Campbell Scientific CS650 and CS655 Water Content Reflectometers - Manual (Page 35)

CS650 and CS655 Water Content Reflectometers
7.4 Error Sources in Water Content Reflectometer
Measurement
7.4.1 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.
7.4.2 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.
7.5 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.
The effect of temperature on probe electronics is minimal with period average
readings varying by less than 0.5% of the 20 °C reading over the range of 10°
to 30°C and less than 2% of the 20°C reading over the range of -10° to 70°C.
The larger error is caused by the change in dielectric permittivity of soil with
temperature. This is mostly due to the high temperature dependence of the
permittivity of water, which varies from a value of 88 at 0°C to 64 at 70°C.
Since water is the major contributor to bulk dielectric permittivity of soil,
temperature related changes to the permittivity of water will lead to
overestimation of volumetric water content at temperatures below 20°C and
underestimation of volumetric water content at temperatures above 20°C.
The Topp equation does not account for soil temperature. The effect of
temperature on the soil permittivity is related to soil specific properties such as
porosity and the permittivity of the soil solid phase with temperature.
Consequently, a general equation that corrects volumetric water content for
temperature for all soils is not available.
A temperature correction equation that works well in quartz sand is given by:
θ
Corr
= θ - 0.0044*Tθ 3 + 0.0014*Tθ 2 + 0.0029*Tθ – 0.0002*T + 2.4*θ 3 –
1.6*θ 2 + 0.32*θ – 0.046
where θ
Corr
is the temperature corrected volumetric water content, T is soil
temperature in °C, and θ is the volumetric water content value at soil
temperature T.
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