10HS
3
THEORY
3
Theory
The 10HS sensor measures the volumetric water content of the soil
using a capacitance technique. Rapidly charging and discharging
a positive and ground electrode (capacitor) in the soil generates an
electromagnetic field whose charge time
t
is related to the capacitance
(
C
) of the soil by Equation 1.
t
=
RC
ln
V
−
V
f
V
i
−
V
f
(1)
where
R
is the series resistance,
V
is voltage at time
t
, V
i
is the
starting voltage and
V
f
is the applied or supply voltage. Further,
for a capacitor with a geometrical factor of
F
, the capacitance is
related to the dielectric permittivity (
ε
) of the medium between the
capacitor electrodes by
C
=
ε
0
εF
(2)
where
ε
0
is the permittivity of free space. Thus, the
ε
of the soil can
be determined by measuring the change time
t
of a sensor buried in
the soil. Consequently, as water has a dielectric permittivity that is
much greater than soil minerals or air, the charge time
t
in the soil
of Equation 1 can be correlated with soil volumetric water content.
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