probe
output frequency or period is empirically related to water content using a
calibration equation.
8.1.1 Response Curves
Figure
8-1
shows calibration data collected during laboratory
measurements in a loam soil with bulk density 1.4 g cm
–3
and bulk
electrical conductivity at saturation of 0.4 dS m
–1
. For this soil, the
saturation bulk electrical conductivity of 0.4 dS m
–1
corresponds to
laboratory electrical conductivity using extraction methods of about 2
dS m
–1
.
The response is accurately described over the entire water content range by
a quadratic equation. However, in the typical water content range of about
10% to about 35% volumetric water content, the response can be described
with slightly less accuracy by a linear calibration equation. The
manufacturer supplied quadratic provides accuracy of
2.5% volumetric
water content for soil electrical conductivity
0.5 dS m
–1
and bulk density
1.55 g cm
–3
in a measurement range of 0% to 50% VWC.