Calibration
The
Profile Probe
detects soil moisture by responding to the
permittivity (
ε′
) of the damp soil (see illustration on page 8) – or
more accurately to the refractive index of the damp soil, which is
~ equivalent to
√ε
.
As a result, the performance of the Profile Probe is best
understood if it is split into these two stages:
Soil calibration
: soil moisture (
θ)
determines
√ε
Profile Probe response
:
√ε
determines PR2 output (Volts)
Soil calibration
Soil calibration
0
1
2
3
4
5
6
0
10
20
30
40
50
Water content of soil (%vol)
√ε
damp soil
Slope
(
a
1
)
Soil offset
(
a
0
)
This method of detection is very sensitive and accurate, but of
course soils can be enormously different one from another.
The soil offset and the slope of the line in the graph above both
depend slightly on soil type, varying with density, clay content,
organic matter etc.
This can be usefully summed up in a simple equation describing
the relationship between
√ε
and the soil water content,
θ
, which
contains two parameters (
a
0
and
a
1
) that reflect the influence of
the soil:
θ
ε
×
+
=
1
0
a
a
[1]
The accuracy of your
Profile Probe
readings can be improved if
you choose appropriate values for
a
0
and
a
1
.
This is usually very
simple…
Profile Probe User Manual
3.0a
Operation
z
17
