Campbell Scientific CS650 and CS655 Water Content Reflectometers - Manual (Page 33)

CS650 and CS655 Water Content Reflectometers
7. The Water Content Reflectometer Method for
Measuring Volumetric Water Content
7.1 Description of Measurement Method
For the water content measurement, a differential emitter-coupled logic (ECL)
oscillator on the circuit board is connected to the two parallel stainless steel
rods. The differentially driven rods form an open-ended transmission line in
which the wave propagation velocity is dependent upon the dielectric
permittivity of the media surrounding the rods. An ECL oscillator state change
is triggered by the return of a reflected signal from the end of one of the rods.
The fundamental principle for CS650 water content measurement is that the
velocity of electromagnetic wave propagation along the probe rods is
dependent on the dielectric permittivity of the material surrounding the rods.
As water content increases, the propagation velocity decreases because of
increasing dielectric permittivity. Therefore, the two-way travel time of the
rod signal is dependent upon water content, hence the name water content
reflectometer. Digital circuitry scales the high-speed oscillator output to an
appropriate frequency for measurement by an onboard microprocessor.
Increases in oscillation period resulting from signal attenuation are corrected
using an electrical conductivity measurement. A calibration equation converts
period and electrical conductivity to bulk dielectric permittivity. The Topp
equation is used to convert from permittivity to volumetric water content.
7.2 The Topp Equation
The relationship between dielectric permittivity and volumetric water content
in mineral soils has been described by Topp et al. (1980) in an empirical
fashion using a 3 rd degree polynomial. With θ
v
the volumetric water content
and K
a
the bulk dielectric permittivity of the soil, the equation presented by
Topp et al. is
θ
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 θ
v
following the methods described in Section 8, Water
Content Reflectomer User-Calibration.
7.3 Electrical Conductivity
7.3.1 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.
25