Campbell Scientific CS616 and CS625 Water Content Reflectometers - Manual (Page 30)

CS616 and CS625 Water Content Reflectometers
53 VWC_2 9 1 1
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99 VWC_48 17 1 1
7. The Water Content Reflectometer Method for
Measuring Volumetric Water Content
7.1 Description of Measurement Method
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 (1) has a high value for dielectric permittivity and (2) 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 2 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.
7.2 Response Curves
FIGURE 7-1 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% VWC to 50% VWC.
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