8.2
Measurements
at
fast
scan
rates
Using
the
SlowSequence
function
allows
the
SDI-12
instruction
to
run
as
a
background
process,
causing
minimum
interference
to
other
measurements
that
use
the
analog
ue
hardware.
Measuring
the
SoilVUE
10
in
a
SlowSequence
section
of
the
program
allows
faster
programs
to
run
as
the
main
scan.
However,
if
the
data
logger
is
too
busy
to
complete
all
of
its
tasks,
some
slow
sequence
commands
may
be
skipped
resulting
in
NANs
(not
a
number)
instead
of
measurements.
8.3
Measurement
theory
The
SoilVUE
10
is
a
multiparameter
soil
profile
probe
that
measures
soil
volumetric
water
content
using
a
time-domain
reflectometry
(TDR)
method.
The
probe
consists
of
TDR
circuitry
connected
to
a
series
of
six
or
nine
helical
waveguides
that
makeup
part
of
the
overall
threaded
design.
The
travel
time
for
a
pulsed
electromagnetic
signal
along
a
waveguide
is
dependent
on
the
velocity
of
the
signal
and
the
waveguide
length.
The
velocity
is
dependent
on
the
dielectric
constant
of
the
material
surrounding
the
waveguide.
This
relationship
can
be
expressed
by:
Eq. 1
Where,
K
a
is the apparent permittivity
c is the velocity of electromagnetic signals in free space
Δt is the travel time
L is the waveguide length
(p. 16) can be simplified to express the apparent permittivity as the ratio of the apparent
probe length (L
a
= c•Δt/2) to the real probe length.
K
a
= (L
a
/L)
2
Eq. 2
The permittivity of water relative to other soil constituents is high. Consequently, changes in
volumetric water content can be directly related to the change in the permittivity of bulk soil
material. The relationship between permittivity and volumetric water content has been described
by, among others, Topp et al. (1980) and Ledieu et al. (1986) in an empirical fashion using both
polynomial and linear forms.
SoilVUE™10 Complete Soil Profiler
16
