sensor specific values are written to the sensor operating system and minimize sensor-to-sensor
variability.
8.3.4.2 Insertion error
The method used for sensor insertion can affect the accuracy of the measurement. The sensor
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. Sensors inserted in a manner that generates air
voids around the rods indicate lower water content than actual. In some applications, the CS650G
insertion and pilot tool helps maintain the proper spacing and parallel orientation of the rods
during sensor insertion.
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 sensor electronics and the effect of temperature on
the dielectric permittivity of the soil.
The effect of temperature on sensor 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 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.
CS650 and CS655 Water Content Reflectometers
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